I'm doing deferred lighting, where I have a gbuffer pass, followed by a main pass. For alpha tested objects, I would like to do the alpha test in the gbuffer pass, and then skip alpha test in the main pass by using a depth compare function of EQUAL. This way the main pass should only render on top of pixels that passed the alpha test during the gbuffer pass.

The problem is that I can't get the depth values to match exactly between passes. It works sometimes, on some machines, in some situations, but not all the time. As far as I know, I am passing the exact same data for all verts and relevant shader parameters (matrices, etc). Each pass has a different vertex shader, but they compute the output position in exactly the same way. Is there any way to make this work reliably in D3D9? Or will I just have to use LESSEQUAL in the main pass, with a possible depth bias?

Thanks for the replies.

I'm targeting D3D9 on the PC, so sadly no precise keyword. No trickery with changing primitive types. I've looked at the D3D bytecode, and it appears to match, but I could be missing something.

I'll keep looking.

Each pass has a different vertex shader, but they compute the output position in exactly the same way. Is there any way to make this work reliably in D3D9?

Just to verify one thing - you don't mix shaders and fixed function pipeline? For some reason I had lots of accuracy problems when doing it.

Cheers!

No fixed function.

I ended up just using LEQUAL for the main pass, and running alpha test again, at least on PC D3D9. I'd still be curious if anyone has any ideas.

Does it work if you use the same vertex shader on both passes?