In the vertex shader, we compute the lighting equation using both the vertex normal and the inverted version, and pass each color to the fragment shader. The fragment shader chooses and applies the appropriate color depending on the orientation of the face.
The evaluation of the reflection model is placed within a function named phongModel. The function is called twice, first using the normal vector (transformed into camera coordinates), and second using the inverted normal vector. The combined results are stored in FrontColor and BackColor, respectively.
There are a few aspects of the shading model that are independent of the orientation of the normal vector (such as the ambient component). One could optimize this code by ...
