When rendering a mesh that is completely closed, the back faces of polygons are hidden. However, if a mesh contains holes, it might be the case that the back faces would become visible. In this case, the polygons may be shaded incorrectly due to the fact that the normal vector is pointing in the wrong direction. To properly shade those back faces, one needs to invert the normal vector and compute the lighting equations based on the inverted normal.

The following image shows a teapot with the lid removed. On the left, the Phong model is used. On the right, the Phong model is augmented with the two-sided rendering technique discussed in this recipe:

In this recipe, we'll look at an example that uses the Phong model discussed in the previous recipes, augmented with the ability to correctly shade back faces.