David's Wiki - User contributions [en]

Investing

2026-05-10T04:09:12Z

David: /* References */

In general, you should save money in the following accounts in order:

# 401k, 403b, or thrift savings
# Traditional or Roth IRA (up to IRS limit)
# Investing account
# High-yield Savings, Cash account, or CD
# Checking account

* 1-3 will be tax-advantaged (either tax-free or tax-deferred) since they are retirement accounts
* 4-6 will not be tax-advantaged

==401(k)/403(b)==
A 401(k) or 403(b) is an employer-sponsored retirement plan. 
That means, it is managed by a company cooperating with your employer. 
If you work for a for-profit company, you will have a 401k.
If you work for a public school or non-profit company, you get a 403b. 
If you work for the federal government, you will have a thrift savings plan. 
For the most part, 401k and 403b rules are identical.

* Employees can contribute up to $19,500 for 2020 ($19,000 in 2019) [https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sharing-plan-contribution-limits]
** These are called "elective deferrals" which are tax-deferred. You may also contribute after-tax deferrals subject to the total maximum contribution below.
* In total, you can add up to $57,000 to your 401k each year, or up to 100% of your income, whichever is lower. This includes both your contributions and your employer's contributions

==Individual Retirement Account (IRA)==
A tax-advantaged retirement account you can control. 
The IRS allows you to deposit up to $6000 ($7000 if 50 or older) per year or up to your income, whichever is lower.
Note that this limit is for all your IRAs combined.
Typically, you should save in a Roth IRA unless you surpass the income limit.
If you're not eligible for the Roth IRA, you may consider the backdoor Roth IRA.

===Traditional IRA===
In a traditional IRA, you deposit pre-tax money (see notes).
Thus, your deposit counts as a tax-deduction.
You pay taxes when you withdraw your money.

;Notes
* Early withdraws (before age 59.5) are subject to a 10% penalty plus taxes
* You must start taking required minimum distributions (RMDs) at age 72
* No more contributions after age 70.5
* While there are no income limits to contributing to a traditional IRA, there are income limits to deducting from your taxes
** See [https://www.irs.gov/retirement-plans/ira-deduction-limits IRA Deduction Limits]
** To avoid being double taxed on your Traditional IRA contributions, be sure to complete [https://www.irs.gov/forms-pubs/about-form-8606 IRS Form 8606]

===Roth IRA===
In a Roth IRA, you deposit post-tax money. However, your money grows tax-free. 
There is an income limit to the Roth IRA of $124,000 in 2020.

;Notes
* No required minimum distributions on your own Roth IRAs
** There are RMDs on inherited Roth IRAs
* You can withdraw your contributions (but not earnings) without penalty
** Note that any further contributions will count towards your annual limit so you cannot "borrow" from your Roth IRA.
* No age limits on contributions

===Backdoor Roth IRA===
If you are a high-earner and believe you will earn more money in retirement,
you may want to do a backdoor Roth IRA to grow your retirement account tax-free rather than tax-deferred.

;Basic Idea
* Contribute to a traditional IRA
* Convert the traditional IRA into a Roth IRA

;Notes
* To avoid tax complications, you should eliminate all other pre-tax IRAs beforehand by rolling them over to a 401k. Otherwise, you will be subject to the pro-rata rule.

===Mega Backdoor Roth===
Requirements: Your 401k allows after-tax contributions and non-hardship withdrawals.

;Basic Idea:
* Max out after-tax contributions to your 401k
* Rollover or withdraw the after-tax portion to a Roth IRA or Roth 401K

==ETFs==
{{main | Exchange-traded fund}}

Exchange-traded funds. Typically these will have a fee called an expense ratio. 
The expense ratio is measured in basis points. 
25 basis points is an annual fee of 0.25%.

Investing

2026-05-10T04:09:02Z

David: /* Brokerages */

Investing

2026-05-04T06:08:43Z

David: /* ETFs */

Investing

2026-05-04T06:08:06Z

David:

In general, you should save money in the following accounts in order:

# 401k, 403b, or thrift savings
# Traditional or Roth IRA (up to IRS limit)
# Investing account
# High-yield Savings, Cash account, or CD
# Checking account

* 1-3 will be tax-advantaged (either tax-free or tax-deferred) since they are retirement accounts
* 4-6 will not be tax-advantaged

==401(k)/403(b)==
A 401(k) or 403(b) is an employer-sponsored retirement plan. 
That means, it is managed by a company cooperating with your employer. 
If you work for a for-profit company, you will have a 401k.
If you work for a public school or non-profit company, you get a 403b. 
If you work for the federal government, you will have a thrift savings plan. 
For the most part, 401k and 403b rules are identical.

* Employees can contribute up to $19,500 for 2020 ($19,000 in 2019) [https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sharing-plan-contribution-limits]
** These are called "elective deferrals" which are tax-deferred. You may also contribute after-tax deferrals subject to the total maximum contribution below.
* In total, you can add up to $57,000 to your 401k each year, or up to 100% of your income, whichever is lower. This includes both your contributions and your employer's contributions

==Individual Retirement Account (IRA)==
A tax-advantaged retirement account you can control. 
The IRS allows you to deposit up to $6000 ($7000 if 50 or older) per year or up to your income, whichever is lower.
Note that this limit is for all your IRAs combined.
Typically, you should save in a Roth IRA unless you surpass the income limit.
If you're not eligible for the Roth IRA, you may consider the backdoor Roth IRA.

===Traditional IRA===
In a traditional IRA, you deposit pre-tax money (see notes).
Thus, your deposit counts as a tax-deduction.
You pay taxes when you withdraw your money.

;Notes
* Early withdraws (before age 59.5) are subject to a 10% penalty plus taxes
* You must start taking required minimum distributions (RMDs) at age 72
* No more contributions after age 70.5
* While there are no income limits to contributing to a traditional IRA, there are income limits to deducting from your taxes
** See [https://www.irs.gov/retirement-plans/ira-deduction-limits IRA Deduction Limits]
** To avoid being double taxed on your Traditional IRA contributions, be sure to complete [https://www.irs.gov/forms-pubs/about-form-8606 IRS Form 8606]

===Roth IRA===
In a Roth IRA, you deposit post-tax money. However, your money grows tax-free. 
There is an income limit to the Roth IRA of $124,000 in 2020.

;Notes
* No required minimum distributions on your own Roth IRAs
** There are RMDs on inherited Roth IRAs
* You can withdraw your contributions (but not earnings) without penalty
** Note that any further contributions will count towards your annual limit so you cannot "borrow" from your Roth IRA.
* No age limits on contributions

===Backdoor Roth IRA===
If you are a high-earner and believe you will earn more money in retirement,
you may want to do a backdoor Roth IRA to grow your retirement account tax-free rather than tax-deferred.

;Basic Idea
* Contribute to a traditional IRA
* Convert the traditional IRA into a Roth IRA

;Notes
* To avoid tax complications, you should eliminate all other pre-tax IRAs beforehand by rolling them over to a 401k. Otherwise, you will be subject to the pro-rata rule.

===Mega Backdoor Roth===
Requirements: Your 401k allows after-tax contributions and non-hardship withdrawals.

;Basic Idea:
* Max out after-tax contributions to your 401k
* Rollover or withdraw the after-tax portion to a Roth IRA or Roth 401K

==ETFs==
{{main | Exchange-traded fund}}

Exchange-traded funds. Typically these will have a fee called an expense ratio. 
However, since they are not usually actively managed, their fees are often lower than mutual funds. 
The expense ratio is measured in basis points. 
25 basis points is an annual fee of 0.25%. 
The following classifications are stolen from Wealthfront and Acorns 

==Brokerages==

===Fidelity===

===Wealthfront===
{{main | Wealthfront }}
; [https://www.wealthfront.com/c/affiliates/invited/AFFA-BMT8-AIJ3-FS7E Referral] 

; Cash Account
Wealthfront offers a cash account.
This account is distributed between up to several banks so it is FDIC Insured up to $3 million <ref name="wealthfrontfdic" />.

==References==
{{reflist|refs=
<ref name="wealthfrontfdic">https://www.wealthfront.com/blog/wealthfront-fdic-insurance/</ref>
}}

NixOS

2025-12-24T03:40:12Z

David: /* Generate a config and install */

== Install ==
The graphical installer is trivial. However, the minimal ISO doesn't handle partitioning for you.

=== Partitioning and Formatting ===
<syntaxhighlight lang="bash>
# Create a new GPT partition table
parted /dev/sda -- mklabel gpt

# Create the EFI Boot partition
parted /dev/sda -- mkpart ESP fat32 1MB 512MB
parted /dev/sda -- set 1 esp on

# Create the Root partition (ext4)
parted /dev/sda -- mkpart primary ext4 512MB 100%

# Format partitions
mkfs.fat -F 32 -n boot /dev/sda1
mkfs.ext4 -L nixos /dev/sda2

# Mount the root partition
mount /dev/disk/by-label/nixos /mnt

# Mount the boot partition
mkdir -p /mnt/boot
mount /dev/disk/by-label/boot /mnt/boot

# Create a 4GB swapfile (adjust size as needed)
dd if=/dev/zero of=/mnt/swapfile bs=1M count=4096
chmod 600 /mnt/swapfile
mkswap /mnt/swapfile
swapon /mnt/swapfile
</syntaxhighlight>

=== Generate a config and install===
<syntaxhighlight lang="bash">
nixos-generate-config --root /mnt
# Edit the config to your desire
nano /mnt/etc/nixos/configuration.nix

nixos-install
</syntaxhighlight>

Example config:
<pre>
{ config, pkgs, ... }:

{
imports = [ ./hardware-configuration.nix ];

# Bootloader settings
boot.loader.systemd-boot.enable = true;
boot.loader.efi.canTouchEfiVariables = true;

# 1. ZFS Support for additional drives
boot.supportedFilesystems = [ "zfs" ];
networking.hostId = "8425e349"; # Required for ZFS; use 8 random hex digits

# 2. Static IP Configuration
# Replace 'eth0' with your actual interface name found via 'ip link'
networking.useDHCP = false;
networking.interfaces.eth0.ipv4.addresses = [{
address = "192.168.1.50";
prefixLength = 24;
}];
networking.defaultGateway = "192.168.1.1";
networking.nameservers = [ "1.1.1.1" "8.8.8.8" ];

# 3. Swapfile declaration
swapDevices = [ {
device = "/swapfile";
size = 4096; # In MB
} ];

# Standard System Settings
system.stateVersion = "24.11"; # Check the current version on the ISO
}
</pre>

=== Install on VPS ===
For a VPS, first install Ubuntu or Debian.
Then run https://github.com/elitak/nixos-infect.

== Resources==
* https://nixos.org/manual/nixos/stable/
* https://wiki.nixos.org/wiki/NixOS_Wiki

NixOS

2025-12-21T23:16:02Z

David: /* Generate a config and install */

Linux

2025-12-21T23:12:51Z

David:

A collection of notes on using Linux systems.
Notes here are for Ubuntu but should work on similar debian derivative distros.
See also [[NixOS]].

==Basic Terminal Commands==
{{see also | Bash (Unix shell)}}
===List===
<code>ls</code>
* <code>-l</code> shows long format
* <code>-a</code> shows all files including hidden files, current directory <code>.</code>, and parent directory <code>..</code>.
** <code>-A</code> omits <code>.</code> and <code>..</code>
* <code>-h</code> human readable file sizes
* <code>-s</code> shows blocks taken up by the file (i.e. size on disk)

There are also other commands like <code>lsblk</code>, <code>lscpu</code>, <code>lshw</code>.

===Disk Space===
* <code>du</code> Disk Usage
** <code>du -sh</code> Show size of current directory
** <code>du -h --max-depth=1</code> Show size of files and folders in current directory. I have <code>du</code> aliased to this.
** Flags:
*** <code>-h</code> human readable (adds M or G)
*** <code>--max-depth</code> depth to recurse. Default is <code>N</code>.
* <code>df</code> Disk Filesystems
** Shows usage, total space available, and mount position
** <code> df -Ph .</code> See free space in current directory

If looking to free up space, I recommend installing <code>ncdu</code>.

===Monitoring===
* <code>htop</code> - basic terminal system monitor, enhanced version of <code>top</code>
* <code>watch -n 0.5 <program></code> - repeatedly call <program> every 0.5 seconds

===Standard Streams===
* <code>|</code> will pipe stdout to the stdin of another process
* <code>></code> will redirect stdout to a file
* <code>2>&1</code> will redirect stderr (2) to stdout (1)
* [https://www.gnu.org/software/coreutils/manual/html_node/tee-invocation.html <code>tee</code>] will redirect stdout to multiple files and show it in the terminal

===Shutdown===
<pre>
shutdown -h [now | -t <time>]
</pre>

* <code>-h</code> poweroff, the default
* <code>-t time</code> schedule a shutdown in ''time'' seconds
* <code>-r</code> restart
* <code>-c</code> cancel pending shutdown

==Package Management==
See [https://www.digitalocean.com/community/tutorials/package-management-basics-apt-yum-dnf-pkg DigitalOcean: Package management basics]

===apt===
<syntaxhighlight lang="bash">
# List all installed packages
apt list --installed

# Search repos for package
apt search libdpkg-dev
</syntaxhighlight>

;Repositories
Repository sources are saved in
* A line in <code>/etc/apt/sources.list</code>
* A file in <code>/etc/apt/sources.list.d/</code>

Application desktop icons are stored in <code>/usr/share/applications/</code>.

The update notifications are in <code>/etc/apt/apt.conf.d/99update-notifier</code>. Comment these out to disable them. 
Unattended-updates are in <code>/etc/apt/apt.conf.d/50unattended-upgrades</code>.

