Name Strings

SPV_EXT_fragment_invocation_density

Contact

To report problems with this extension, please open a new issue at:

https://github.com/KhronosGroup/SPIRV-Registry

Contributors

  • Daniel Koch, NVIDIA

  • Pat Brown, NVIDIA

  • Matthew Netsch, Qualcomm

  • Jan-Harald Fredriksen, Arm

  • Jeff Leger, Qualcomm

Status

  • Complete

Version

Last Modified Date

2018-11-07

Revision

1

Dependencies

This extension is written against the SPIR-V Specification, Version 1.3, Revision 2, Unified.

This extension requires SPIR-V 1.0.

Overview

This extension provides SPIR-V support for the GLSL GL_EXT_fragment_invocation_density extension.

In the corresponding API extensions, applications can use a texture (such as a shading rate image, or fragment density map) to control the number of fragment shader invocations that will be spawned for a particular neighborhood of covered pixels. We refer to the density of fragment shader invocations as the "fragment invocation density".

This extension adds support for two new fragment shader built-ins under the new FragmentDensityEXT capability. These built-ins can be used to determine the density of fragment shader invocations.

A FragSizeEXT decorated variable will represent the width and height of a rectangle of pixels that is being shaded by this fragment shader invocation.

A FragInvocationCountEXT decorated variable will represent the maximum number of fragment shader invocations executed for each fragment.

Extension Name

To use this extension within a SPIR-V module, the following OpExtension must be present in the module:

OpExtension "SPV_EXT_fragment_invocation_density"

Modifications to the SPIR-V Specification, Version 1.3

(Modify Section 3.21, BuiltIn)

(add new rows to the Builtin table)

BuiltIn Enabling Capabilities Enabled by Extension

5292

FragSizeEXT
Input that represents the width and height of a rectangle of pixels corresponding to this invocation. Only valid in the Fragment Execution Model. See the API specification for more detail.

FragmentDensityEXT

SPV_EXT_fragment_invocation_density

5293

FragInvocationCountEXT
Input that represents the maximum number of fragment shader invocations executed for each fragment, as derived from the effective invocation density for the fragment. Only valid in the Fragment Execution Model. See the API specification for more detail.

FragmentDensityEXT

SPV_EXT_fragment_invocation_density
(Modify Section 3.31, Capability, adding a new row to the Capability table)
Capability Depends On Enabled by Extension

5291

FragmentDensityEXT
Uses the FragSizeEXT or FragInvocationCountEXT Builtins.

Shader

SPV_EXT_fragment_invocation_density

Validation Rules

An OpExtension must be added to the SPIR-V for validation layers to check legal use of this extension:

OpExtension "SPV_EXT_fragment_invocation_density"

Issues

  1. How does this extension compare to SPV_NV_shading_rate?

    RESOLVED: SPV_NV_shading rate was to support the VK_NV_shading_rate_image extension. This version is a little bit more generic and is intended to support both VK_NV_shading_rate_image and VK_EXT_fragment_density_map. However, neither of those extensions is strictly needed for this extension to be of interest to applications.

    This extension uses the slightly more generic term "Fragment invocation density" instead of "shading rate" and the names of various tokens are different, per the following table, but otherwise the extensions are intended to provide equivalent functionality.

    SPV_NV_shading_rate SPV_EXT_fragment_invocation_density

    ShadingRateNV

    FragmentDensityEXT

    FragmentSizeNV

    FragSizeEXT

    InvocationsPerPixelNV

    FragInvocationCountEXT

  2. Should we re-use the tokens from SPV_NV_shading_rate or do we need to assign new ones?

    RESOLVED: Re-using the tokens from SPV_NV_shading_rate as this is meant to be a drop-in replacement.

Revision History

Rev Date Author Changes

1

2018-11-07

Daniel Koch

Initial draft