include::meta/VK_AMD_shader_fragment_mask.txt[]

*Last Modified Date*::
    2017-08-16
*IP Status*::
    No known IP claims.
*Dependencies*::
  - Requires the
    https://www.khronos.org/registry/spir-v/extensions/AMD/SPV_AMD_shader_fragment_mask.html[`SPV_AMD_shader_fragment_mask`]
    SPIR-V extension.
*Contributors*::
  - Aaron Hagan, AMD
  - Daniel Rakos, AMD
  - Timothy Lottes, AMD

This extension provides efficient read access to the fragment mask in
compressed multisampled color surfaces.
The fragment mask is a lookup table that associates color samples with color
fragment values.

From a shader, the fragment mask can be fetched with a call to
code:fragmentMaskFetchAMD, which returns a single code:uint where each
subsequent four bits specify the color fragment index corresponding to the
color sample, starting from the least significant bit.
For example, when eight color samples are used, the color fragment index for
color sample 0 will be in bits 0-3 of the fragment mask, for color sample 7
the index will be in bits 28-31.

The color fragment for a particular color sample may then be fetched with
the corresponding fragment mask value using the code:fragmentFetchAMD shader
function.

=== New Object Types

None.

=== New Enum Constants

None.

=== New Enums

None.

=== New SPIR-V Capabilities

  * <<spirvenv-capabilities-table-fragmentmaskamd, code:FragmentMaskAMD>>

=== New Structures

None.

=== New Functions

None.

=== Examples

This example shows a shader that queries the fragment mask from a
multisampled compressed surface and uses it to query fragment values.

[source,c++]
----------------------------------------
#version 450 core

#extension GL_AMD_shader_fragment_mask: enable

layout(binding = 0) uniform sampler2DMS       s2DMS;
layout(binding = 1) uniform isampler2DMSArray is2DMSArray;

layout(binding = 2, input_attachment_index = 0) uniform usubpassInputMS usubpassMS;

layout(location = 0) out vec4 fragColor;

void main()
{
    vec4 fragOne = vec4(0.0);

    uint fragMask = fragmentMaskFetchAMD(s2DMS, ivec2(2, 3));
    uint fragIndex = (fragMask & 0xF0) >> 4;
    fragOne += fragmentFetchAMD(s2DMS, ivec2(2, 3), 1);

    fragMask = fragmentMaskFetchAMD(is2DMSArray, ivec3(2, 3, 1));
    fragIndex = (fragMask & 0xF0) >> 4;
    fragOne += fragmentFetchAMD(is2DMSArray, ivec3(2, 3, 1), fragIndex);

    fragMask = fragmentMaskFetchAMD(usubpassMS);
    fragIndex = (fragMask & 0xF0) >> 4;
    fragOne += fragmentFetchAMD(usubpassMS, fragIndex);

    fragColor = fragOne;
}
----------------------------------------

=== Version History

  * Revision 1, 2017-08-16 (Aaron Hagan)
    - Initial draft