APPLE_clip_distance.md

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Hash : dadd1986
Author :
Date : 2020-04-21T01:50:00

Implement GL_APPLE_clip_distance

- Built-in variable gl_ClipDistance has been added to compiler.
- Desktop GL: gl_ClipDistance is supported since GL 3.0. Enable/Disable
each gl_ClipDistances[i] works out of the box via glEnable().
- Vulkan/Metal: Use uniform variable to control writing to each
gl_ClipDistance. One bit flag controls one element in the gl_ClipDistance
array. The writing to the disabled element in vertex shader will be
ignored, and turned into zero assignment instead.
- Direct3D/Mobile GL: Not implemented yet.
- Added ClipDistanceTest to gl_tests and compiler unittests.
- GL_APPLE_clip_distance is a subset of GL_EXT_clip_cull_distance, so
GL_EXT_clip_cull_distance could be implemented in future if needed.

Bug: angleproject:4452
Change-Id: I571ac8b56826989808a680226a04bec4cf59988e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2084324
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

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gl_ClipDistance extension support in Vulkan back-end

OpenGL GLSL’s gl_ClipDistance is supported by Vulkan. However, OpenGL supports disabling/enabling individual gl_ClipDistance[i] on the API level side. Writing to gl_ClipDistance[i] in shader will be ignored if it is disabled. Vulkan doesn’t have any equivalent API to disable/enable the writing, though writing to a gl_ClipDistance[i] variable automatically enables it.

To implement this enabling/disabling API in Vulkan back-end:

The shader compiler will translate each gl_ClipDistance[i] assignment to an assignment to ANGLEClipDistance[i] variable.
A special driver uniform variable clipDistancesEnabled will contain one bit flag for each enabled gl_ClipDistance[i]. This variable supports up to 32 gl_ClipDistance indices.
At the end of vertex shader, the enabled gl_ClipDistance[i] will be assigned the respective value from ANGLEClipDistance[i]. On the other hand, those disabled elements will be assigned zero value. This step is described in the following code:
```
  for (int index : arraylength(gl_ClipDistance))
  {
      if (ANGLEUniforms.clipDistancesEnabled & (0x1 << index))
          gl_ClipDistance[index] = ANGLEClipDistance[index];
      else
          gl_ClipDistance[index] = 0;
  }
```
Some minor optimizations:
- Only those indices that are referenced in the original code will be used in if else block above.
- Those elements whose index not referenced in the original code will be zeroised instead.
- If the original code only uses up to an index < gl_MaxClipDistances, then the loop will have at most index+1 iterations. If there is at least one index not being integral constant value known at compile time then declared size of gl_ClipDistance will be the loop size.
- Finally, if the original code doesn’t use gl_ClipDistance, then all the steps above will be omitted.

Source

src/libANGLE/renderer/vulkan/doc/APPLE_clip_distance.md

# gl_ClipDistance extension support in Vulkan back-end

OpenGL GLSL's `gl_ClipDistance` is supported by Vulkan. However, OpenGL supports disabling/enabling
individual `gl_ClipDistance[i]` on the API level side. Writing to `gl_ClipDistance[i]` in shader
will be ignored if it is disabled. Vulkan doesn't have any equivalent API to disable/enable the
writing, though writing to a `gl_ClipDistance[i]` variable automatically enables it.

To implement this enabling/disabling API in Vulkan back-end:

- The shader compiler will translate each `gl_ClipDistance[i]` assignment to an assignment to
  `ANGLEClipDistance[i]` variable.
- A special driver uniform variable `clipDistancesEnabled` will contain one bit flag for each
  enabled `gl_ClipDistance[i]`. This variable supports up to 32 `gl_ClipDistance` indices.
- At the end of vertex shader, the enabled `gl_ClipDistance[i]` will be assigned the respective
  value from `ANGLEClipDistance[i]`. On the other hand, those disabled elements will be assigned
  zero value. This step is described in the following code:
    ```
    for (int index : arraylength(gl_ClipDistance))
    {
        if (ANGLEUniforms.clipDistancesEnabled & (0x1 << index))
            gl_ClipDistance[index] = ANGLEClipDistance[index];
        else
            gl_ClipDistance[index] = 0;
    }
    ```
- Some minor optimizations:
    - Only those indices that are referenced in the original code will be used in if else block
      above.
    - Those elements whose index not referenced in the original code will be zeroised instead.
    - If the original code only uses up to an index < `gl_MaxClipDistances`, then
      the loop will have at most `index+1` iterations. If there is at least one index not being
      integral constant value known at compile time then declared size of `gl_ClipDistance`
      will be the loop size.
    - Finally, if the original code doesn't use `gl_ClipDistance`, then all the steps above will be
      omitted.

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