It can sometimes be useful to visually determine which faces are front-facing and which are back-facing (based on winding). For example, when working with arbitrary meshes, polygons may not be specified using the appropriate winding. As another example, when developing a mesh procedurally, it can sometimes be helpful to determine which faces have proper winding in order to help with debugging. We can easily tweak our fragment shader to help us solve these kinds of problems by mixing a solid color with all back (or front) faces. For example, we could change the else clause within our fragment shader to the following:

FragColor = mix( vec4(BackColor,1.0), 
                vec4(1.0,0.0,0.0,1.0), 0.7 );

This would mix a solid red color with all back faces, helping them stand out, as shown in the following image. In the image, back faces are mixed with 70 percent red, as shown in the preceding code: