How to get depth for each sample in MSAA?

Question

I'm wondering if there is a method to get depth for each sample in fragment-shader. I have tried GL_ARB_sample_shading extension, which can give sample ID and sub-pixel coordinate but I haven't found any route for depth. I have tried gl_FragCoord, but it's a constant among all samples within a pixel, giving something like (?.5,?.5,?,?).

It is supposed that depth is calculated and tested for each sample in MSAA. My question is whether it's available in a fragment-shader.

I want to discard fragments out of a depth range defined by two depth maps. That's why I need depth for each sample to perform correct per-sample discard. My depth maps are also generated with MSAA and I use GL_ARB_texture_multisample extension to fetch multi-sampled depth in fragment-shader.

Here's sample code but I have no idea how to implement getSampleDepth.

#version 430
#extension GL_ARB_sample_shading:enable
#extension GL_ARB_texture_multisample:enable

uniform sampler2DMS tex;
out vec4 vFragColor;

float getSampleDepth(int idx){
    //TODO: I have no idea how to get depth for each sample
}

void main(void){
    ivec2 icoord = ivec2(gl_FragCoord.xy);
    int id = gl_SampleID;
    if(getSampleDepth(id) < texelFetch(tex,icoord,id).x)
        discard;
    vFragColor = vec4(1.f);
    // It seems sample can be correctly discarded
    // Keeping part of samples leads to dim color
    // if(gl_SampleID < 2)
    //     discard;
}

Have you actually provoked per-sample shading? That is, have you done something that requires per-sample shading, like access `gl_SampleID`? — Nicol Bolas, Mar 31 '21 at 13:17
@Nicol Bolas Yes, I access `gl_SampleID` as shown in sample code. — Shengjie MA, Apr 02 '21 at 04:49

Nicol Bolas · Accepted Answer · 2021-04-02T05:14:41.683

If you want the current fragment's depth, just use gl_FragCoord.z. This is just as true of per-sample shading as for regular rendering.

If you're getting the same value for gl_FragCoord, it would be because you have not successfully provoked per-sample shading somehow. Or a driver bug. One possible workaround might be to explicitly activate per-sample shading by glEnable(GL_SAMPLE_SHADING) and glMinSampleShading(1.0f).

score 0 · Answer 2 · answered Apr 14 '21 at 03:12

I finally figured it out by writing a test code. gl_FragCoord.z does work. I wrote vec4(gl_FragCoord.xyz,float(gl_SampleID)) to each sample, and then read them to memory. I got something like this:

(x,y):(0,0)
[sample 00] RGBA 0.500000,0.500000,0.281250,0.000000
[sample 01] RGBA 0.500000,0.500000,0.343750,1.000000
[sample 02] RGBA 0.500000,0.500000,0.406250,2.000000
[sample 03] RGBA 0.500000,0.500000,0.468750,3.000000
(x,y):(1,0)
[sample 00] RGBA 1.500000,0.500000,0.281250,0.000000
[sample 01] RGBA 1.500000,0.500000,0.343750,1.000000
[sample 02] RGBA 1.500000,0.500000,0.406250,2.000000
[sample 03] RGBA 1.500000,0.500000,0.468750,3.000000
(x,y):(0,1)
[sample 00] RGBA 0.500000,1.500000,0.531250,0.000000
[sample 01] RGBA 0.500000,1.500000,0.593750,1.000000
[sample 02] RGBA 0.500000,1.500000,0.656250,2.000000
[sample 03] RGBA 0.500000,1.500000,0.718750,3.000000
(x,y):(1,1)
[sample 00] RGBA 1.500000,1.500000,0.531250,0.000000
[sample 01] RGBA 1.500000,1.500000,0.593750,1.000000 
[sample 02] RGBA 1.500000,1.500000,0.656250,2.000000
[sample 03] RGBA 1.500000,1.500000,0.718750,3.000000

How to get depth for each sample in MSAA?

2 Answers2