Question

I am using Nvidia CG and Direct3D9 and have the question about the following code.

It compiles, but doesn’t “loads” (using cgLoadProgram wrapper) and the resulting failure is described simplyas D3D failure happened.

It’s a part of the pixel shader compiled with shader model set to 3.0

What may be interesting is that this shader loads fine in the following cases:

1) Manually unrolling the while statement (to many if { } statements).

2) Removing the line with the tex2D function in the loop.

3) Switching to shader model 2_X and manually unrolling the loop.

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Problem part of the shader code:

float2 tex = float2(1, 1);
float2 dtex = float2(0.01, 0.01);

float h = 1.0 - tex2D(height_texture1, tex);
float height = 1.00;

while ( h < height )
{
    height -= 0.1;
    tex += dtex;

    // Remove the next line and it works (not as expected,
    // of course)
    h = tex2D( height_texture1, tex );
}

If someone knows why this can happen or could test the similiar code in non-CG environment or could help me in some other way, I’m waiting for you 😉

Thanks.

Answer 1

I think you need to determine the gradients before the loop using ddx/ddy on the texture coordinates and then use tex2D(sampler2D samp, float2 s, float2 dx, float2 dy)

The GPU always renders quads not pixels (even on pixel borders – superfluous pixels are discarded by the render backend). This is done because it allows it to always calculate the screen space texture derivates even when you use calculated texture coordinates. It just needs to take the difference between the values at the pixel centers.

But this doesn’t work when using dynamic branching like in the code in the question, because the shader processors at the individual pixels could diverge in control flow. So you need to calculate the derivates manually via ddx/ddy before the program flow can diverge.