Using GL_ANY_SAMPLES_PASSED may allow your driver to perform the more efficient test you're looking for
This isn't a solution for what I'm requesting. This will simply draw or not draw the entire thing based on a *previous* draw result. I need to not draw specific fragments based on a single fragment passing in *the same* draw call.
The only difference between samples-passed and any-samples-passed is that the first returns an integer counter of how many pixels passed the depth test, while the latter returns a Boolean indicating whether any pixels passed the depth test (basically, returning 'counter>0').
If the GPU is capable of short-circuiting a draw call as you're requesting, then using this 'any' version query is a hint to the driver that it should go ahead and perform this short-circuit optimization.
The any-conservative query is the same, but tells the driver that it's allowed to perform the test against the Hi-Z buffer instead of the Z buffer, which will be quicker but less accurate (may return true when the ground-truth answer is false).
So, mhagain has answered the original question perfectly ;-D
The other solutions of implementing an atomic test in the fragment shader will always be slower, because in a typical occlusion querying situation the fragment shader does absolutuely zero work anyway.
FWIW though, in my experience, GPU occlusion queries are a terrible solution for occlusion culling if you're after performance. I'd personally still recommend CPU based solutions...
