How put different colors at front and back sides of quad

Question

Actually, my question is in header. I use "modern opengl" approach with shaders and buffers and I wonder is it exist any way to get different colors on two sides of one quad.

Desired result (back and front sides)

Answer 1

GLSL provides the built-in gl_FrontFacing variable that determines if the fragment is a front-facing fragment or a back-facing fragment (is true if front facing) e.g. if it is a fragment of the front side of the quad or a fragment of the back side of the quad.

Within the fragment shader you can then check for this variable and set the corresponding colors per face direction:

if(gl_FrontFacing)
{
    outColor = color1;
}
else    // Fragment is back facing fragment
{
    outColor = color2;
}