Support for 'FullyCoveredEXT' from 'VK_EXT_conservative_rasterization' in GLSL

[Matheo-Moinet]

Hello,

Using VK_EXT_conservative_rasterization on a Linux Vulkan-GLSL project, I am trying to use the FullyCoveredEXT builtin/decorator in the fragment shader, defined as:

• From VK_EXT_conservative_rasterization:

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.

• From FullyCoveredEXT:

Decorating a variable with the FullyCoveredEXT built-in decoration will make that variable indicate whether the fragment area is fully covered by the generating primitive. This variable is non-zero if conservative rasterization is enabled and the current fragment area is fully covered by the generating primitive, and is zero if the fragment is not covered or partially covered, or conservative rasterization is disabled.

There doesn't seem to be any builtin or decorated variable called FullyCoveredEXT without enabling an extension, but I couldn't find a working GLSL extension to do so. (I have tried GL_NV_conservative_raster without success.)

There seem to be a SPIR-V extension implementing this (SPV_EXT_fragment_fully_covered), and there already seem to be hardware support for this on my card NVIDIA RTX 4070 (in the struct VkPhysicalDeviceConservativeRasterizationPropertiesEXT fullyCoveredFragmentShaderInputVariable is set to true).

fullyCoveredFragmentShaderInputVariable is VK_TRUE if the implementation supports the SPIR-V builtin fragment shader input variable FullyCoveredEXT specifying that conservative rasterization is enabled and the fragment area is fully covered by the generating primitive.

Have I missed something somewhere? Or perhaps is this not implemented yet in GLSL?
If this is not implemented, would it be possible to add it?

Thanks

[gnl21]

Ping @pdaniell-nv

[pdaniell-nv]

You're right there is no "EXT" GLSL extension that exposes this feature. Until one is added, you can use GLSL_NV_conservative_raster_underestimation and the gl_FragFullyCoveredNV builtin. glslang, which is part of the Vulkan SDK, has an implementation of this extension, so should generate SPIR-V with the FullyCoveredEXT decoration, which can target any Vulkan implementation that supports the fullyCoveredFragmentShaderInputVariable feature.

[Matheo-Moinet]

Ahh thank you, I didn't think GLSL_NV_conservative_raster_underestimation would work and compile when GL_NV_conservative_raster didn't!
This works for me for now. In case I later want my code to be AMD compatible, I believe there is no option right now then.
Do you happen to have an ETA for when the "EXT" extension would be added?

[pdaniell-nv]

The SPIR-V generated by glslang should work fine with non-NVIDIA implementations since it uses the multi-vendor FullyCoveredEXT decoration.

[Matheo-Moinet]

Ohh okay, I got confused with all the different brand names in the extensions. I had always thought "NV extension <=> only works on NVIDIA" but I now understand that this is not how it works.
Thank you for your time and explanations!