GLSL Rim Highlight

[Xilef]

I want to do some half-rim highlighting, I want a GLSL vertex+fragment shader that renders the edge of the model with the original full-bright texture.

This is the effect I'm trying to achieve:

Here's a diagram:

The rendering path I'm trying to get is:

• Render 3D model darkened to light level of room
• Find direction of nearest light source
• If light source is not in-front
• Start shader program[light_direction] to highlight the model rim with original, undarkened texture

Ideally I want this to be 3 shader programs, one for behind, one for the left, one for the right so I can select which one to use out of 4 states, no program, left program, right program, behind program.

Thanks if anyone can help, I'm pulling my hair out over this one as the internet lacks GLSL tutorials massively.


I've had a second go at this and at the moment I have this based on vertex normals:

Just ignore that poser on the far right!

EDIT: Completed the tint lighting effect that is added to the shader, the dwarf below has a blue tinge in the darkened areas that make the light (Now yellowish) stand out more.