This extension adds a new rasterization mode called conservative rasterization. There are two modes of conservative rasterization; overestimation and underestimation.

When overestimation is enabled, if any part of the primitive, including its edges, covers any part of the rectangular pixel area, including its sides, then a fragment is generated with all coverage samples turned on. This extension allows for some variation in implementations by accounting for differences in overestimation, where the generating primitive size is increased at each of its edges by some sub-pixel amount to further increase conservative pixel coverage. Implementations can allow the application to specify an extra overestimation beyond the base overestimation the implementation already does. It also allows implementations to either cull degenerate primitives or rasterize them.

When underestimation is enabled, fragments are only generated if the rectangular pixel area is fully covered by the generating primitive. If supported by the implementation, when a pixel rectangle is fully covered the fragment shader input variable builtin called FullyCoveredEXT is set to true. The shader variable works in either overestimation or underestimation mode.

Implementations can process degenerate triangles and lines by either discarding them or generating conservative fragments for them. Degenerate triangles are those that end up with zero area after the rasterizer quantizes them to the fixed-point pixel grid. Degenerate lines are those with zero length after quantization.

VK_EXT_conservative_rasterization

Name String

VK_EXT_conservative_rasterization

Extension Type

Device extension

Registered Extension Number

102

Revision

1

Extension and Version Dependencies

VK_KHR_get_physical_device_properties2 or Version 1.1

Contact

• Piers Daniell pdaniell-nv

Other Extension Metadata

Last Modified Date

2020-06-09

Interactions and External Dependencies

• This extension requires SPV_EXT_fragment_fully_covered if the VkPhysicalDeviceConservativeRasterizationPropertiesEXT::fullyCoveredFragmentShaderInputVariable feature is used.
• This extension requires SPV_KHR_post_depth_coverageif the VkPhysicalDeviceConservativeRasterizationPropertiesEXT::conservativeRasterizationPostDepthCoverage feature is used.
• This extension provides API support for GL_NV_conservative_raster_underestimation if the VkPhysicalDeviceConservativeRasterizationPropertiesEXT::fullyCoveredFragmentShaderInputVariable feature is used.

Contributors

• Daniel Koch, NVIDIA
• Daniel Rakos, AMD
• Jeff Bolz, NVIDIA
• Slawomir Grajewski, Intel
• Stu Smith, Imagination Technologies