# PIX is cheating on me (or the shader)

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Hey,

I have a very strange problem. I've been struggling with SSAO self-occlusion (http://www.gamedev.net/topic/595679-ssao-problem-with-self-occlusion/) and finally decided to see what is actually being done under the hood of the shader. So I used PIX to debug some selected pixel of the screen which in my opinion should be white (unoccluded) and which ends up being pretty dark. Look at the code:
 float occlusion = 0.0f; float3 pixelPosition = tex2D(positionsSampler, input.texCoord).xyz; float3 pixelNormal = tex2D(normalsAndDepthsSampler, input.texCoord).xyz; float pixelDepth = -tex2D(normalsAndDepthsSampler, input.texCoord).w; for (int i = 0; i < 2; i++) { float3 randomVector = tex2D(randomVectorsSampler, input.texCoord * (7.0f + (float)i)).xyz; randomVector = randomVector * 2.0f - 1.0f; for (int j = 0; j < 8; j++) { float3 offsetVector = reflect(kernelVectors1[j], randomVector); if (dot(pixelNormal, offsetVector) < 0.0f) offsetVector *= -1.0f; float3 samplePosition = pixelPosition + offsetVector * radius * pixelDepth / 10.0f; // HERE samplePosition.z *= -1.0f; samplePosition = projectToScreen(samplePosition); float sampleDepth = -tex2D(normalsAndDepthsSampler, samplePosition.xy).w; occlusion += occlusionFunction0(samplePosition.z - sampleDepth); } } 
During stepping through each instruction, the debugger does 3 steps in the line commented with "HERE". And during these steps I see in the Variables table in the debugger, that the value of offsetVector changes (!). How can this be? offsetVector is not changed anywhere.
I though that maybe offsetVector is "updated" with "radius * pixelDepth / 10.0f" term. But this doesn't match. Before the change offsetVector is:
 offsetVector ( 0.079, -0.016, -0.012 ) float3 
After the change it becomes:
 offsetVector ( 0.008, -0.002, -0.001 ) float3 
where the term "radius * pixelDepth / 10.0f" is equal to 0.09633 in this case. It is not only that the scale of .x component doesn't match (0.079*0.09633!=0.008). Other components are scaled by a different value than the .x component! So if the .x component changes from 0.079 to 0.008 via some "magic constant" k, the relationships -0.016*k equal to -0.002 and -0.012*k equal to -0.001 do not hold. So every component of offsetVector is scaled by a different value.

Who is guilty? PIX or me cause I do not something very obvious? I'd appreciate your help on that issue.

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Did you disable optimizations? If not then your HLSL code won't really correspond to the assembly code. And even then you'll probably have to drop down to assembly for certain cases. For instance ps_3_0 doesn't support indexing into constant registers...this means that the compiler either has to unroll the loop, or in the loop it has to do a bunch of compares each iteration to "select" the right constant. Either way looking at the assembly will tell you what's really going on.

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Yeah, I did disable optimizations. And I looked closer at what is happening during asm instruction exectuion. Indeed, the debugger much more pays attention to the asm code rather than HSLS one. Huh, shame a bit that you cannot rely on what the debugger displays about high level variables. That makes the debugging much harder. Anyway, thanks for your help.

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