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VK_KHR_fragment_shader_barycentric.proposal

This document details the VK_KHR_fragment_shader_barcentric extension, which adds a cross-vendor way to access barycentric coordinates in a fragment shader.

Problem Statement

Barycentric coordinates are widely used in computer graphics, and are an important building block for various algorithms. By being able to access the location of a pixel within a primitive and the non-interpolated attributes at the vertices, pixel shaders are able to perform things such as custom attribute interpolation or effects based on the pixel’s location within a primitive.

Solution Space

Two options have been considered:

  • Take VK_NV_fragment_shader_barycentric and add any new required properties and behaviors.
  • Start afresh

This proposal focuses on the first option.

Proposal

New SPIR-V decorations

A new SPIR-V extension SPV_KHR_fragment_shader_barycentric adds three fragment shader variable decorations:

  • PerVertexKHR, which indicates that a fragment shader input will not have interpolated values, but instead must be accessed with an extra array index that identifies one of the vertices of the primitive producing the fragment
  • BaryCoordKHR, which indicates that the variable is a three-component floating-point vector holding barycentric weights for the fragment produced using perspective interpolation
  • BaryCoordNoPerspKHR, which indicates that the variable is a three-component floating-point vector holding barycentric weights for the fragment produced using linear interpolation

Barycentric weights

  • For point primitives, BaryCoordKHR and BaryCoordNoPerspKHR are assigned the value (1,0,0).
  • For line primitives, BaryCoordKHR and BaryCoordNoPerspKHR are assigned the values (1,0,0) and (0,1,0) at the ends of the primitive.
  • For polygon primitives, BaryCoordKHR and BaryCoordNoPerspKHR are assigned the values (1,0,0), (0,1,0), and (0,0,1) at the three vertices.

Per-vertex attributes

Per-vertex attributes for "missing" vertices, such as the third vertex of a line primitive, will return values from the valid vertex with the highest index.

Vertices are numbered as follows:

Primitive TopologyVertex 0Vertex 1Vertex 2

VK_PRIMITIVE_TOPOLOGY_POINT_LIST

i

i

i

VK_PRIMITIVE_TOPOLOGY_LINE_LIST

2i

2i+1

2i+1

VK_PRIMITIVE_TOPOLOGY_LINE_STRIP

i

i+1

i+1

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST

3i

3i+1

3i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP (even)

i

i+1

i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP (odd)

i

i+2

i+1

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN

i+1

i+2

0

VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY

4i+1

4i+2

4i+2

VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY

i+1

i+2

i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY

6i

6i+2

6i+4

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY (even)

2i

2i+2

2i+4

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY (odd)

2i

2i+4

2i+2

When the provoking vertex mode is VK_PROVOKING_VERTEX_MODE_LAST_VERTEX_EXT, the original vertex numbers used are the same as above except as indicated in the table below.

Primitive TopologyVertex 0Vertex 1Vertex 2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP (odd, and triStripVertexOrderIndependentOfProvokingVertex is VK_FALSE)

i+1

i

i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN

0

i+1

i+2

VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY (odd)

2i+2

2i

2i+4

Properties

A new property structure is added:

typedef struct VkPhysicalDeviceFragmentShaderBarycentricPropertiesKHR {
    VkStructureType    sType;
    void*              pNext;
    VkBool32           triStripVertexOrderIndependentOfProvokingVertex;
} VkPhysicalDeviceFragmentShaderBarycentricPropertiesKHR;

The triStripVertexOrderIndependentOfProvokingVertex property indicates that the implementation does not change its vertex numbering for triangle strip primitives when the provoking vertex mode is VK_PROVOKING_VERTEX_MODE_LAST_VERTEX_EXT.

GLSL mapping

The following variables from GL_EXT_fragment_shader_barycentric map to these SPIR-V built-in decorations:

  • in vec3 gl_BaryCoordEXT; → BaryCoordKHR
  • in vec3 gl_BaryCoordNoPerspEXT; → BaryCoordNoPerspKHR

GLSL variables declared using the __pervertexEXT GLSL qualifier are expected to be decorated with PerVertexKHR in SPIR-V.

HLSL mapping

  • in float3 barycentrics : SV_Barycentrics; → BaryCoordKHR
  • in noperspective float3 barycentrics : SV_Barycentrics; → BaryCoordNoPerspKHR

Values of per-vertex attributes provided by GetAttributeAtVertex are expected to be decorated with PerVertexKHR in SPIR-V.

Issues

None