Z-fighting Direct3D9, only with dynamic buffer

Question

I lock and fill a vertex buffer every frame in Direct3d9 with data from my blendshape code. My shading uses two steps, so I render once with one shader, then draw an additive blend with my other shader.

For reasons beyond me, the data in my vertex buffer is (apparently) slightly different between those two drawing calls, because I have flickering z-fighting where the second pass sometimes renders 'behind' the first.

This is all done in one thread, and the buffer is unlocked a long time before the render calls. Additionally, no changes to any shader instruction take place, so the data should be exactly the same in both calls. If the blendshape happens not to change, no z-fighting takes place.

For now I 'push' the depth a little in my shader, but this is a very inelegant solution.

Why might this data be changed? Why may DirectX make changes to the data in my buffer after I unlock it? Can I force it not to change it?

Answer 1

1st. Are you sure the data is really changed by D3D, or this is just assumption? I'm sure D3D doesn't change your data

2nd. As you said, you have two different shaders drawing your geometry. They mave have different transformation operations. Or because of optimization the transformation in your shaders could be different, thats why your transformed vertices may differ slightly (but enough for z-fighting). I suggest using two passes in one shader/technique. Or if your still want to use two shaders, you better use shared code for transformation and other identitcal operation.

Answer 2

I can sure that the D3D runtime will not change any data you passed in by a vertex buffer, I did the same thing like you when render two layers terrain, no Z-fighting. But there are indeed some render states will change it while rasterizing the triangles into pixels, they're D3DRS_DEPTHBIAS and D3DRS_SLOPESCALEDEPTHBIAS in D3D9, or the equal values in D3D10_RASTERIZER_DESC structure. If these render states were changed, you should check them.

You also need to be sure that all of the transform matrices or other constants which do calculation with position in the shader are precisely equal, otherwise there will be z-fighting.

I suggest you use some graphic debugging tools to check it. You can use PIX, or PerfHUD or Nsight if you were using NVIDIA card.

I'm sorry for my poor English, it must be hard to understand. But I wish this could help you, thanks.