GLSL - optimize if-else

Question

I have GLSL mobile shader (ES 2.0) and inside I have this:

float b = texture2D(t, texCoord).r;
float s = 0.0;
if (b > 240.0) s = b - 2322.2;
else if (b > 90.0) s = b - 74.0;    
else if (b < 10.0) s = b / 10.0;
else s = b - 10.0;  

Is it going to be slow? I know, that branching in shader should be avoided. Is there any way, how to rewrite this to remove ifs?

Answer 1

I suggest to use the GLSL functions mix and step.

mix interpolates between 2 values according to a floating point interpolation value a in the range [0.0, 1.0]. If the a is equal 0.0 then the 1st value is returned and if the a is equal 1.0 then the 2nd value is returned.

step tests whether a value is less than an edge value. If it is less then 0.0 is returned, else 1.0 is returned.

If you combine the 2 functions your code will look like this:

float b = texture2D(t, texCoord).r;
float s = mix(
    mix( b/10.0, b-10.0,   step(10.0, b) ),
    mix( b-74.0, b-2322.2, step(240.0, b) ),
    step(90.0, b) );

Note, the result of step is either exactly 0.0 or exactly 1.0, this causes that mix either returns the 1st value or returns the 2nd value.