Beyond Grading - Blender Compositing and Post Processing - page 47

The Fresnel effect
The Fresnel option available in shader parameters is used to modify the reflection in-
tensity, based on the viewing angle, to simulate a metallic behavior. After 3D render-
ing, altering this property requires rerendering. Blender provides an alternate method
to build and modify the Fresnel effect in compositing, using the Normal node.
The following screenshot illustrates the Fresnel workflow. In this procedure, the dot
output of a Normal node is connected to the Map Range node and the To Min /
To Max values are tweaked to obtain a black-and-white mask map, as shown in the
screenshot. A Math node is connected to this mask input to clamp information to the
0-1 range.
The 3D-combined render output is rebuilt using the diffuse, specular, and reflection
passes from the 3D render. While rebuilding, the mask created using the Normal node
should be applied as a mask to the factor input of the reflection Add node. This results
in applying reflection only to the white areas of the mask, thereby exhibiting the Fres-
nel effect. A similar technique can be used to add edge highlights, using the mask as
a factor input to the grade nodes.