OpenGL Render front and back faces with different shading for 3d modeller

Question

I'm working on a simple 3d modeller and I would like to render the back facing triangles a bit darker, to easier differentiate between front and back faces. Like you can see in most modelling applications. How does one achieve is?

Should I enable back face culling and render the primitives and then enable front face culling, rotate the normals by 180 degrees and render the primitives again? Is this a practical approach?

Answer 1

A fragment shader receives the gl_FrontFacing system-defined input value. It will be false if the fragment was generated from a back-facing triangle, and true otherwise. So you can use that in your shader to determine how to generate a color value.