Okay, let me explain my use case a little bit, I recognize it’s a little unusual perhaps there is a trivial way I’m missing to get what I want.
The characters will be animated in toonboom harmony and exported as an image sequence with an alpha channel, they will then be placed into the scene on a surface.
I would like the lights in the scene to light the characters in a pleasing way, ie not like they are just on a plane.
When I shine a light on the plane I would like to create a rim lighting around the characters wrapping around the alpha channel on the image.
The way I normally do this with after effects is by coloring the character in black and compositing it with a solid color color card for the light and offsetting it then adding it to the original.
It produces a nice effect and is very easy.
What I am trying to do in blender is control the “offset” of the UV map based on the amount of light falling on the surface.
So if I had a surface that was illuminated like this
I would expect feeding the ShaderToRGB into the bump node would produce something like this:
I could then subtract this from the UV map of the character to produce something that is distorted “away” from the brightest light.
That I could mix with the original image
to produce something like this
Notice the nice taper on the arm that is only partly in the light
However, when I actualy do it,
I just get Blue.
Well, ALMOST blue If I mess around with it I can get some artifacts to show up, so it’s doing /something/ just not what I expect.
This is certainly undocumented behavior and I will file a bug report on it.