Precision problem on AMD

Question

I have a problem with precision on AMD in shaders (hlsl). In vertex shader I calculate UVs for particle using modulus operator on float. With simple % there is a problem on Nvidia and on AMD (e.g. 7 % 7 = 7) so I have written modulus as:

float Mod( float x, float y )    
{       
     return x - floor(x / y) * y;    
}

This have fixed the problem on Nvidia but AMD still returns wrong values.

Any idea what is going on? And how to fixed it?

I know I can use:

float( int( x ) % int( y ) )

but I think it is slower and I wonder if there is any better way.

Answer 1

In HLSL, you can use the built-in fmod function for this.

I'm not sure what's going wrong with your implementation, though. It looks mathematically correct. It's conceivable you're encountering a precision issue with the subtraction, depending on the inputs.

BTW, an alternative implementation which avoids the subtraction is:

float Mod(float x, float y)
{
    return frac(x / y) * y;
}