What happens to the depth buffer if I discard a fragment in a shader using early_fragment_tests?

Question

I'm using a fragment shader which discards some fragments using the discard keyword. My shader also uses the early_fragment_tests ( image store load obliges ).

EDIT : I do not write the gl_FragDepth, I let the standard OpenGL handle the depth value.

Will my depth buffer be updated with the fragment's depth before the discard keyword is executed?

EDIT : It does not seems like it on my NVidia Quadro 600 and K5000.

Any clue where I could find that information? FYI, I searched http://www.opengl.org/registry/specs/ARB/shader_image_load_store.txt. I found close enough topics but not that particular one.

Answer 1

Will my depth buffer be updated with the fragment's depth before the discard keyword is executed?

No, this sort of behavior is explicitly forbidden in a shader that contains discard or that writes an arbitrary value to gl_FragDepth. This is because in such a shader, the depth of your fragment after it is shaded may be unrelated the position generated during initial rasterization (pre-shading).

Without writing to gl_FragDepth or discarding, the depth of a fragment is actually known long before the actual fragment shader executes and this forms the foundation for early depth tests. Rasterization/shading can be skipped for some (individual tiled regions) or all of a primitive if it can be determined that it would have failed a depth test before the fragment shader is evaluated, but if the fragment shader itself is what determines a fragment's depth, then all bets are off.

There is an exception to this rule in DX11 / OpenGL 4.x. If you write your shaders in such a way that you can guarantee the output depth will always preserve the result of a depth test (same result as the depth generated during rasterization), early fragment tests can be enabled in a shader that uses discard or writes to gl_FragDepth. This feature is known as conservative depth, and unless you use this it is generally understood that discard is going to break early depth optimizations across the board.

Now, since you should never write to the depth buffer before you know whether the value you are writing passes or fails a depth test (gl_FragDepth may be different) or if the fragment even survives (discard may be used), you can see why a primitive shaded by a fragment shader that contains discard cannot write to the depth buffer before the shader is evaluated.