I don't know if back lighting is the correct term, but I'm after the effect where a strong light behind the object appears to give it a glow around the edges (something like this). I figure if I can find how much light would pass 'through' any given pixel in a sprite, and encode this in a texture, then I could use it in a shader to change the amount of backlighting depending on lighting conditions and position.
Here's the result of the first step - analysing the input sprite(s) and determining how much light should be let through each individual pixel. Below you can see the result from several sprites (Tails, a cloud and the trees, pinched from the ice reflection test). Until I can come up with a better name I'm calling this the 'scatter map', as it represents how much light scatters around an object.
Next I'll need to generate a normal map from this so I can shade it directionally. Then I can write a shader to use the original sprite, the normal map and the greyscale scatter map, plus the lighting environment, to draw the final lit sprite. Hopefully I'll be able to get something drawing pretty pictures in a couple of days. [grin]