{{hidden | dpkg |
===dpkg===
<syntaxhighlight lang="bash">
# List everything
sudo dpkg -l

# List things with apache in the name
sudo dpkg -l | grep apache
</syntaxhighlight>
}}
{{hidden | yum |
===yum===
<syntaxhighlight lang="bash">
# Update package lists, typically not necessary
yum check-update

# Upgrade packages
yum update
</syntaxhighlight>
}}

==SSH==
===SSH Keys===
Generate an ssh-key for every client
<syntaxhighlight lang="bash">
ssh-keygen -t ed25519 [-C "comment your client name"] [-f output_path]
</syntaxhighlight>

Some older software such as Solid file explorer require RSA keys in PEM key format
<syntaxhighlight lang="bash">
ssh-keygen -t rsa -b 4096 -m PEM [-C "comment your client name"] [-f output_path]
</syntaxhighlight>

You can also convert existing keys to PEM format
<syntaxhighlight lang="bash">
ssh-keygen -p -m PEM [-C "comment your client name"] [-f output_path]
</syntaxhighlight>

If you want to change the comment on your key
<syntaxhighlight lang="bash">
ssh-keygen -c -C "New comment" -f path_to_key
</syntaxhighlight>

Manage ssh keys
<syntaxhighlight lang="bash">
# On the client
ssh-copy-id <host>
</syntaxhighlight>
<syntaxhighlight lang="bash">
# On the server
vim ~/.ssh/authorized_keys
</syntaxhighlight>

Notes: 
* According to [https://security.stackexchange.com/questions/143442/what-are-ssh-keygen-best-practices this] you should avoid using ECDSA and DSA keys.

===Disable password authentication===
# Edit <code>/etc/ssh/sshd_config</code>
# Set <code>PasswordAuthentication</code> to <code>no</code>
# Set <code>ChallengeResponseAuthentication</code> to <code>no</code>
# Test by ssh'ing into the machine using <code>-o PreferredAuthentications=password -o PubkeyAuthentication=no</code>

===Port Forwarding===
Also known as: SSH Tunneling, SSH Proxy, SSH Reverse Proxy

If you need to access a port on the remote computer, you can use the <code>-L</code> option to forward ports from the remote to the local machine.
<syntaxhighlight lang="bash">
ssh -L <localport>:localhost:<remoteport> <remoteurl>
# E.g. ssh -L 8080:localhost:80 david@davidl.me
</syntaxhighlight>

You can also do the reverse, giving the remote access to a local port using <code>-R</code>
<syntaxhighlight lang="bash">
ssh -R <localport>:host:<remoteport> <remoteurl>
# E.g. ssh -R 8080:localhost:80 david@davidl.me
</syntaxhighlight>

;Notes
* You can also run this without creating a shell using <code>-N</code>. This will block your shell. See [https://unix.stackexchange.com/questions/100859/ssh-tunnel-without-shell-on-ssh-server SE Answer].
* Adding <code>-f</code> pushes ssh to the background.
** This will implicitly add <code>-n</code> which redirects <code>stdin</code> from <code>/dev/null</code>.
** If you want to be able to foreground this again, the use <code>&</code> or <kbd>Ctrl</kbd>+<kbd>z</kbd> instead.

===alias===
You can create aliases in your <code>.ssh/config</code>
<pre>
Host my_alias
User my_username
Hostname my_server@my_domain.com
Port 52
</pre>

==VNC==
===x11vnc===
[https://askubuntu.com/questions/1033274/ubuntu-18-04-connect-to-login-screen-over-vnc Reference]

I recommend not exposing VNC. Set it to localhost only and use ssh port forwarding.

===Remmina===
If using a wired connection, you can save a preset to <code>localhost:5901</code> or similar.

Note that the Remmina which ships with Ubuntu 18.04 is outdated and buggy.
You can upgrade it by adding the Remmina PPA.
See [https://remmina.org/how-to-install-remmina/ https://remmina.org/how-to-install-remmina/] for details.
<pre>
sudo apt-add-repository ppa:remmina-ppa-team/remmina-next
sudo apt update
sudo apt install remmina remmina-plugin-rdp remmina-plugin-secret
</pre>

==Nvidia==

===Driver Installation===
# Run <code>ubuntu-drivers list</code> to get a list of drivers
# Install the latest driver
#* E.g. <code>sudo apt install nvidia-driver-460</code>
# If you have secure boot enabled, you will be asked for a password during installation
#* This is because the driver is a DKMS module.
#* After installation, reboot your computer and select "Enroll MOK" and enter that password in.
#* '''Note''' Failure to do this will result in the driver not working
# Validate your installation by running <code>nvidia-smi</code>.
#* <code>nvidia-smi</code> shows the latest cuda version supported by the driver, not the cuda version installed.

===Cuda Installation===
Download cuda from the nvidia website or add the cuda repo to your apt sources.

===Switching between Nvidia and Intel===
[https://www.linuxbabe.com/desktop-linux/switch-intel-nvidia-graphics-card-ubuntu Reference]

Make sure the Nvidia graphics drivers are installed. Then you can select between Nvidia and Intel GPUs using the Nvidia X Server Settings application <code>nvidia-settings</code>. Alternatively, you can use the following commands in the terminal. 
To switch to the Nvidia GPU:
<pre>
sudo prime-select nvidia
</pre>
To switch back to the Intel GPU:
<pre>
sudo prime-select intel
</pre>
<code>prime-select query</code> will print either <code>nvidia</code> or <code>intel</code> to stdout.

===Fix tearing on laptops===
[https://ubuntuhandbook.org/index.php/2018/07/fix-screen-tearing-ubuntu-18-04-optimus-laptops/ Reference] 
# Add <code>options nvidia-drm modeset=1</code> to <code>/etc/modprobe.d/nvidia-drm-nomodeset.conf</code>
# Run <code>sudo update-initramfs -u</code>

==Environment Variables==
[https://help.ubuntu.com/community/EnvironmentVariables Ubuntu Help Reference]

==Tmux==
[https://tmuxcheatsheet.com/ Tmux cheat sheet]

Tmux, or Terminal Multiplexer is an alternative to screen. 
Use it to keep terminals open and tasks running after you disconnect your SSH connection. 
Getting Started:
<syntaxhighlight lang="bash">
# Make a new session
tmux
# Make a new named session
tmux new -s my_session
# Rename a session
# Keybinding: Ctrl + b, $
tmux rename-session [-t current-name] [new-name]
# Detach from a session
# Keybinding: Ctrl + b, d
tmux detach
# List windows
tmux ls
# Attach to a session
tmux attach -t my_session
# Renumber windows
:movew
</syntaxhighlight>

===Mouse scrolling===
Set <code>set -g mouse on</code> in your <code>~/.tmux.conf</code>

==File Manager==
The default file manager in Ubuntu is Nautilus

===Add to context menu===
[https://askubuntu.com/questions/1030940/nautilus-actions-in-18-04 AskUbuntu]

;22.04
See [https://github.com/harry-cpp/code-nautilus https://github.com/harry-cpp/code-nautilus]

;20.04
<pre>
sudo add-apt-repository universe
sudo apt update
sudo apt install filemanager-actions nautilus-actions nautilus-extension-fma
</pre>

==Etcher==
[https://github.com/balena-io/etcher Github] 
Installing etcher
<syntaxhighlight lang="bash">
echo "deb https://deb.etcher.io stable etcher" | sudo tee /etc/apt/sources.list.d/balena-etcher.list
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 379CE192D401AB61
sudo apt update
sudo apt install balena-etcher-electron
</syntaxhighlight>

==Logs==
Logs are stored under <code>/var/log</code>. These can end up taking up a lot of space. 
You can delete logs in the journal folder [https://unix.stackexchange.com/questions/130786/can-i-remove-files-in-var-log-journal-and-var-cache-abrt-di-usr Reference] 

==Default gcc/g++ version==
See [https://askubuntu.com/questions/26498/how-to-choose-the-default-gcc-and-g-version https://askubuntu.com/questions/26498/how-to-choose-the-default-gcc-and-g-version]. 
<syntaxhighlight lang="bash">
# Install
sudo update-alternatives --remove-all gcc
sudo update-alternatives --remove-all g++
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 10
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 20
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 10
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 20
sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30
sudo update-alternatives --set cc /usr/bin/gcc
sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30
sudo update-alternatives --set c++ /usr/bin/g++

# Select
sudo update-alternatives --config gcc
sudo update-alternatives --config g++
</syntaxhighlight>

==Power Management==
===tlp===
[https://linrunner.de/en/tlp/docs/tlp-linux-advanced-power-management.html Website] 
Battery power management

==Virtual Machines (VM)==
===Guest VMs===
Using Ubuntu as a guest:
* Install <code>open-vm-tools-desktop</code>

===KVM===
{{main | Archwiki: KVM}}

===Docker===
{{main | Docker (software)}}

==Services and Scheduling==
===crontab===
The following will open a list of cron jobs you have.
<pre>
crontab -e
</pre>
The default editor is nano. You can change it to vim using <code>VISUAL=vim</code> env variable or with <code>select-editor</code>.

===systemd service===
See [https://wiki.debian.org/systemd/Services debian/systemd Services] 
[https://www.freedesktop.org/software/systemd/man/systemd.service.html manual] 

System-wide services are in <code>/etc/systemd/system/</code> 
User services are in <code>~/.config/systemd/user/</code>

{{hidden|A basic systemd service file|
<pre>
# Contents of /etc/systemd/system/myservice.service
[Unit]
Description=My Service
After=network.target

[Service]
Type=simple
Restart=always
WorkingDirectory=/usr/local/bin
ExecStart=/usr/local/bin/myservice

[Install]
WantedBy=multi-user.target
</pre>

Enable with <code>sudo systemctl enable myservice</code>

}}

;Usage
* <code>sudo systemctl enable <my_service></code>
* <code>sudo systemctl status <my_service></code>
* <code>sudo systemctl start <my_service></code>
* <code>sudo systemctl stop <my_service></code>
* <code>sudo systemctl restart <my_service></code>
* <code>sudo systemctl disable <my_service></code>

;Notes
* Type should be <code>forking</code> if your service runs and then ends
* See service log with <code>sudo journalctl myservice</code>

==File Management==
===rsync===
{{main | rsync}}

[https://linux.die.net/man/1/rsync Documentation]

Use this to sync folders between directories of across networks

;Common Flags
* <code>-a, --archive</code> archive mode; equals -rlptgoD
* <code>--info=progress2</code> show progress

See [[ArchWiki: rsync]] to learn how to use rclone for incremental backups (a la time machine).

===rclone===
{{ main | rclone }}
Similar to rsync but for cloud services such as Dropbox and Google Drive. 
I recommend installing from their website to get the latest version.

===scp===
Usage
<syntaxhighlight lang="bash">
scp [source_machine]:[source_file] [target_machine]:[target_file]
</syntaxhighlight>
;Flags
* <code>-r</code> recursive, needed to scp directories
* <code>-P [port]</code>

;Notes
* The machine can be an alias or user@domain

===7z===
7zip CLI 
Install with <code>sudo apt install p7zip-full</code>
<syntaxhighlight lang="bash>
# Archive
7z a <output_file> <input_file/folder>
# Archive with password
7z a <output_file> <input_file> -p -mhe=on

# Extract
7z x <file> [-o{dir}]
</syntaxhighlight>

* <code>-mhe=on</code> hides file stuctures

===zip/unzip===
Note that p7zip-full also includes the ability to zip/unzip .zip files. 
;Zip a folder
<code>zip -r file.zip folder</code>
;Unzip an archive
<code>unzip file.zip [-d destination]</code>

===diff===
[https://www.geeksforgeeks.org/diff-command-linux-examples/ diff examples]

;Important flags
* <code>--strip-trailing-cr</code> Ignores <code>\r</code>

===tar===
{{ main | tar (computing) }}
;Extraction
<pre>
tar xzvf archive.tar.gz
</pre>

;Archive
<pre>
tar czpvf archive.tar.gz files
</pre>

===find===
Find files by their filename
<pre>
find <folder> [args] -name <name>
</pre>

* <code>-maxdepth <num></code>

===grep===
Find files containing a pattern
<pre>
grep -r <pattern> *
</pre>

==Dual Booting==
===Fix time difference between Windows===
By default, Windows stores the local time in the hardware clock while Ubuntu stores UTC time.

Set ubuntu to store UTC time:
<syntaxhighlight lang="bash">
timedatectl set-local-rtc 0 --adjust-system-clock
</syntaxhighlight>

Set Windows to store UTC time:
https://wiki.archlinux.org/title/System_time#UTC_in_Microsoft_Windows

===Recover GRUB after installing Windows===
[https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows Ubuntu Help] 
If you install windows after installing Ubuntu

===GrubReboot===
[https://wiki.debian.org/GrubReboot GrubReboot] 
Allows you to reboot into an OS one time. 
i.e. If you are ssh'd into linux and want to boot into Windows one time. 

===Encryption===
[https://www.mikekasberg.com/blog/2020/04/08/dual-boot-ubuntu-and-windows-with-encryption.html https://www.mikekasberg.com/blog/2020/04/08/dual-boot-ubuntu-and-windows-with-encryption.html]

==Users and Groups==
===Users===
<syntaxhighlight lang="bash">
# Make a new user
adduser <user>

# Add user to admins
usermod -aG sudo <user>

# Change the password of a user
passwd
passwd <user>

# Delete a user
# -r will also delete their home directory
userdel -r <user>
</syntaxhighlight>

===Groups===
<syntaxhighlight lang="bash">
# Make a group
groupadd <group>

# Delete a group
groupdel <group>

# List members in groups
getent group <group>

# Add user to group
usermod -a -G <group> <user>

# Remove user from group
gpasswd -d <user> <group>
</syntaxhighlight>

==Permissions==
In unix filesystems, files and folders have individual permissions. 
You can set permissions for each file/folder independently and for the following sets of users:
* User/Owner <code>u</code>
* Group <code>g</code>
* Other <code>o</code>
You can also set permissions for all of the above with:
* All <code>a</code>
Each file and folder can have the following permission for each set of user:
* Read <code>r</code>
* Write <code>w</code>
* Execute <code>x</code>
The above totals 9 bits (3 sets of users times 3 permissions).

