Order independent transparency: Difference between revisions

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~~Methods for~~ order independent transparency

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:

# Approximate transparency using a commutative operation, i.e. addition.

# Render fragments (per-pixel colors) from back to front.

==Additive Transparency==

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This builds upon additive transparency with two buffers:

1. Accum buffer: weighted average of all premultiplied-alpha RGB colors.

# Accum buffer: weighted average of all premultiplied-alpha RGB colors.

2. Revealage buffer: buffer representing how much of the opaque background is visible through the transparent layers.

# Revealage buffer: buffer representing how much of the opaque background is visible through the transparent layers.

==Dual Depth Peeling==

==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.

@@ Line 1: / Line 1: @@
-Methods for order independent transparency
+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:
+# Approximate transparency using a commutative operation, i.e. addition.
+# Render fragments (per-pixel colors) from back to front.
 ==Additive Transparency==
@@ Line 8: / Line 14: @@
 This builds upon additive transparency with two buffers:
-. Accum buffer: weighted average of all premultiplied-alpha RGB colors.
+# 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.
+# Revealage buffer: buffer representing how much of the opaque background is visible through the transparent layers.
-==Dual Depth Peeling==
+==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.