Shader optimization - Cg/HLSL pseudo and via multiplication

Question

HLSL/Cg do not allow texture fetching inside conditional blocks. To get around this I am first checking a variable and performing some computations, afterwards I set a float flag to 0.0 or 1.0, depending on the computations. I'd like to trigger a texture fetch only if the flag is 1.0 or not null. I kind of hoped this would do the trick:

float4 TU0_atlas_colour=pseudoBool*tex2Dlod(TU0_texture, float4(tileCoord, 0, mipLevel));

I want to know if pseudoBool is 0, will the texture fetch function still be called and produce overhead? I was hoping to prevent it from getting executed via this trick that usually works in plain C/C++.

Answer 1

Firstly you can do texture fetching inside conditional blocks in HLSL. tex2Dlod() and tex2Dgrad() will work fine inside one. It's just tex2D() that won't compile, and you can work round that by computing ddx() and ddy() outside the conditional and using tex2Dgrad().

To reliably stop the texture fetch (or any other block of code) being executed in HLSL, use an if statement prefixed with the [branch] attribute. This requires shader model 3 or higher.

You can also of course also make two separate shaders and switch between them from the calling code, but that isn't as flexible. Where it's possible it will be quicker though.

Also note that unless the branch is skipping over significantly more lines of code than a single texture read you will probably lose performance and not gain it. If in doubt test with both [branch] and [flatten] and compare performance between the two. Without either attribute the compiler will guess at which option is best.