In addition to the above, there are 3 special bits:
* [https://en.wikipedia.org/wiki/Sticky_bit Sticky bit <code>t</code>] - only allow the owners of subfiles/subfolders to modify them
** Useful for shared folders such as /tmp
* Setuid - automatically elevate execution of this file to the owner's priviledges
* Setgid - automatically elevate execution of this file to the group's priviledges

In total, permissions for each file and folder can be stored in 16 bits or 2 bytes.

===chmod===
change mode

===chown===
change owner
<pre>
chown [-r] <user>[:<group>] <item>
</pre>

===chgrp===

===Access Control Lists (ACL)===

==Display Scaling (HiDPI)==
See [https://wiki.archlinux.org/index.php/HiDPI Arch Wiki HiDPI]

Fractional scaling is natively available in Ubuntu 20.04+.

{{hidden | Ubuntu 18.04 |
;Xorg
<pre>
# Find your display
xrandr
xrandr --output <display> --scale 1.25x1.25
</pre>

;Wayland
<pre>
gsettings set org.gnome.mutter experimental-features "['scale-monitor-framebuffer']"
</pre>

I have the following script run at startup
<pre>
#!/bin/bash

gsettings set org.gnome.desktop.interface scaling-factor 2
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides "{'Gdk/WindowScalingFactor': <2>}"
xrandr --output DP-2 --scale 1.3x1.3
</pre>
}}

==Clock==
See [https://help.ubuntu.com/lts/serverguide/NTP.html Ubuntu Time Synchronization] 
<pre>
# Install chrony
sudo apt install chrony
# Synchronize time
sudo chronyd -q
# Check time synchronization
sudo chronyd -Q
</pre>

Notes
* Syncing over the internet will be off by a few milliseconds (e.g. 0.003 seconds).
* Syncing with another computer over lan

===Syncing with another computer===
See [https://askubuntu.com/questions/787855/how-to-use-chrony-to-synchronize-timestamp-on-two-computers/1018204 askubuntu] 
;On the server
Add the following to <code>/etc/chrony.conf</code>
<pre>
# make it serve time even if it is not synced (as it can't reach out)
local stratum 8
# allow the IP of your peer to connect (192.168 subnet)
allow 192.168
# Or
# allow all
</pre>

;On the client
Add the following to <code>/etc/chrony.conf</code>
<pre>
# set the servers IP here to sync to it
server <Server_IP> iburst
# remove the default servers in the config
</pre>

==<code>/dev/</code>==
See [[Wikipedia: Device file#Pseudo-devices]]

===<code>null</code>===
Discards all input.
Produces EOF.

===<code>random</code>===
See [https://security.stackexchange.com/questions/3936/is-a-rand-from-dev-urandom-secure-for-a-login-key/3939#3939 stackexchange] 
See [https://www.2uo.de/myths-about-urandom/ Myths about urandom] 

;TLDR: Use <code>/dev/urandom</code> instead of <code>/dev/random</code>

===<code>urandom</code>===
Produces random numbers.

On my system, it's limited to about 60 MB/s. If you need faster randomness, you can encrypt from <code>/dev/zero</code> to get 2.7 GB/s.
See [https://serverfault.com/questions/6440/is-there-an-alternative-to-dev-urandom/415962#415962 reference].
<pre>
# Using urandom
pv < /dev/urandom > /dev/ull

# Using encryption
openssl enc -pbkdf2 -iter 100000 -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero | pv > /dev/null

# Create a 4 GB file.
dd if=/dev/zero bs=4M count=1024 | openssl enc -pbkdf2 -iter 100000 -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt | pv > random.bin
</pre>

==Gnome==
===Tweaks===
<pre>
sudo apt install gnome-tweaks
sudo apt install chrome-gnome-shell
</pre>

==Auto Reboot==
[https://unix.stackexchange.com/questions/141095/automatically-reboot-if-no-wifi-connection-for-a-certain-time reference]

{{hidden | Auto Reboot Script |
Auto reboot if no internet is detected:
<syntaxhighlight lang="bash">
#!/bin/bash

TMP_FILE=/tmp/inet_up

# Edit this function if you want to do something besides reboot
no_inet_action() {
if [ "$1" -eq 1 ]; then
systemctl restart network-manager
elif [ "$1" -ge 2 ]; then
rm -f $TMP_FILE
shutdown -r now "No Internet"
fi
}

increment_tmp_file() {
if [ ! -f $TMP_FILE ]; then
echo 0 > $TMP_FILE
fi
oldnum=$(cut -d ',' -f2 $TMP_FILE)
newnum=$(("$oldnum" + 1))
sed -i "s/$oldnum\$/$newnum/g" $TMP_FILE
}

if ping -c5 google.com; then
echo 0 > $TMP_FILE
date > /tmp/inet_up_last_check
else
increment_tmp_file
oldnum=$(cut -d ',' -f2 $TMP_FILE)
no_inet_action "$oldnum"
fi

</syntaxhighlight>

Add to sudo's crontab to run every 10 minutes
<pre>
*/10 * * * * /home/david/bin/check_inet.sh
</pre>
}}

==Encryption==
For encrypting entire drives, I recommend LUKS. 
If you want encrypt a directly, you can use fscrypt (ext4 only).

Note that ecryptfs is deprecated and shouldn't be used.

===Encrypt Home After Install===
See [[Archwiki: Fscrypt#Encrypt_a_home_directory]].
See [https://tlbdk.github.io/ubuntu/2018/10/22/fscrypt.html https://tlbdk.github.io/ubuntu/2018/10/22/fscrypt.html].

This uses the newer fscrypt and requires Ubuntu 18.10+.
<ol>
<li>
Install fscrypt and do setup
<pre>
sudo apt-get install fscrypt libpam-fscrypt
sudo fscrypt setup
sudo fscrypt setup /
sudo tune2fs -O encrypt /dev/<yourdevice>
# E.g. sudo tune2fs -O encrypt /dev/sda5
</pre>
</li>
<li>
Create a new temp sudo user and login to it
</li>
<li>
Create the encrypted home folder
<pre>
export USERNAME=david
# Move old home folder
sudo mv /home/$USERNAME /home/$USERNAME.bak

# Create a new home folder and encrypt it
mkdir /home/$USERNAME
chown $USERNAME:$USERNAME /home/$USERNAME
fscrypt encrypt /home/$USERNAME --user=$USERNAME

# Copy files to the new home folder using cp or rsync
# cp -a -T /home/$USERNAME.bak /home
rsync -aHX --info=progress2 /home/$USERNAME.bak/ /home/$USERNAME/
</pre>
</li>
<li>
Test the encrypted home folder by logging into your user
</li>
<li>
Cleanup by removing the temporary user and deleting the old home folder
<pre>
shred /home/$USERNAME.bak/
</pre>
</li>
</ol>

;Notes and Caveats
* <code>systemd</code> will no longer have access to your home so all startup apps should be placed elsewhere
** E.g. Move all startup scripts in your <code>~/.local/bin</code> to <code>/usr/local/bin</code>
* <code>ssh</code> will not work until home has been decrypted since the authorized keys are in <code>~/.ssh/authorized_keys</code>

{{hidden | SSH Workaround |
Getting SSH to work with an encrypted home dir is a giant pain.
Also things like tmux still won't work.
Overall I do not recommend doing this on a server.

# Move ssh keys elsewhere such as <code>/etc/ssh/authorized_keys/<user></code>.
#* Add <code>/etc/ssh/authorized_keys/%u</code> to the <code>AuthorizedKeysFile</code> line in <code>/etc/ssh/sshd_config</code>.
# Create a sudo user with and unencrypted home directory.
# After every restart, ssh into the unencrypted sudo user and decrypt your home directory:
#* <code><nowiki>sudo fscrypt unlock /home/david --user=david</nowiki></code>
# Then ssh into your account.
}}

==SFTP==
You can create a specific user with a chroot to limit SFTP to specific folders.
See [[Archwiki: SFTP chroot]] for details.
/etc/ssh/sshd_config
<pre>
Subsystem sftp /usr/lib/ssh/sftp-server

Match Group sftponly
ChrootDirectory %h
ForceCommand internal-sftp
AllowTcpForwarding no
X11Forwarding no
PasswordAuthentication no
</pre>

==Hardware Info==

;Benchmarking
Basic CPU benchmark
<pre>
sysbench cpu --threads=2 run
</pre>

==MOTD==
Message of the day is the text you see when you login via SSH.
Ubuntu stores its MOTD in <code>/etc/update-motd.d/</code>. Other distros use <code>/etc/motd/</code>.
You can disable the Ubuntu news motd in <code>/etc/default/motd-news</code>.

==System Administration==
{{main | Linux Administration}}

==Installing Binaries==
# Copy your binary to <code>/usr/local/bin/</code> or <code>~/.local/bin/</code>
# Copy your man page to <code>/usr/local/share/man/man1/</code> or <code>~/.local/share/man/man1/</code>

==Network Troubleshooting==
On one of my OptiPlex 5060 servers, the network adapter would reset on git ssh clones. 
This would appear in <code>/var/log/syslog</code> as:
<pre>
Feb 8 22:22:01 optiplex5060-2 kernel: [ 4378.992607] e1000e 0000:00:1f.6 eno1: Reset adapter unexpectedly
</pre>
This was resolved by disabling TCP Segmentation Offload:
<syntaxhighlight lang="bash">
sudo ethtool -K eno1 tso off

# Verify tso is disabled
ethtool -k eno1 | grep tcp
</syntaxhighlight>

To make this persist across reboots:
{{hidden | Script |
If you're using netplan (default for Ubuntu): 
[https://michael.mulqueen.me.uk/2018/08/disable-offloading-netplan-ubuntu/ Reference] 
<syntaxhighlight lang="bash">
output_path=/usr/lib/networkd-dispatcher/routable.d/10-disable-offloading
sudo tee $output_path <<EOF> /dev/null
#!/bin/bash
ethtool -K eno1 tso off
EOF
sudo chmod +x $output_path
</syntaxhighlight>

If using ifupdown:
<syntaxhighlight lang="bash">
output_path=/etc/network/if-up.d/disable-tso
sudo tee $output_path <<EOF> /dev/null
#!/bin/bash
ethtool -K eno1 tso off
EOF
sudo chmod +x $output_path
</syntaxhighlight>
}}

==Cloning to a new disk==
The easiest way is to use gparted.

{{hidden | Terminal Guide |
To do this in the terminal:
<syntaxhighlight lang="bash">
OLD_DRIVE=/dev/sda
NEW_DRIVE=/dev/sdb

# Show old drive partitions in sectors
parted $OLD_DRIVE unit s print free

# Apply GPT
parted $NEW_DRIVE mklabel gpt
# Copy new EFI partition with start 1024s and end 1050623s
parted $NEW_DRIVE mkpart primary fat32 2048s 1050623s
# Apply boot and esp flags.
parted $NEW_DRIVE set 1 boot on
parted $NEW_DRIVE set 1 esp on
parted $NEW_DRIVE name 1 'EFI System Partition'
# dd the old to the new
dd if=${OLD_DRIVE}1 of=${NEW_DRIVE}1 bs=4k

# Make a new partition. Make sure start and end sectors are aligned.
# i.e. start % 8 == 0 and end % 8 == 7 if your physical sector size is 4096 bytes, typical for new HDDs and SSDs.
parted $NEW_DRIVE mkpart primary btrfs 1050624s 488396791s
parted $NEW_DRIVE align-check opt 2

# Copy the filesystem
mkfs.btrfs ${NEW_DRIVE}2
mkdir /media/${NEW_DRIVE}
mount -t btrfs -o compress=zstd /media/${NEW_DRIVE}2
rsync -axHAWXS --numeric-ids --info=progress2 /media/${NEW_DRIVE}2
</syntaxhighlight>

[https://superuser.com/questions/307541/copy-entire-file-system-hierarchy-from-one-drive-to-another rsync reference]

;rsync options
* -a archive mode
* -x one file system
* -H preserve hard links
* -A preserve ACLs
* -W copy whole files instead of deltas
* -X preserve extended attributes
* -S handle sparse files efficiently
* --numeric-ids use id instead of uid/gid

To copy a root partition, make sure you change the following on the new drive:
* Update the UUID and mount options in <code>/etc/fstab</code>
* Update the UUID in <code>/boot/grub/grub.cfg</code> and run <code>update-grub</code>
* Update the UUID in <code>/boot/EFI/ubuntu/grub.cfg</code>
* Run [https://help.ubuntu.com/community/Boot-Repair boot-repair] from a live disk if you run into any issues.
}}

==Ubuntu==
Ubuntu-specific notes

===Disable ESM message===
[https://askubuntu.com/questions/1453749/inhibit-esm-messages-at-login Reference]
<syntaxhighlight lang="bash">
# Disable MOTD
sudo chmod -x /etc/update-motd.d/88-esm-announce
sudo chmod -x /etc/update-motd.d/91-contract-ua-esm-status

# Disable APT check
sudo sed -Ezi.orig \
-e 's/(def _output_esm_service_status.outstream, have_esm_service, service_type.:\n)/\1 return\n/' \
-e 's/(def _output_esm_package_alert.*?\n.*?\n.:\n)/\1 return\n/' \
/usr/lib/update-notifier/apt_check.py
sudo /usr/lib/update-notifier/update-motd-updates-available --force
</syntaxhighlight>

Main Page

2025-12-21T23:12:07Z

David:

Welcome to David Li's wiki.

