GLSL Checkerboard Pattern

Question

i want to shade the quad with checkers:

f(P)=[floor(Px)+floor(Py)]mod2.

My quad is:

glBegin(GL_QUADS);    
  glVerte         


        

Answer 1

What your code does is calculate the factor 4 times (once for each vertex, since it's vertex shader code) and then interpolate those values (because it's written into a varying varible) and then output that variable as color in the fragment shader.

So it doesn't work that way. You need to do that calculation directly in the fragment shader. You can get the fragment position using the gl_FragCoord built-in variable in the fragment shader.