How can I "add" Depth information to the main frame buffer
Jean-Simon Brochu
Let's say I have this scene 
And I want to add depth information from a custom made fragment shader.
Now the intuitive thing to do would be to draw a quad over my teapot without depth test enabled but with glDepthMask( 1 ) and glColorMask( 0, 0, 0, 0 ). Write some fragments gl_FragDepth and discard some other fragments.
if ( gl_FragCoord.x < 100 )
gl_FragDepth = 0.1;
else
discard;
For some reason, on a NVidia Quadro 600 and K5000 it works as expected but on a NVidia K3000M and a Firepro(dont't remember which one), all the area covered by my discarded fragments is given the depth value of the quad.
Can't I leave the discarded fragments depth values unmodified?
EDIT I have found a solution to my problem. It turns out that as Andon M. Coleman and Matt Fishman pointed out, I have early_fragment_test enabled but not because I enabled it, but because I use imageStore, imageLoad.
With the little time I had to address the problem, I simply copied the content of my current depth buffer just before the "add depth pass" to a texture. Assigned it to a uniform sampler2D. And this is the code in my shader:
if ( gl_FragCoord.x < 100 )
gl_FragDepth = 0.1;
else
{
gl_FragDepth = texture( depthTex, gl_PointCoord ).r;
color = vec4(0.0,0.0,0.0,0.0);
return;
}
This writes a completely transparent pixel with an unchanged z value.
Matt Fichman
Well, it sounds like a driver bug to me -- discarded fragments should not hit the depth buffer. You could bind the original depth buffer as a texture, sample it using the gl_FragCoord, and then write the result back instead of using discard. That would add an extra texture lookup -- but it might be a suitable workaround.
EDIT: From section 6.4 of the GLSL 4.40 Specification:
The discard keyword is only allowed within fragment shaders. It can be used within a fragment shader to abandon the operation on the current fragment. This keyword causes the fragment to be discarded and no updates to any buffers will occur. Control flow exits the shader, and subsequent implicit or explicit derivatives are undefined when this exit is non-uniform. It would typically be used within a conditional statement, for example:
if (intensity < 0.0) discard;A fragment shader may test a fragment’s alpha value and discard the fragment based on that test. However, it should be noted that coverage testing occurs after the fragment shader runs, and the coverage test can change the alpha value.if
(emphasis mine)
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