Inspired by a paper about ilumination for cell animation I have written a plugin for XSI’s FX-Tree to map a texture onto a rendered image based on its normal map.
This way you can add or rather paint in light effects (especially rim lights) in post (see video below).

The main idea was to utilize normal map passes/channels to fine-tune the lighting (adding rim lights, etc.) in post. The plugin renders extremely fast, since it only depends on the image size and not on the complexity of the rendered geometry. Using object, world or camera based normal maps allows for advanced applications.

The rim light in the video comes from a gradient of an orthographically rendered sphere. By switching the projection in the plugin you can use rectangular textures as well. The reason why it works well with spherical rendered textures is, that it maps the normals of the normal map to normals of a unit sphere before these vectors are projected to calculate the color from the lookup texture.

The next video shows the result of blending mappings using camera based normals (used for rim lights) and object based normals (view dependant) to render a shading for a constant pass.