Fragment Operations

Fragments produced by rasterization go through a number of operations to determine whether or how values produced by fragment shading are written to the framebuffer.

The following fragment operations adhere to rasterization order, and are typically performed in this order:

1. Scissor test
2. Sample mask test
3. Certain Fragment shading operations:
   • Sample Mask Accesses
   • Depth Replacement
4. Multisample coverage
5. Depth bounds test
6. Stencil test
7. Depth test
8. Sample counting
9. Coverage reduction

The coverage mask generated by rasterization describes the initial coverage of each sample covered by the fragment. Fragment operations will update the coverage mask to add or subtract coverage where appropriate. If a fragment operation results in all bits of the coverage mask being 0, the fragment is discarded, and no further operations are performed. Fragments can also be programmatically discarded in a fragment shader by executing one of

• OpTerminateInvocation
• OpDemoteToHelperInvocationEXT
• OpKill.

When one of the fragment operations in this chapter is described as “replacing” a fragment shader output, that output is replaced unconditionally, even if no fragment shader previously wrote to that output.

If there is a fragment shader and it declares the EarlyFragmentTests execution mode, fragment shading and multisample coverage operations should instead be performed after sample counting, and sample mask test may instead be performed after sample counting.

For a pipeline with the following properties:

• a fragment shader is specified
• the fragment shader does not write to storage resources;
• the fragment shader specifies the DepthReplacing execution mode; and
• either
  • the fragment shader specifies the DepthUnchanged execution mode;
  • the fragment shader specifies the DepthLess execution mode and the pipeline uses a VkPipelineDepthStencilStateCreateInfo::depthCompareOp of VK_COMPARE_OP_GREATER or VK_COMPARE_OP_GREATER_OR_EQUAL; or
  • the fragment shader specifies the DepthGreater execution mode and the pipeline uses a VkPipelineDepthStencilStateCreateInfo::depthCompareOp of VK_COMPARE_OP_LESS or VK_COMPARE_OP_LESS_OR_EQUAL

the implementation may perform depth bounds test before fragment shading and perform an additional depth test immediately after that using the interpolated depth value generated by rasterization.

Once all fragment operations have completed, fragment shader outputs for covered color attachment samples pass through framebuffer operations.