Choosing between multiple shaders based on uniform variable

Question

I want to choose from 2 fragment shaders based on the value of an uniform variable. I want to know how to do that.

I have onSurfaceCreated function which does compile and link to create program1 and glGetAttribLocation of that program1

In my onDrawFrame I do glUseProgram(program1). This function runs for every frame.

My problem is that, in the function onDrawFrame(), I get value of my uniform variable. There I have to choose between program1 or program2. But program1 is already compiled linked and all. How to do this? How will I change my program accordingly and use that since it is already done in onSurfaceCreated.?

Answer 1

You can think of glUseProgram as a temporary function. You call it and it "binds" the program to the current context. What you need to do is call glUseProgram(program2) if the uniform variable is a certain value, though I'd highly recommend you avoid this if at all possible. You usually would compile program1 and program2 separately and then just bind whichever one whenever you need it. Example:

//compile shader 1
//compile shader 2
glUseProgram(prog1);
//Draw stuff that you want to draw with prog1
glUseProgram(prog2);
//Draw more stuff using prog2
glUseProgram(0);

Or if you insist on changing it based off of a uniform value:

//compile shaders
glUseProgram(prog1);
//get the uniform value here (but this will be slow)
if(uniformVal == whatever)
    glUseProgram(prog1);
else
    glUseProgram(prog2);
//Draw stuff
glUseProgram(0);

Again, I highly recommend you do NOT use the uniform value, because communicating from the GPU towards the CPU is harder. Instead solve which shader you require before ever binding the shader at all.