<div style="display:flex;flex-direction:row;">
<div class="main-card-container">
<div class="main-flex-card">
<div class="main-card-header">Programming</div>
<ul>
<li>[[C++]]</li>
<li>[[JavaScript]], [[TypeScript]]</li>
<li>[[NodeJS]]</li>
<li>[[Python]]</li>
<li>[[Bash (Unix shell) | Bash]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Libraries</div>
<ul>
<li>[[PyTorch]]</li>
<li>[[TensorFlow]]</li>
<li>[[OpenCV]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Frameworks</div>
<ul>
<li>[[Laravel]]</li>
<li>[[Unity]]</li>
<li>[[React (JavaScript library) | React]]</li>
<li>[[Angular (web framework) | Angular]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Software Tools</div>
<ul>
<li>[[Git]]</li>
<li>[[FFmpeg]]</li>
<li>[[Rclone]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Theory</div>
<ul>
<li>[[Geometric Computer Vision | 3D Vision]], [[Visual Learning and Recognition | Recognition]]</li>
<li>[[Computer Graphics | Graphics]], [[Advanced Computer Graphics | Advanced Graphics]]</li>
<li>[[Machine Learning]], [[Deep Learning]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Typesetting</div>
<ul>
<li>[[Latex]]</li>
<li>[[Markdown]]</li>
<li>[[Wikitext]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Operating Systems</div>
<ul>
<li>[[Linux]]</li>
<li>[[macOS]]</li>
<li>[[NixOS]]</li>
<li>[[Windows]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Editors</div>
<ul>
<li>[[Visual Studio Code]]</li>
<li>[[Vim (text editor) | Vim]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Containers</div>
<ul>
<li>[[Docker (software) | Docker]]</li>
<li>[[Kubernetes]]</li>
</ul>
</div>
</div>
</div>

==Programming==
===General Purpose===
* [[C (programming_language)]]
* [[C++]]
* [[Rust (programming language)]]
* [[C Sharp|C#]]
* [[Java]]
* [[Kotlin]]
* [[Bash (Unix shell)]]

===Web Programming===
* [[JavaScript]]
* [[NodeJS]]
* [[WebGL]]
* [[PHP]]
* [[HTML]]
* [[Cascading Style Sheets | Cascading Style Sheets (CSS)]]
* [[Apache HTTP Server]]
* [[Caddy (web server)]]

===Numerical Computation===
* [[Julia]]
* [[MATLAB]]
* [[Python]]
* [[Cython]]
* [[R]]

===GPU Programming===
* [[CUDA]]
* [[OpenCL]]
* [[ROCm]]
* [[SYCL]]

====Shader Languages====
* [[GLSL]]
* [[HLSL]]

===Typesetting===
* [[Latex]]
** [[PGF/TikZ | TikZ]]
* [[Markdown]]
* [[Markdeep]]

==Frameworks==

===Game Development===
* [[Unity]]

===Web Development===

;Front-end Frameworks
* [[Angular (web framework)]]
* [[React (JavaScript library)]] See ([[Next.js]])
* [[Vue.js]]

;Back-end Frameworks
* [[Laravel]]
* [[Express.js]]

;CSS Libraries
* [[Bootstrap]]
* [https://bulma.io/ Bulma]

;Databases
* [[MySQL]]

===App Development===
* [[Android System Development]]
* [[Android App Development]]
* [[Android Native Development]]

==Machine Learning==
* [[Hyperparameters]]
* [[Machine Learning Glossary]]

===Libraries===
* [[Flux]]
* [[TensorFlow]]
* [[PyTorch]]

====Visualization Tools====
* [[TensorBoard]]

===Applications===
* [[Neural Physics]]
* [https://joelgrus.com/2016/05/23/fizz-buzz-in-tensorflow/ Fizz Buzz]

===Architectures===
* [[Convolutional neural network | Convolutional neural network (CNNs)]]
* [[Generative adversarial network | Generative Adversarial Networks (GANs)]]
* [[Long short-term memory | Long short-term memory (LSTM)]]
* [[Transformer (machine learning model)]]
* [[Capsule neural network]]
* [[Siamese neural network]]
* [[Graph neural network]]

===Research Areas===
* [[Adversarial machine learning]]
** [[Adversarial Examples]]
** [[Poisoning Attacks]]
* [[Zero-Shot Learning]]

==Computer Vision==
* [[List of Feature Descriptors]]
* [[Essential Matrix]]

==Libraries==
;Video and Audio
* [[FFmpeg]]

;Graphics
* [[GLFW]]
* [[three.js]]
* [[Open3D]]

;Vision
* [[OpenCV]]

;Math
* [[Eigen (C++ library)]]

;Web Development
* [[Webpack]]

==Software and Tools==

===Software Engineering===
* [[CMake]]
* [[Makefile]]
* [[Git]], [[GitLab]]
* [[GNU Compiler Collection | GNU Compiler Collection (GCC)]]

===Text Editors and IDEs===
* [[Visual Studio Code]]
* [[Emacs]]
* [[Vim (text editor)]]

===Photo/Video Editing===
* [[Adobe Premiere Pro]]
* [[DaVinci Resolve]]

===3D Modeling===
* [[Blender (software)]]

===Office===
* [[Microsoft Powerpoint]]

===Misc===
* [[Useful Node Programs]]
* [[Google Chrome]]
* [[Conda (package manager)]]

==Operating Systems==
* [[Linux]]
* [[macOS]]
* [[Windows]]
* [[Android (operating system) | Android]]

===Environments===
* [[Cygwin]]
* [[MSYS2]]
* [[Kernel-based Virtual Machine | Kernel-based Virtual Machine (KVM)]]

===Containers===
* [[Docker (software)]]
* [[Kubernetes]]

==Research==

===Surveys===
* [[Steganography]]
* [[Natural language processing]]
* [[Spherical Harmonics]]
* [[Image Compression]]
* [[Video Compression]]
* [[Depth Estimation]]
* [[Single Image Animation]]
* [[Foveated Rendering]]
* [[Texture Synthesis]]
* [[Image Registration]]
* [[Image quality assessment]]
* [[Image-based rendering]]

===Communication===
* [[Computer Science Conferences]]
* [[Giving Talks]]
* [[Writing Papers]]
* [[Reading Papers]]
* [[Reviewing Papers]]
* [[Writing Rebuttals]]

==Math and Theory==

===Machine Learning===
* [[Machine Learning]]
** [[Supervised Learning]]
** [[Unsupervised Learning]]
** [[Reinforcement Learning]]
** [[Transfer Learning]]
** Domain Adaptation
** [[Ensemble Learning]]

* [[Deep Learning]]

===Mathematics===
* [[Advanced Calculus]]
* [[Linear Algebra]]
* [[Probability]]
** [[Stochastic Processes]]

* [[Statistics]]
* [[Numerical Analysis]]
** [[Numerical Optimization]]

===Computer Science===
* [[Algorithms]]
** [[Interview Algorithms]]
** [[Parallel Algorithms]]
** [[Graph Theory]]

* [[Computer Graphics]]
** [[Advanced Computer Graphics]]

* [[Computer Vision]]
** [[Geometric Computer Vision]]
** [[Visual Learning and Recognition]]

* [[Data Structures]]
** [[Multidimensional Data Structures]]

* Computer Systems
** Computer Architecture
** [[Computer Networking]]

==Glossaries==
* [[Machine Learning Glossary]]
* [[New Technology Glossary]]

==Misc==
* [[MediaWiki]]
* [[Regular Expressions|Regular Expressions (Regex)]]
* [[Preprocessor Macros]]
* [[Credit Cards]]
* [[Investing]]
* [[Technical Interviews]]
* [[Datasets]]
* [[Homelab]]

Main Page

2025-12-21T23:11:46Z

David:

Welcome to David Li's wiki.

<div style="display:flex;flex-direction:row;">
<div class="main-card-container">
<div class="main-flex-card">
<div class="main-card-header">Programming</div>
<ul>
<li>[[C++]]</li>
<li>[[JavaScript]], [[TypeScript]]</li>
<li>[[NodeJS]]</li>
<li>[[Python]]</li>
<li>[[Bash (Unix shell) | Bash]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Libraries</div>
<ul>
<li>[[PyTorch]]</li>
<li>[[TensorFlow]]</li>
<li>[[OpenCV]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Frameworks</div>
<ul>
<li>[[Laravel]]</li>
<li>[[Unity]]</li>
<li>[[React (JavaScript library) | React]]</li>
<li>[[Angular (web framework) | Angular]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Software Tools</div>
<ul>
<li>[[Git]]</li>
<li>[[FFmpeg]]</li>
<li>[[Rclone]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Theory</div>
<ul>
<li>[[Geometric Computer Vision | 3D Vision]], [[Visual Learning and Recognition | Recognition]]</li>
<li>[[Computer Graphics | Graphics]], [[Advanced Computer Graphics | Advanced Graphics]]</li>
<li>[[Machine Learning]], [[Deep Learning]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Typesetting</div>
<ul>
<li>[[Latex]]</li>
<li>[[Markdown]]</li>
<li>[[Wikitext]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Operating Systems</div>
<ul>
<li>[[Linux]]</li>
<li>[[macOS]]</li>
<li>[[NixOS]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Editors</div>
<ul>
<li>[[Visual Studio Code]]</li>
<li>[[Vim (text editor) | Vim]]</li>
</ul>
</div>
<div class="main-flex-card">
<div class="main-card-header">Containers</div>
<ul>
<li>[[Docker (software) | Docker]]</li>
<li>[[Kubernetes]]</li>
</ul>
</div>
</div>
</div>

==Programming==
===General Purpose===
* [[C (programming_language)]]
* [[C++]]
* [[Rust (programming language)]]
* [[C Sharp|C#]]
* [[Java]]
* [[Kotlin]]
* [[Bash (Unix shell)]]

===Web Programming===
* [[JavaScript]]
* [[NodeJS]]
* [[WebGL]]
* [[PHP]]
* [[HTML]]
* [[Cascading Style Sheets | Cascading Style Sheets (CSS)]]
* [[Apache HTTP Server]]
* [[Caddy (web server)]]

===Numerical Computation===
* [[Julia]]
* [[MATLAB]]
* [[Python]]
* [[Cython]]
* [[R]]

===GPU Programming===
* [[CUDA]]
* [[OpenCL]]
* [[ROCm]]
* [[SYCL]]

====Shader Languages====
* [[GLSL]]
* [[HLSL]]

===Typesetting===
* [[Latex]]
** [[PGF/TikZ | TikZ]]
* [[Markdown]]
* [[Markdeep]]

==Frameworks==

===Game Development===
* [[Unity]]

===Web Development===

;Front-end Frameworks
* [[Angular (web framework)]]
* [[React (JavaScript library)]] See ([[Next.js]])
* [[Vue.js]]

;Back-end Frameworks
* [[Laravel]]
* [[Express.js]]

;CSS Libraries
* [[Bootstrap]]
* [https://bulma.io/ Bulma]

;Databases
* [[MySQL]]

===App Development===
* [[Android System Development]]
* [[Android App Development]]
* [[Android Native Development]]

==Machine Learning==
* [[Hyperparameters]]
* [[Machine Learning Glossary]]

===Libraries===
* [[Flux]]
* [[TensorFlow]]
* [[PyTorch]]

====Visualization Tools====
* [[TensorBoard]]

===Applications===
* [[Neural Physics]]
* [https://joelgrus.com/2016/05/23/fizz-buzz-in-tensorflow/ Fizz Buzz]

===Architectures===
* [[Convolutional neural network | Convolutional neural network (CNNs)]]
* [[Generative adversarial network | Generative Adversarial Networks (GANs)]]
* [[Long short-term memory | Long short-term memory (LSTM)]]
* [[Transformer (machine learning model)]]
* [[Capsule neural network]]
* [[Siamese neural network]]
* [[Graph neural network]]

===Research Areas===
* [[Adversarial machine learning]]
** [[Adversarial Examples]]
** [[Poisoning Attacks]]
* [[Zero-Shot Learning]]

==Computer Vision==
* [[List of Feature Descriptors]]
* [[Essential Matrix]]

==Libraries==
;Video and Audio
* [[FFmpeg]]

;Graphics
* [[GLFW]]
* [[three.js]]
* [[Open3D]]

;Vision
* [[OpenCV]]

;Math
* [[Eigen (C++ library)]]

;Web Development
* [[Webpack]]

==Software and Tools==

===Software Engineering===
* [[CMake]]
* [[Makefile]]
* [[Git]], [[GitLab]]
* [[GNU Compiler Collection | GNU Compiler Collection (GCC)]]

===Text Editors and IDEs===
* [[Visual Studio Code]]
* [[Emacs]]
* [[Vim (text editor)]]

===Photo/Video Editing===
* [[Adobe Premiere Pro]]
* [[DaVinci Resolve]]

===3D Modeling===
* [[Blender (software)]]

===Office===
* [[Microsoft Powerpoint]]

===Misc===
* [[Useful Node Programs]]
* [[Google Chrome]]
* [[Conda (package manager)]]

==Operating Systems==
* [[Linux]]
* [[macOS]]
* [[Windows]]
* [[Android (operating system) | Android]]

===Environments===
* [[Cygwin]]
* [[MSYS2]]
* [[Kernel-based Virtual Machine | Kernel-based Virtual Machine (KVM)]]

===Containers===
* [[Docker (software)]]
* [[Kubernetes]]

==Research==

===Surveys===
* [[Steganography]]
* [[Natural language processing]]
* [[Spherical Harmonics]]
* [[Image Compression]]
* [[Video Compression]]
* [[Depth Estimation]]
* [[Single Image Animation]]
* [[Foveated Rendering]]
* [[Texture Synthesis]]
* [[Image Registration]]
* [[Image quality assessment]]
* [[Image-based rendering]]

===Communication===
* [[Computer Science Conferences]]
* [[Giving Talks]]
* [[Writing Papers]]
* [[Reading Papers]]
* [[Reviewing Papers]]
* [[Writing Rebuttals]]

==Math and Theory==

===Machine Learning===
* [[Machine Learning]]
** [[Supervised Learning]]
** [[Unsupervised Learning]]
** [[Reinforcement Learning]]
** [[Transfer Learning]]
** Domain Adaptation
** [[Ensemble Learning]]

* [[Deep Learning]]

===Mathematics===
* [[Advanced Calculus]]
* [[Linear Algebra]]
* [[Probability]]
** [[Stochastic Processes]]

* [[Statistics]]
* [[Numerical Analysis]]
** [[Numerical Optimization]]

===Computer Science===
* [[Algorithms]]
** [[Interview Algorithms]]
** [[Parallel Algorithms]]
** [[Graph Theory]]

* [[Computer Graphics]]
** [[Advanced Computer Graphics]]

* [[Computer Vision]]
** [[Geometric Computer Vision]]
** [[Visual Learning and Recognition]]

* [[Data Structures]]
** [[Multidimensional Data Structures]]

* Computer Systems
** Computer Architecture
** [[Computer Networking]]

==Glossaries==
* [[Machine Learning Glossary]]
* [[New Technology Glossary]]

==Misc==
* [[MediaWiki]]
* [[Regular Expressions|Regular Expressions (Regex)]]
* [[Preprocessor Macros]]
* [[Credit Cards]]
* [[Investing]]
* [[Technical Interviews]]
* [[Datasets]]
* [[Homelab]]

2025-10-08T21:45:46Z

David:

Transparent objects need to be rendered from back to front. This is simple to do each frame on CPU when you have a limited number of convex meshes which are small relative to their distance to each other.

