GLSL - Test Fragment Values

问题

Say you have a vec3 colourIn going from a vertex shader to a frag shader, is there a way to test a value and overwrite it if you want to?

For example, set any fragment that has a blue value larger than 0.5 to the colour white?

In my Shader.frag I implemented this test:

    if(colourIn.b>0.5){ //or if(greaterThan(colourIn.b,0.5))
     colourIn.b=0.0;
    }

It compiles and renders the scene, but I can't tell if it's worked because I'm colourblind (lol)... Have I got the theory right and implemented it correctly?

回答1:

You could directly write the conditional if you wanted, and your example should be correct, but a smarter move might be something like:

float mixValue = clamp(floor(colourIn.b * 2.0), 0.0, 1.0);
colourIn.b = mix(colourIn.b, 0.0, mixValue);

// the floor will be:
//     0.0 for [0.0 — 0.5);
//     1.0 for [0.5, 1.0)
//     2.0 1.0
//
// so the clamp will make mixValue:
//     0.0 for [0.0, 0.5]
//     1.0 for (0.5, 1.0]
//
// if you were to multiply by 1.99 then you could
// get rid of the clamp but if the input is a 
// GLubyte then that'd move 128 into the low group
// instead of the high one

Which avoids the conditional and therefore any related pipeline stalls or parallelisation breakdowns.

来源：https://stackoverflow.com/questions/19844547/glsl-test-fragment-values