VK_KHR_fragment_shader_barycentric

Other Extension Metadata

Last Modified Date

2022-03-10

IP Status

No known IP claims.

Interactions and External Dependencies

• This extension provides API support for GL_EXT_fragment_shader_barycentric

Contributors

• Stu Smith, AMD
• Tobias Hector, AMD
• Graeme Leese, Broadcom
• Jan-Harald Fredriksen, Arm
• Slawek Grajewski, Intel
• Pat Brown, NVIDIA
• Hans-Kristian Arntzen, Valve
• Contributors to the VK_NV_fragment_shader_barycentric specification

Description

This extension is based on the VK_NV_fragment_shader_barycentric extension, and adds support for the following SPIR-V extension in Vulkan:

• SPV_KHR_fragment_shader_barycentric

The extension provides access to three additional fragment shader variable decorations in SPIR-V:

• 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

When using GLSL source-based shader languages, 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.

New Structures

• Extending VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:
  • VkPhysicalDeviceFragmentShaderBarycentricFeaturesKHR
• Extending VkPhysicalDeviceProperties2:
  • VkPhysicalDeviceFragmentShaderBarycentricPropertiesKHR

New Enum Constants

• VK_KHR_FRAGMENT_SHADER_BARYCENTRIC_EXTENSION_NAME
• VK_KHR_FRAGMENT_SHADER_BARYCENTRIC_SPEC_VERSION
• Extending VkStructureType:
  • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_FRAGMENT_SHADER_BARYCENTRIC_FEATURES_KHR
  • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_FRAGMENT_SHADER_BARYCENTRIC_PROPERTIES_KHR

New Built-In Variables

• BaryCoordKHR
• BaryCoordNoPerspKHR

New SPIR-V Decorations

• PerVertexKHR

New SPIR-V Capabilities

• FragmentBarycentricKHR

Issues

1) What are the interactions with MSAA and how are BaryCoordKHR and BaryCoordNoPerspKHR interpolated?

RESOLVED: The inputs decorated with BaryCoordKHR or BaryCoordNoPerspKHR may also be decorated with the Centroid or Sample qualifiers to specify interpolation, like any other fragment shader input. If the shaderSampleRateInterpolationFunctions feature is enabled, the extended instructions InterpolateAtCentroid, InterpolateAtOffset, and InterpolateAtSample from the GLSL.std.450 may also be used with inputs decorated with BaryCoordKHR or BaryCoordNoPerspKHR.

Version History

• Revision 1, 2022-03-10 (Stu Smith)
  • Initial revision