However, when parts of a mesh overlap each other, you have large meshes which overlap each other, or you have a huge number of transparent objects, it may be necessary to switch to an order-independent transparency.

The main idea of order-independent transparency is to either:
1. Approximate transparency using a commutative operation, i.e. addition.
2. Render fragments (per-pixel colors) from back to front.

==Additive Transparency==
Take all the transparent colors and add them up. Addition is commutative so the order doesn't matter.

==Weighted Blended Order Independent Transparency==
See http://casual-effects.blogspot.com/2014/03/weighted-blended-order-independent.html

This builds upon additive transparency with two buffers:
# Accum buffer: weighted average of all premultiplied-alpha RGB colors.
# Revealage buffer: buffer representing how much of the opaque background is visible through the transparent layers.

==Depth Peeling==
Do several render passes. In each render pass, set a range of z values from back to front and render only z values within the threshold for each render pass.

===Dual Depth Peeling===
https://developer.download.nvidia.com/SDK/10/opengl/src/dual_depth_peeling/doc/DualDepthPeeling.pdf

This is a performance optimization over depth peeling which allows using half the number of passes.

==Stochastic Transparency==
https://research.nvidia.com/publication/2011-08_stochastic-transparency

The idea is to apply dithering, rendering objects as opaque.
When objects overlap, the depth test prevents objects behind from rendering over objects in front.

==Per-Pixel Linked Lists==
This method requires an atomic counter or storage buffers with atomic operations, available in OpenGL 4.2+, Vulkan, and WebGPU. It also requires a small amount of overdraw since sorting needs to happen per pixel.

The idea is to have each pixel build a linked list of each fragment being drawn. Then the linked list will be sorted and rendered back to front.

Order independent transparency

2025-10-08T21:39:10Z

David: /* Per-Pixel Linked Lists */

Methods for order independent transparency

==Additive Transparency==
Take all the transparent colors and add them up. Addition is commutative so the order doesn't matter.

==Weighted Blended Order Independent Transparency==
See http://casual-effects.blogspot.com/2014/03/weighted-blended-order-independent.html

This builds upon additive transparency with two buffers:
# Accum buffer: weighted average of all premultiplied-alpha RGB colors.
# Revealage buffer: buffer representing how much of the opaque background is visible through the transparent layers.

==Depth Peeling==
Do several render passes. In each render pass, set a range of z values from back to front and render only z values within the threshold for each render pass.

===Dual Depth Peeling===
https://developer.download.nvidia.com/SDK/10/opengl/src/dual_depth_peeling/doc/DualDepthPeeling.pdf

This is a performance optimization over depth peeling which allows using half the number of passes.

==Stochastic Transparency==
https://research.nvidia.com/publication/2011-08_stochastic-transparency

The idea is to apply dithering, rendering objects as opaque.
When objects overlap, the depth test prevents objects behind from rendering over objects in front.

==Per-Pixel Linked Lists==
This method requires an atomic counter or storage buffers with atomic operations, available in OpenGL 4.2+, Vulkan, and WebGPU. It also requires a small amount of overdraw since sorting needs to happen per pixel.

The idea is to have each pixel build a linked list of each fragment being drawn. Then the linked list will be sorted and rendered back to front.

2025-10-08T21:22:26Z

David:

Order independent transparency

2025-10-08T21:21:11Z

David: /* Weighted Blended Order Independent Transparency */

Order independent transparency

2025-10-08T21:21:03Z

David:

Order independent transparency

2025-10-08T21:06:49Z

David:

Methods for order independent transparency

==Weighted Blended Order Independent Transparency==
See http://casual-effects.blogspot.com/2014/03/weighted-blended-order-independent.html

==Dual Depth Peeling==
https://developer.download.nvidia.com/SDK/10/opengl/src/dual_depth_peeling/doc/DualDepthPeeling.pdf

==Stochastic Transparency==
https://research.nvidia.com/publication/2011-08_stochastic-transparency

==Per-Pixel Linked Lists==

Order independent transparency

2025-10-08T21:06:36Z

David:

Dyson Cordless Vacuum

2025-09-14T18:55:31Z

David: Created page with "https://www.techradar.com/home/vacuums/dyson-vacuum-tools-guide"

https://www.techradar.com/home/vacuums/dyson-vacuum-tools-guide

Tesla Model Y Sounds

2025-09-06T20:05:52Z

David: /* Rear seat rattle */

List of resources to deal with rattle and creaking sounds in the Tesla Model Y.

===Seat belt adjuster rattle===

See https://www.youtube.com/watch?v=KctTaFCZhWE for the fix. 
This one works for me.

===Rear seat rattle===
See https://www.youtube.com/watch?v=nn2mBrJVefo

See https://www.reddit.com/r/ModelY/comments/1l1yhib/backseat_rattle_found/?share_id=GL0NVJwcQksqU9wUtxDnN&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=1

===Hatch creaking===

# Adjust the rubber bumpers
# https://www.youtube.com/watch?v=OBPwmvFn-5c
# For non-US cars, see https://teslamotorsclub.com/tmc/threads/creaking-tailgate-mobile-technician-visit.306684/page-2

;Hatch rattle
In the hatch, there is a place where two metal pieces join along the bottom of the window. You can insert popsicle sticks there to prevent metal creaking.

Tesla Model Y Sounds

2025-09-06T20:05:46Z

David: /* Hatch creaking */

List of resources to deal with rattle and creaking sounds in the Tesla Model Y.

===Seat belt adjuster rattle===

See https://www.youtube.com/watch?v=KctTaFCZhWE for the fix. 
This one works for me.

===Rear seat rattle===
See https://www.youtube.com/watch?v=nn2mBrJVefo
See https://www.reddit.com/r/ModelY/comments/1l1yhib/backseat_rattle_found/?share_id=GL0NVJwcQksqU9wUtxDnN&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=1

===Hatch creaking===

# Adjust the rubber bumpers
# https://www.youtube.com/watch?v=OBPwmvFn-5c
# For non-US cars, see https://teslamotorsclub.com/tmc/threads/creaking-tailgate-mobile-technician-visit.306684/page-2

;Hatch rattle
In the hatch, there is a place where two metal pieces join along the bottom of the window. You can insert popsicle sticks there to prevent metal creaking.

Tesla Model Y Sounds

2025-09-06T20:05:37Z

David:

List of resources to deal with rattle and creaking sounds in the Tesla Model Y.

===Seat belt adjuster rattle===

See https://www.youtube.com/watch?v=KctTaFCZhWE for the fix. 
This one works for me.

===Rear seat rattle===
See https://www.youtube.com/watch?v=nn2mBrJVefo
See https://www.reddit.com/r/ModelY/comments/1l1yhib/backseat_rattle_found/?share_id=GL0NVJwcQksqU9wUtxDnN&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=1

===Hatch creaking===

# Adjust the rubber bumpers
# https://www.youtube.com/watch?v=OBPwmvFn-5c
# For non-US cars, see https://teslamotorsclub.com/tmc/threads/creaking-tailgate-mobile-technician-visit.306684/page-2

# Hatch rattle
In the hatch, there is a place where two metal pieces join along the bottom of the window. You can insert popsicle sticks there to prevent metal creaking.

2025-06-07T06:40:40Z

David: /* Homogeneous Coordinates */

Basics of Computer Graphics

==Homogeneous Coordinates==
[http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/ http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/]

Points and vectors are represented using homogeneous coordinates in computer graphics.
This allows affine transformations in 3D (i.e. rotation and translation) to be represented as a matrix multiplication.
While rotations can typically be represented in a 3x3 matrix multiplication, a translation requires a [[wikipedia:Shear mapping | ''shear'']] in 4D.

Points are <math>(x,y,z,1)</math> and vectors are <math>(x,y,z,0)</math>.
The last coordinate in points allow for translations to be represented as matrix multiplications.

;Notes
* The point <math>(kx, ky, kz, k)</math> is equivalent to <math>(x, y, z, 1)</math>.

Affine transformations consist of translations, rotations, and scaling

===Translation Matrix===
<math>
T =
\begin{bmatrix}
1 & 0 & 0 & X\\
0 & 1 & 0 & Y\\
0 & 0 & 1 & Z\\
0 & 0 & 0 & 1
\end{bmatrix}
</math>

===Rotation Matrix===
Rotations can be about the X, Y, and Z axis. 
Below is a rotation about the Z axis by angle <math>\theta</math>. 
<math>
R = \begin{bmatrix}
\cos(\theta) & -\sin(\theta) & 0 & 0\\
\sin(\theta) & \cos(\theta) & 0 & 0\\
0 & 0 & 1 & 0\\
0 & 0 & 0 & 1
\end{bmatrix}
</math>

To formulate a rotation about a specific axis, we use [[Wikipedia:Rodrigues' rotation formula]]. 
Suppose we want to rotate by angle <math>\theta</math> around axis <math>\mathbf{k}=(k_x, k_y, k_z)</math>. 
Let <math>
\mathbf{K} = [\mathbf{k}]_{\times} =
\begin{bmatrix}
0 & -k_z & k_y\\
k_z & 0 & -k_x\\
-k_y & k_x & 0
\end{bmatrix}</math> 
Then the rotation matrix is <math>\mathbf{R} = \mathbf{I}_{3} + (\sin \theta)\mathbf{K} + (1 - \cos \theta)\mathbf{K}^2</math> 
Here the 4x4 form is:
<math>
R = \begin{bmatrix}
\mathbf{R} & \mathbf{0}\\
\mathbf{0}^T & 1
\end{bmatrix}
</math>

===Scaling Matrix===
<math>
S = \begin{bmatrix}
X & 0 & 0 & 0\\
0 & Y & 0 & 0\\
0 & 0 & Z & 0\\
0 & 0 & 0 & 1
\end{bmatrix}
</math>

===Transformation matrix===
<math>
L = T * R * S
</math>

Depending on implementation, it may be more memory-efficient or compute-efficient to represent affine transformations as their own class rather than 4x4 matrices. For example, a rotation can be represented with 3 floats in angle-axis or 4 floats in quaternion coordinates rather than a 3x3 rotation matrix.

For example, see
* [https://eigen.tuxfamily.org/dox/classEigen_1_1Transform.html Eigen::Transform]

===Barycentric Coordinates===

==MVP Matrices==

To convert from model coordinates <math>v</math> to screen coordinates <math>w</math>, you do multiply by the MVP matrices <math>w=P*V*M*v</math>
* The model matrix <math>M</math> applies the transform of your object. This includes the position and rotation. <math>M*v</math> is in world coordinates.
* The view matrix <math>V</math> applies the inverse transform of your camera. <math>V*M*v</math> is in camera or view coordinates (i.e. coordinates relative to the camera).
* The projection matrix <math>P</math> applies the projection of your camera, typically an orthographic or a perspective camera. The perspective camera shrinks objects in the distance.

