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.