===Model Matrix===
[https://gamedev.stackexchange.com/questions/16719/what-is-the-correct-order-to-multiply-scale-rotation-and-translation-matrices-f Order of matrices] 
The model matrix is the product of the element's scale, rotation, and translation matrices. 
<math>M = T * R * S</math>

===View Matrix===
[https://webglfactory.blogspot.com/2011/06/how-to-create-view-matrix.html Reference] 
[https://www.scratchapixel.com/lessons/mathematics-physics-for-computer-graphics/lookat-function Lookat function] 
The view matrix is a 4x4 matrix which encodes the position and rotation of the camera. 
Given a camera at position <math>\mathbf p</math> looking at target <math>\mathbf t</math> and up vector <math>\mathbf u</math>. 
We can calculate the forward vector (from target to position) as <math>\mathbf{f}=\mathbf{p} - \mathbf{t}</math>. 
We can calculate the right vector as <math>\mathbf u \times \mathbf f</math>. 
Then the view matrix is written as:
<pre>
r_x r_y r_z 0
u_x u_y u_z 0
f_x f_y f_z 0
p_x p_y p_z 1
</pre>
<pre>
Matrix lookAt(camera_pos, target, up) {
forward = normalize(camera - target)
up_normalized = normalize(up)
right = normalize(cross(up, forward)
// Make sure up is perpendicular to forward
up = normalize(cross(forward, right)
m = stack([right, up, forward, camera], 0)
return m
}
</pre>

===Perspective Projection Matrix===
[https://www.scratchapixel.com/lessons/3d-basic-rendering/perspective-and-orthographic-projection-matrix/building-basic-perspective-projection-matrix https://www.scratchapixel.com/lessons/3d-basic-rendering/perspective-and-orthographic-projection-matrix/building-basic-perspective-projection-matrix]

https://www.songho.ca/opengl/gl_projectionmatrix.html

The projection matrix applies a perspective projection based on the field of view of the camera. This is done dividing the x,y view coordinates by the z-coordinate so that further object appear closer to the center. Note that the output is typically in normalized device coordinates (NDC) <math>[-1, 1]\times[-1, 1]</math> rather than image coordinates <math>{0, ..., W-1} \times {0, ..., H-1}</math>. Additionally, in NDC, the y-coordinate typically points upwards unlike image coordinates.

The Z-coordinate in the projection matrix represents a remapped version of the z-depth, i.e. depth along the camera forward axis. In OpenGL, this maps z=-f to 1 and z=-n to -1 where -z is forward.

Notes: In computer vision, this is analogous to the calibration matrix <math>K</math>.
It contains the intrinsic parameters of your pinhole camera such as field of view and focal length. The focal length determines the resolution of your output.

===Inverting the projection===
If you have the depth (either z-depth or euclidean depth), you can invert the projection operation.
The idea is to construct a ray from the camera to the pixel on a plane of the viewing frustrum and scale the distance accordingly.

See [https://gamedev.stackexchange.com/questions/108856/fast-position-reconstruction-from-depth/111885#111885 stackexchange].

==Shading==
{{main | Wikipedia:Shading}}
===Interpolation===

* Flat shading - color is computed for each face/triangle.
* Gourard shading - color is computed for each vertex and interpolated.
* Phong shading - color is computed for each pixel with the normal vector interpolated from each vertex.

===Lambert reflectance===
{{main | Wikipedia: Lambertian reflectance}}
This is a way to model diffuse (matte) materials.

<math>I_D = (\mathbf{L} \cdot \mathbf{N}) * C * I_{L}</math>
* <math>\mathbf{N}</math> is the normal vector.
* <math>\mathbf{L}</math> is the vector to the light.
* <math>C</math> is the color.
* <math>I_{L}</math> is the intensity of light.

===Phong reflection model===
{{main | Wikipedia: Phong reflection model}}
See [https://www.scratchapixel.com/lessons/3d-basic-rendering/phong-shader-BRDF scratchapixel phong shader BRDF].
This is a way to model specular (shiny) materials.

Here, the image is a linear combination of ambient, diffuse, and specular colors.

If <math>\mathbf{N}</math> is the normal vector, <math>\mathbf{V}</math> is a vector from the vertex to the viewer, <math>\mathbf{L}</math> from the light to the vertex, and <math>\mathbf{R}</math> the incident vector (i.e. <math>\mathbf{L}</math> rotated 180 around <math>\mathbf{N}</math>) then
* Ambient is a constant color for every pixel.
* The diffuse coefficient is <math>\mathbf{N} \cdot \mathbf{L}</math>.
* The specular coefficient is <math>(\mathbf{R} \cdot \mathbf{V})^n</math> where <math>n</math> is the ''shininess''.
The final color is <math>k_{ambient} * ambientColor + k_{diffuse} * (\mathbf{N} \cdot \mathbf{L}) * diffuseColor + k_{specular} * (\mathbf{R} \cdot \mathbf{V})^n * specularColor</math>.

;Notes
* The diffuse and specular components need to be computed for every visible light source.

===Physically Based===
See [https://static1.squarespace.com/static/58586fa5ebbd1a60e7d76d3e/t/593a3afa46c3c4a376d779f6/1496988449807/s2012_pbs_disney_brdf_notes_v2.pdf pbs disney brdf notes] and the [http://www.pbr-book.org/ pbr-book]
In frameworks and libraries, these are often refered to as ''standard materials'' or in Blender, ''Principled BSDF''.

==Blending and Pixel Formats==

===Pixel Formats===

===Blending===

To output transparent images, i.e. images with alpha, you'll generally want to blend using [[Premultiplied Alpha]]. Rendering in premultiplied alpha prevents your RGB color values from getting mixed with the background color empty pixels.

===Rendering===

For rasterization, the render loop typically consists of:
# Render the shadow map.
# Render all opaque objects front-to-back.
## Opaque objects write to the depth buffer.
# Render all transparent objects back-to-front
## Transparent objects do not write to the depth buffer.

Rendering opaque objects front to back minimizes overdraw, where a pixel gets drawn to multiple times in a single frame.
Rendering transparent objects back to front is needed for proper blending of transparent materials.

==Anti-aliasing==
For a high-quality anti-aliasing, you'll generally want to multiple multi-sampling (MSAA).
This causes the GPU to render the depth buffer at a higher resolution to determine the contribution of your fragment shader's color to the final image.

See https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing#:~:text=How%20MSAA%20really%20works%20is,buffer%20to%20determine%20subsample%20coverage for more details.

==More Terms==
* [[Wikipedia: Diffuse reflection | Diffuse reflection]] - reflection scattered in many directions (i.e. matte)
* [[Wikipedia: Specular reflection | Specular reflection]] - mirror reflection
* [[Wikipedia: Refraction | Refraction]] - change in direction of light as it passes through a material

==Resources==
* [https://www.udacity.com/course/interactive-3d-graphics--cs291 Udacity Interactive 3D Graphics]

FFmpeg

2025-06-05T06:01:23Z

David: /* My Preferences */

[https://ffmpeg.org/ FFmpeg] (Fast Forward MPEG) is a library for encoding and decoding multimedia.

You can interact with FFmpeg using their command-line interface or using their [https://ffmpeg.org/doxygen/trunk/index.html C API].
Note that a lot of things involving just decoding or encoding can be done by calling their CLI application and piping things to stdin or from stdout.

==CLI==
You can download static builds of FFmpeg from
* Linux: [https://johnvansickle.com/ffmpeg/ https://johnvansickle.com/ffmpeg/]
* Windows: [https://ffmpeg.zeranoe.com/builds/ https://ffmpeg.zeranoe.com/builds/]

If you need nvenc support, you can build FFmpeg with https://github.com/markus-perl/ffmpeg-build-script.

Basic usage is as follows:
<pre>
ffmpeg [-ss start_second] -i input_file [-s resolution] [-b bitrate] [-t time] [-r output_framerate] output.mp4
</pre>

* Use <code>-pattern_type glob</code> for wildcards (e.g. all images in a folder)

===x264===
x264 is a software h264 decoder and encoder. 
[https://trac.ffmpeg.org/wiki/Encode/H.264]

===Changing Pixel Format===
Encode to h264 with YUV420p pixel format
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -c:v libx264 -profile:v high -pix_fmt yuv420p output.mp4
</syntaxhighlight>

===Images to Video===
[https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/image_sequence Reference] 
Assuming 60 images per second and you want a 30 fps video.
<syntaxhighlight lang="bash">
# Make sure -framerate is before -i
ffmpeg -framerate 60 -i image-%03d.png -r 30 video.mp4
</syntaxhighlight>

===Video to Images===
Extracting frames from a video

<syntaxhighlight lang="bash">
ffmpeg -i video.mp4 frames/%d.png
</syntaxhighlight>

* Use <code>-ss H:M:S</code> to specify where to start before you input the video
* Use <code>-vframes 1</code> to extract one frames
* Use <code>-vf "select=not(mod(n\,10))"</code> to select every 10th frame

===Get a list of encoders/decoders===
[https://superuser.com/questions/1236275/how-can-i-use-crf-encoding-with-nvenc-in-ffmpeg Reference]
<syntaxhighlight lang="bash">
for i in encoders decoders filters; do
echo $i:; ffmpeg -hide_banner -${i} | egrep -i "npp|cuvid|nvenc|cuda"
done
</syntaxhighlight>

===PSNR/SSIM===
[https://github.com/stoyanovgeorge/ffmpeg/wiki/How-to-Compare-Video Reference] 
FFmpeg can compare two videos and output the psnr or ssim numbers for each of the y, u, and v channels. 
<syntaxhighlight lang="bash">
ffmpeg -i distorted.mp4 -i reference.mp4 \
-lavfi "ssim;[0:v][1:v]psnr" -f null –

ffmpeg -i distorted.mp4 -i reference.mp4 -lavfi psnr -f null -
ffmpeg -i distorted.mp4 -i reference.mp4 -lavfi ssim -f null -
</syntaxhighlight>

===Generate Thumbnails===
[https://superuser.com/questions/1099491/batch-extract-thumbnails-with-ffmpeg Reference] 
Below is a bash script to generate all thumbnails in a folder
{{hidden|Script|
<syntaxhighlight lang="bash">
#!/usr/bin/env bash

OUTPUT_FOLDER="thumbnails"

mkdir -p $OUTPUT_FOLDER
for file in *.mp4;
do ffmpeg -i "$file" -vf "select=gte(n\,300)" -vframes 1 "$OUTPUT_FOLDER/${file%.mp4}.png";
done
</syntaxhighlight>
}}

===MP4 to GIF===
Normally you can just do
<syntaxhighlight lang="bash">
ffmpeg -i my_video.mp4 my_video.gif
</syntaxhighlight>

If you want better quality, you can use the following filter_complex:
<pre>
[0]split=2[v1][v2];[v1]palettegen=stats_mode=full[palette];[v2][palette]paletteuse=dither=sierra2_4a
</pre>

Here is another script from [https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality]
{{hidden | mp4 to gif script |
<syntaxhighlight lang="bash">
#!/bin/sh
ffmpeg -i $1 -vf "fps=15,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" -loop 0 $2
</syntaxhighlight>
}}

===Pipe to stdout===
Below is an example of piping the video only to stdout:
<pre>
ffmpeg -i video.webm -pix_fmt rgb24 -f rawvideo -
</pre>

In Python, you can read it as follows:
<syntaxhighlight lang="python">
video_width = 1920
video_height = 1080
ffmpeg_process = subprocess.Popen(ffmpeg_command,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
raw_image = ffmpeg_process.stdout.read(
video_width * video_height * 3)
image = (np.frombuffer(raw_image, dtype=np.uint8)
.reshape(video_height, video_width, 3))
</syntaxhighlight>

==Filters==
Filters are part of the CLI 
[https://ffmpeg.org/ffmpeg-filters.html https://ffmpeg.org/ffmpeg-filters.html]

===Crop===
<syntaxhighlight lang="bash">
ffmpeg -i input_filename -vf "crop=w:h:x:y" output_filename
</syntaxhighlight>

* Here <code>x</code> and <code>y</code> are the top left corners of your crop. <code>w</code> and <code>h</code> are the height and width of the final image or video.

===Resizing/Scaling===
[https://trac.ffmpeg.org/wiki/Scaling FFMpeg Scaling] 
[https://ffmpeg.org/ffmpeg-filters.html#scale scale filter]

<syntaxhighlight lang="bash">
ffmpeg -i input.avi -vf scale=320:240 output.avi

ffmpeg -i input.jpg -vf scale=iw*2:ih input_double_width.png
</syntaxhighlight>

* If the aspect ratio is not what you expect, try using the <code>setdar</code> filter.
** E.g. <code>setdar=ratio=2/1</code>

;Resizing with transparent padding
Useful for generating logos
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=128:w=128:force_original_aspect_ratio=decrease,pad=128:128:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

{{hidden | More sizes |

;256
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=256:w=256:force_original_aspect_ratio=decrease,pad=256:256:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

;512
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=512:w=512:force_original_aspect_ratio=decrease,pad=512:512:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

}}

===Rotation===
[https://ffmpeg.org/ffmpeg-filters.html#transpose transpose filter]

To rotate 180 degrees
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -vf "transpose=1,transpose=1" output.mp4
</syntaxhighlight>
* 0 – Rotate by 90 degrees counter-clockwise and flip vertically.
* 1 – Rotate by 90 degrees clockwise.
* 2 – Rotate by 90 degrees counter-clockwise.
* 3 – Rotate by 90 degrees clockwise and flip vertically.

===360 Video===
See [https://ffmpeg.org/ffmpeg-filters.html#v360 v360 filter]

=====Converting EAC to equirectangular=====
Youtube sometimes uses an EAC format. You can convert this to the traditional equirectangular format as follows:
<pre>
ffmpeg -i input.mp4 -vf "v360=eac:e" output.mp4
</pre>

Sometimes you may run into errors where height or width is not divisible by 2. 
Apply a scale filter to fix this issue.
<pre>
ffmpeg -i input.mp4 -vf "v360=eac:e,scale=iw:-2" output.mp4
</pre>

====Converting to rectilinear====
<pre>
ffmpeg -i input.mp4 -vf "v360=e:rectilinear:h_fov=90:v_fov=90" output.mp4
</pre>

====Metadata====
To add 360 video metadata, you should use [https://github.com/google/spatial-media Google's spatial-media].
This will add the following sidedata which you can see using <code>ffprobe</code>:
<pre>
Side data:
spherical: equirectangular (0.000000/0.000000/0.000000)
</pre>

===Removing Duplicate Frames===
[https://stackoverflow.com/questions/37088517/remove-sequentially-duplicate-frames-when-using-ffmpeg Reference] 
[https://ffmpeg.org/ffmpeg-filters.html#mpdecimate mpdecimate filter]

Useful for extracting frames from timelapses.
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -vf mpdecimate,setpts=N/FRAME_RATE/TB out.mp4
</syntaxhighlight>

===Stack and Unstack===
To stack, see [https://ffmpeg.org/ffmpeg-all.html#hstack <code>hstack</code>], [https://ffmpeg.org/ffmpeg-all.html#vstack <code>vstack</code>].
To unstack, see <code>crop</code>.

===Filter-Complex===
Filter complex allows you to create a graph of filters.

Suppose you have 3 inputs: $1, $2, $3.
Then you can access them as streams [0], [1], [3].
The filter syntax allows you to chain multiple filters where each filter is an edge.
For example, <code>[0]split[t1][t2]</code> creates two vertices t1 and t2 from input 0.
The last statement in your edge will be the output of your command:
E.g. <code>[t1][t2]vstack</code>

<pre>
ffmpeg -i $1 -i $2 -i $3 -filter_complex "[0]split[t1][t2];[t1][t2]vstack" output.mkv -y
</pre>

===Concatenate Videos===
<pre>
ffmpeg -i part_1.mp4 \
-i part_2.mp4 \
-i part_3.mp4 \
-filter_complex \
"[0]scale=1920:1080[0s];\
[1]scale=1920:1080[1s];\
[2]scale=1920:1080[2s];\
[0s][0:a][1s][1:a][2s][2:a]concat=n=3:v=1:a=1[v][a]" \
-map "[v]" -map "[a]" \
-vsync 2 \
all_parts.mp4 -y
</pre>

===Replace transparency===
[https://superuser.com/questions/1341674/ffmpeg-convert-transparency-to-a-certain-color Reference] 
Add a background to transparent images. 
<pre>
ffmpeg -i in.mov -filter_complex "[0]format=pix_fmts=yuva420p,split=2[bg][fg];[bg]drawbox=c=white@1:replace=1:t=fill[bg];[bg][fg]overlay=format=auto" -c:a copy new.mov
</pre>

===Draw Text===
https://stackoverflow.com/questions/15364861/frame-number-overlay-with-ffmpeg
<pre>
ffmpeg -i input -vf "drawtext=fontfile=Arial.ttf: text='%{frame_num}': start_number=1: x=(w-tw)/2: y=h-(2*lh): fontcolor=black: fontsize=20: box=1: boxcolor=white: boxborderw=5" -c:a copy output
</pre>

==C API==
A doxygen reference manual for their C api is available at [https://ffmpeg.org/doxygen/trunk/index.html]. 
Note that FFmpeg is licensed under GPL. 
If you only need to do encoding and decoding, you can simply pipe the inputs and outputs of the FFmpeg CLI to your program [https://batchloaf.wordpress.com/2017/02/12/a-simple-way-to-read-and-write-audio-and-video-files-in-c-using-ffmpeg-part-2-video/]. 

===Getting Started===
Best way to get started is to look at the [https://ffmpeg.org/doxygen/trunk/examples.html official examples].

====Structs====
* [https://www.ffmpeg.org/doxygen/trunk/structAVInputFormat.html <code>AVInputFormat</code>]/[https://www.ffmpeg.org/doxygen/trunk/structAVOutputFormat.html <code>AVOutputFormat</code>] Represents a container type.
* [https://www.ffmpeg.org/doxygen/trunk/structAVFormatContext.html <code>AVFormatContext</code>] Represents your specific container.
* [https://www.ffmpeg.org/doxygen/trunk/structAVStream.html <code>AVStream</code>] Represents a single audio, video, or data stream in your container.
* [https://www.ffmpeg.org/doxygen/trunk/structAVCodec.html <code>AVCodec</code>] Represents a single codec (e.g. H.264)
* [https://www.ffmpeg.org/doxygen/trunk/structAVCodecContext.html <code>AVCodecContext</code>] Represents your specific codec and contains all associated paramters (e.g. resolution, bitrate, fps).
* [https://www.ffmpeg.org/doxygen/trunk/structAVPacket.html <code>AVPacket</code>] Compressed Data.
* [https://www.ffmpeg.org/doxygen/trunk/structAVFrame.html <code>AVFrame</code>] Decoded audio or video data.
* [https://www.ffmpeg.org/doxygen/trunk/structSwsContext.html <code>SwsContext</code>] Used for image scaling and colorspace and pixel format conversion operations.

====Pixel Formats====
[https://www.ffmpeg.org/doxygen/4.0/pixfmt_8h.html Reference] 
Pixel formats are stored as <code>AVPixelFormat</code> enums. 
Below are descriptions for a few common pixel formats. 
Note that the exact sizes of buffers may vary depending on alignment. 

;AV_PIX_FMT_RGB24
* This is your standard 24 bits per pixel RGB. 
* In your AVFrame, data[0] will contain your single buffer RGBRGBRGB. 
* Where the linesize is typically <math>3 * width</math> bytes per row and <math>3</math> bytes per pixel.

;AV_PIX_FMT_YUV420P
* This is a planar YUV pixel format with chroma subsampling. 
* Each pixel will have its own luma component (Y) but each <math>2 \times 2</math> block of pixels will share chrominance components (U, V) 
* In your AVFrame, data[0] will contain your Y image, data[1] will contain your . 
* Data[0] will typically be <math>width * height</math> bytes. 
* Data[1] and data[2] will typically be <math>width * height / 4</math> bytes. 

===Muxing to memory===
You can specify a custom <code>AVIOContext</code> and attach it to your <code>AVFormatContext->pb</code> to mux directly to memory or to implement your own buffering.

===NVENC===
[https://superuser.com/questions/1296374/best-settings-for-ffmpeg-with-nvenc Options Reference]

When encoding using NVENC, your <code>codec_ctx->priv_data</code> is a pointer to a <code>NvencContext</code>.

To list all of the things you can set in the private data, you can type the following in bash
<syntaxhighlight lang="bash">
ffmpeg -hide_banner -h encoder=h264_nvenc
</syntaxhighlight>

{{ hidden | NVENC Codec Ctx |
<syntaxhighlight lang="c++">
if ((ret = av_hwdevice_ctx_create(&hw_device_ctx, AV_HWDEVICE_TYPE_CUDA, NULL,
NULL, 0)) < 0) {
cerr << "[VideoEncoder::VideoEncoder] Failed to create hw context" << endl;
return;
}

if (!(codec = avcodec_find_encoder_by_name("h264_nvenc"))) {
cerr << "[VideoEncoder::VideoEncoder] Failed to find h264_nvenc encoder"
<< endl;
return;
}
codec_ctx = avcodec_alloc_context3(codec);
codec_ctx->bit_rate = 2500000;
codec_ctx->width = source_codec_ctx->width;
codec_ctx->height = source_codec_ctx->height;
codec_ctx->codec_type = AVMEDIA_TYPE_VIDEO;
codec_ctx->time_base = source_codec_ctx->time_base;
input_timebase = source_codec_ctx->time_base;
codec_ctx->framerate = source_codec_ctx->framerate;
codec_ctx->pix_fmt = AV_PIX_FMT_CUDA;
codec_ctx->profile = FF_PROFILE_H264_CONSTRAINED_BASELINE;
codec_ctx->max_b_frames = 0;
codec_ctx->delay = 0;
codec_ctx->gop_size = 0;
// Todo: figure out which ones of these do nothing
av_opt_set(codec_ctx->priv_data, "cq", "23", AV_OPT_SEARCH_CHILDREN);
av_opt_set(codec_ctx->priv_data, "preset", "llhp", 0);
av_opt_set(codec_ctx->priv_data, "tune", "zerolatency", 0);
av_opt_set(codec_ctx->priv_data, "look_ahead", "0", 0);
av_opt_set(codec_ctx->priv_data, "zerolatency", "1", 0);
av_opt_set(codec_ctx->priv_data, "nb_surfaces", "0", 0);
</syntaxhighlight>
}}

==C++ API==
FFmpeg does not have an official C++ API. 
There are wrappers such as [https://github.com/Raveler/ffmpeg-cpp Raveler/ffmpeg-cpp] which you can use. 
However, I recommend just using the C API and wrapping things in smart pointers.

==Python API==
You can try [https://github.com/PyAV-Org/PyAV pyav] which contains bindings for the library. However I haven't tried it.
If you just need to call the CLI, you can use [https://github.com/kkroening/ffmpeg-python ffmpeg-python] to help build calls.

==JavaScript API==
To use FFmpeg in a browser, see [https://ffmpegwasm.netlify.app/ ffmpegwasm].
This is used in https://davidl.me/apps/media/index.html.

==My Preferences==
My preferences for encoding video

===AV1===
Prefer AV1 for encoding video on on modern devices.

===H265/HEVC===
H264/HEVC is now a good tradeoff between size, quality, and compatibility.
This has been supported on devices since Android 5.0 (2014).
<syntaxhighlight lang="bash">
ffmpeg -i $1 -c:v libx265 -crf 23 -preset slow -pix_fmt yuv444p10le -c:a libopus -b:a 128K $2
</syntaxhighlight>

;Notes
* The pixel format <code>yuv444p10le</code> is 10 bit color without chroma subsampling. If your source is lower, you can use <code>yuv420p</code> instead for 8-bit color and 4:2:0 chroma subsampling.

===H264===
If you need compatability with very old and low end devices.
<syntaxhighlight lang="bash">
ffmpeg -i $1 -c:v libx264 -crf 28 -preset medium -pix_fmt yuv420p -c:a libfdk_aac -b:a 128K $2
</syntaxhighlight>

===Opus===

For streaming:
<syntaxhighlight lang="bash">
ffmpeg -i input.wav -c:a libopus -b:a 96k output.opus
</syntaxhighlight>

See https://wiki.xiph.org/Opus_Recommended_Settings

FFmpeg

2025-06-05T01:33:15Z

David: /* Opus */

[https://ffmpeg.org/ FFmpeg] (Fast Forward MPEG) is a library for encoding and decoding multimedia.

You can interact with FFmpeg using their command-line interface or using their [https://ffmpeg.org/doxygen/trunk/index.html C API].
Note that a lot of things involving just decoding or encoding can be done by calling their CLI application and piping things to stdin or from stdout.

==CLI==
You can download static builds of FFmpeg from
* Linux: [https://johnvansickle.com/ffmpeg/ https://johnvansickle.com/ffmpeg/]
* Windows: [https://ffmpeg.zeranoe.com/builds/ https://ffmpeg.zeranoe.com/builds/]

If you need nvenc support, you can build FFmpeg with https://github.com/markus-perl/ffmpeg-build-script.

Basic usage is as follows:
<pre>
ffmpeg [-ss start_second] -i input_file [-s resolution] [-b bitrate] [-t time] [-r output_framerate] output.mp4
</pre>

* Use <code>-pattern_type glob</code> for wildcards (e.g. all images in a folder)

===x264===
x264 is a software h264 decoder and encoder. 
[https://trac.ffmpeg.org/wiki/Encode/H.264]

===Changing Pixel Format===
Encode to h264 with YUV420p pixel format
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -c:v libx264 -profile:v high -pix_fmt yuv420p output.mp4
</syntaxhighlight>

===Images to Video===
[https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/image_sequence Reference] 
Assuming 60 images per second and you want a 30 fps video.
<syntaxhighlight lang="bash">
# Make sure -framerate is before -i
ffmpeg -framerate 60 -i image-%03d.png -r 30 video.mp4
</syntaxhighlight>

===Video to Images===
Extracting frames from a video

<syntaxhighlight lang="bash">
ffmpeg -i video.mp4 frames/%d.png
</syntaxhighlight>

* Use <code>-ss H:M:S</code> to specify where to start before you input the video
* Use <code>-vframes 1</code> to extract one frames
* Use <code>-vf "select=not(mod(n\,10))"</code> to select every 10th frame

===Get a list of encoders/decoders===
[https://superuser.com/questions/1236275/how-can-i-use-crf-encoding-with-nvenc-in-ffmpeg Reference]
<syntaxhighlight lang="bash">
for i in encoders decoders filters; do
echo $i:; ffmpeg -hide_banner -${i} | egrep -i "npp|cuvid|nvenc|cuda"
done
</syntaxhighlight>

===PSNR/SSIM===
[https://github.com/stoyanovgeorge/ffmpeg/wiki/How-to-Compare-Video Reference] 
FFmpeg can compare two videos and output the psnr or ssim numbers for each of the y, u, and v channels. 
<syntaxhighlight lang="bash">
ffmpeg -i distorted.mp4 -i reference.mp4 \
-lavfi "ssim;[0:v][1:v]psnr" -f null –

ffmpeg -i distorted.mp4 -i reference.mp4 -lavfi psnr -f null -
ffmpeg -i distorted.mp4 -i reference.mp4 -lavfi ssim -f null -
</syntaxhighlight>

===Generate Thumbnails===
[https://superuser.com/questions/1099491/batch-extract-thumbnails-with-ffmpeg Reference] 
Below is a bash script to generate all thumbnails in a folder
{{hidden|Script|
<syntaxhighlight lang="bash">
#!/usr/bin/env bash

OUTPUT_FOLDER="thumbnails"

mkdir -p $OUTPUT_FOLDER
for file in *.mp4;
do ffmpeg -i "$file" -vf "select=gte(n\,300)" -vframes 1 "$OUTPUT_FOLDER/${file%.mp4}.png";
done
</syntaxhighlight>
}}

===MP4 to GIF===
Normally you can just do
<syntaxhighlight lang="bash">
ffmpeg -i my_video.mp4 my_video.gif
</syntaxhighlight>

If you want better quality, you can use the following filter_complex:
<pre>
[0]split=2[v1][v2];[v1]palettegen=stats_mode=full[palette];[v2][palette]paletteuse=dither=sierra2_4a
</pre>

Here is another script from [https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality]
{{hidden | mp4 to gif script |
<syntaxhighlight lang="bash">
#!/bin/sh
ffmpeg -i $1 -vf "fps=15,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" -loop 0 $2
</syntaxhighlight>
}}

===Pipe to stdout===
Below is an example of piping the video only to stdout:
<pre>
ffmpeg -i video.webm -pix_fmt rgb24 -f rawvideo -
</pre>

In Python, you can read it as follows:
<syntaxhighlight lang="python">
video_width = 1920
video_height = 1080
ffmpeg_process = subprocess.Popen(ffmpeg_command,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
raw_image = ffmpeg_process.stdout.read(
video_width * video_height * 3)
image = (np.frombuffer(raw_image, dtype=np.uint8)
.reshape(video_height, video_width, 3))
</syntaxhighlight>

==Filters==
Filters are part of the CLI 
[https://ffmpeg.org/ffmpeg-filters.html https://ffmpeg.org/ffmpeg-filters.html]

===Crop===
<syntaxhighlight lang="bash">
ffmpeg -i input_filename -vf "crop=w:h:x:y" output_filename
</syntaxhighlight>

* Here <code>x</code> and <code>y</code> are the top left corners of your crop. <code>w</code> and <code>h</code> are the height and width of the final image or video.

===Resizing/Scaling===
[https://trac.ffmpeg.org/wiki/Scaling FFMpeg Scaling] 
[https://ffmpeg.org/ffmpeg-filters.html#scale scale filter]

<syntaxhighlight lang="bash">
ffmpeg -i input.avi -vf scale=320:240 output.avi

ffmpeg -i input.jpg -vf scale=iw*2:ih input_double_width.png
</syntaxhighlight>

* If the aspect ratio is not what you expect, try using the <code>setdar</code> filter.
** E.g. <code>setdar=ratio=2/1</code>

;Resizing with transparent padding
Useful for generating logos
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=128:w=128:force_original_aspect_ratio=decrease,pad=128:128:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

{{hidden | More sizes |

;256
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=256:w=256:force_original_aspect_ratio=decrease,pad=256:256:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

;512
<syntaxhighlight lang="bash">
ffmpeg -i icon.svg -vf "scale=h=512:w=512:force_original_aspect_ratio=decrease,pad=512:512:(ow-iw)/2:(oh-ih)/2:color=0x00000000" -y icon.png
</syntaxhighlight>

}}

===Rotation===
[https://ffmpeg.org/ffmpeg-filters.html#transpose transpose filter]

To rotate 180 degrees
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -vf "transpose=1,transpose=1" output.mp4
</syntaxhighlight>
* 0 – Rotate by 90 degrees counter-clockwise and flip vertically.
* 1 – Rotate by 90 degrees clockwise.
* 2 – Rotate by 90 degrees counter-clockwise.
* 3 – Rotate by 90 degrees clockwise and flip vertically.

===360 Video===
See [https://ffmpeg.org/ffmpeg-filters.html#v360 v360 filter]

=====Converting EAC to equirectangular=====
Youtube sometimes uses an EAC format. You can convert this to the traditional equirectangular format as follows:
<pre>
ffmpeg -i input.mp4 -vf "v360=eac:e" output.mp4
</pre>

Sometimes you may run into errors where height or width is not divisible by 2. 
Apply a scale filter to fix this issue.
<pre>
ffmpeg -i input.mp4 -vf "v360=eac:e,scale=iw:-2" output.mp4
</pre>

====Converting to rectilinear====
<pre>
ffmpeg -i input.mp4 -vf "v360=e:rectilinear:h_fov=90:v_fov=90" output.mp4
</pre>

====Metadata====
To add 360 video metadata, you should use [https://github.com/google/spatial-media Google's spatial-media].
This will add the following sidedata which you can see using <code>ffprobe</code>:
<pre>
Side data:
spherical: equirectangular (0.000000/0.000000/0.000000)
</pre>

===Removing Duplicate Frames===
[https://stackoverflow.com/questions/37088517/remove-sequentially-duplicate-frames-when-using-ffmpeg Reference] 
[https://ffmpeg.org/ffmpeg-filters.html#mpdecimate mpdecimate filter]

Useful for extracting frames from timelapses.
<syntaxhighlight lang="bash">
ffmpeg -i input.mp4 -vf mpdecimate,setpts=N/FRAME_RATE/TB out.mp4
</syntaxhighlight>

===Stack and Unstack===
To stack, see [https://ffmpeg.org/ffmpeg-all.html#hstack <code>hstack</code>], [https://ffmpeg.org/ffmpeg-all.html#vstack <code>vstack</code>].
To unstack, see <code>crop</code>.

===Filter-Complex===
Filter complex allows you to create a graph of filters.

Suppose you have 3 inputs: $1, $2, $3.
Then you can access them as streams [0], [1], [3].
The filter syntax allows you to chain multiple filters where each filter is an edge.
For example, <code>[0]split[t1][t2]</code> creates two vertices t1 and t2 from input 0.
The last statement in your edge will be the output of your command:
E.g. <code>[t1][t2]vstack</code>

<pre>
ffmpeg -i $1 -i $2 -i $3 -filter_complex "[0]split[t1][t2];[t1][t2]vstack" output.mkv -y
</pre>

===Concatenate Videos===
<pre>
ffmpeg -i part_1.mp4 \
-i part_2.mp4 \
-i part_3.mp4 \
-filter_complex \
"[0]scale=1920:1080[0s];\
[1]scale=1920:1080[1s];\
[2]scale=1920:1080[2s];\
[0s][0:a][1s][1:a][2s][2:a]concat=n=3:v=1:a=1[v][a]" \
-map "[v]" -map "[a]" \
-vsync 2 \
all_parts.mp4 -y
</pre>

===Replace transparency===
[https://superuser.com/questions/1341674/ffmpeg-convert-transparency-to-a-certain-color Reference] 
Add a background to transparent images. 
<pre>
ffmpeg -i in.mov -filter_complex "[0]format=pix_fmts=yuva420p,split=2[bg][fg];[bg]drawbox=c=white@1:replace=1:t=fill[bg];[bg][fg]overlay=format=auto" -c:a copy new.mov
</pre>

===Draw Text===
https://stackoverflow.com/questions/15364861/frame-number-overlay-with-ffmpeg
<pre>
ffmpeg -i input -vf "drawtext=fontfile=Arial.ttf: text='%{frame_num}': start_number=1: x=(w-tw)/2: y=h-(2*lh): fontcolor=black: fontsize=20: box=1: boxcolor=white: boxborderw=5" -c:a copy output
</pre>

==C API==
A doxygen reference manual for their C api is available at [https://ffmpeg.org/doxygen/trunk/index.html]. 
Note that FFmpeg is licensed under GPL. 
If you only need to do encoding and decoding, you can simply pipe the inputs and outputs of the FFmpeg CLI to your program [https://batchloaf.wordpress.com/2017/02/12/a-simple-way-to-read-and-write-audio-and-video-files-in-c-using-ffmpeg-part-2-video/]. 

===Getting Started===
Best way to get started is to look at the [https://ffmpeg.org/doxygen/trunk/examples.html official examples].

====Structs====
* [https://www.ffmpeg.org/doxygen/trunk/structAVInputFormat.html <code>AVInputFormat</code>]/[https://www.ffmpeg.org/doxygen/trunk/structAVOutputFormat.html <code>AVOutputFormat</code>] Represents a container type.
* [https://www.ffmpeg.org/doxygen/trunk/structAVFormatContext.html <code>AVFormatContext</code>] Represents your specific container.
* [https://www.ffmpeg.org/doxygen/trunk/structAVStream.html <code>AVStream</code>] Represents a single audio, video, or data stream in your container.
* [https://www.ffmpeg.org/doxygen/trunk/structAVCodec.html <code>AVCodec</code>] Represents a single codec (e.g. H.264)
* [https://www.ffmpeg.org/doxygen/trunk/structAVCodecContext.html <code>AVCodecContext</code>] Represents your specific codec and contains all associated paramters (e.g. resolution, bitrate, fps).
* [https://www.ffmpeg.org/doxygen/trunk/structAVPacket.html <code>AVPacket</code>] Compressed Data.
* [https://www.ffmpeg.org/doxygen/trunk/structAVFrame.html <code>AVFrame</code>] Decoded audio or video data.
* [https://www.ffmpeg.org/doxygen/trunk/structSwsContext.html <code>SwsContext</code>] Used for image scaling and colorspace and pixel format conversion operations.

====Pixel Formats====
[https://www.ffmpeg.org/doxygen/4.0/pixfmt_8h.html Reference] 
Pixel formats are stored as <code>AVPixelFormat</code> enums. 
Below are descriptions for a few common pixel formats. 
Note that the exact sizes of buffers may vary depending on alignment. 

;AV_PIX_FMT_RGB24
* This is your standard 24 bits per pixel RGB. 
* In your AVFrame, data[0] will contain your single buffer RGBRGBRGB. 
* Where the linesize is typically <math>3 * width</math> bytes per row and <math>3</math> bytes per pixel.

;AV_PIX_FMT_YUV420P
* This is a planar YUV pixel format with chroma subsampling. 
* Each pixel will have its own luma component (Y) but each <math>2 \times 2</math> block of pixels will share chrominance components (U, V) 
* In your AVFrame, data[0] will contain your Y image, data[1] will contain your . 
* Data[0] will typically be <math>width * height</math> bytes. 
* Data[1] and data[2] will typically be <math>width * height / 4</math> bytes. 

===Muxing to memory===
You can specify a custom <code>AVIOContext</code> and attach it to your <code>AVFormatContext->pb</code> to mux directly to memory or to implement your own buffering.

===NVENC===
[https://superuser.com/questions/1296374/best-settings-for-ffmpeg-with-nvenc Options Reference]

When encoding using NVENC, your <code>codec_ctx->priv_data</code> is a pointer to a <code>NvencContext</code>.

To list all of the things you can set in the private data, you can type the following in bash
<syntaxhighlight lang="bash">
ffmpeg -hide_banner -h encoder=h264_nvenc
</syntaxhighlight>

{{ hidden | NVENC Codec Ctx |
<syntaxhighlight lang="c++">
if ((ret = av_hwdevice_ctx_create(&hw_device_ctx, AV_HWDEVICE_TYPE_CUDA, NULL,
NULL, 0)) < 0) {
cerr << "[VideoEncoder::VideoEncoder] Failed to create hw context" << endl;
return;
}

if (!(codec = avcodec_find_encoder_by_name("h264_nvenc"))) {
cerr << "[VideoEncoder::VideoEncoder] Failed to find h264_nvenc encoder"
<< endl;
return;
}
codec_ctx = avcodec_alloc_context3(codec);
codec_ctx->bit_rate = 2500000;
codec_ctx->width = source_codec_ctx->width;
codec_ctx->height = source_codec_ctx->height;
codec_ctx->codec_type = AVMEDIA_TYPE_VIDEO;
codec_ctx->time_base = source_codec_ctx->time_base;
input_timebase = source_codec_ctx->time_base;
codec_ctx->framerate = source_codec_ctx->framerate;
codec_ctx->pix_fmt = AV_PIX_FMT_CUDA;
codec_ctx->profile = FF_PROFILE_H264_CONSTRAINED_BASELINE;
codec_ctx->max_b_frames = 0;
codec_ctx->delay = 0;
codec_ctx->gop_size = 0;
// Todo: figure out which ones of these do nothing
av_opt_set(codec_ctx->priv_data, "cq", "23", AV_OPT_SEARCH_CHILDREN);
av_opt_set(codec_ctx->priv_data, "preset", "llhp", 0);
av_opt_set(codec_ctx->priv_data, "tune", "zerolatency", 0);
av_opt_set(codec_ctx->priv_data, "look_ahead", "0", 0);
av_opt_set(codec_ctx->priv_data, "zerolatency", "1", 0);
av_opt_set(codec_ctx->priv_data, "nb_surfaces", "0", 0);
</syntaxhighlight>
}}

==C++ API==
FFmpeg does not have an official C++ API. 
There are wrappers such as [https://github.com/Raveler/ffmpeg-cpp Raveler/ffmpeg-cpp] which you can use. 
However, I recommend just using the C API and wrapping things in smart pointers.

==Python API==
You can try [https://github.com/PyAV-Org/PyAV pyav] which contains bindings for the library. However I haven't tried it.
If you just need to call the CLI, you can use [https://github.com/kkroening/ffmpeg-python ffmpeg-python] to help build calls.

==JavaScript API==
To use FFmpeg in a browser, see [https://ffmpegwasm.netlify.app/ ffmpegwasm].
This is used in https://davidl.me/apps/media/index.html.

==My Preferences==
My preferences for encoding video
===H264===
H264 is best when you need the most compatability, especially with older or low end devices.
<syntaxhighlight lang="bash">
!#/bin/bash

ffmpeg -i $1 -c:v libx264 -crf 28 -preset medium -pix_fmt yuv420p -c:a libfdk_aac -b:a 128K $2
</syntaxhighlight>

;Notes
* MP4 is ok

===H265/HEVC===
H264/HEVC is now a good tradeoff between size, quality, and compatability.
<syntaxhighlight lang="bash">
!#/bin/bash

ffmpeg -i $1 -c:v libx265 -crf 23 -preset slow -pix_fmt yuv444p10le -c:a libopus -b:a 128K $2
</syntaxhighlight>

;Notes
* You need to output to a MKV file
* The pixel format <code>yuv444p10le</code> is 10 bit color without chroma subsampling. If your source is lower, you can use <code>yuv420p</code> instead for 8-bit color and 4:2:0 chroma subsampling.

===AV1===

===Opus===

For streaming:
<syntaxhighlight lang="bash">
ffmpeg -i input.wav -c:a libopus -b:a 96k output.opus
</syntaxhighlight>

See https://wiki.xiph.org/Opus_Recommended_Settings