You should check Fabio Policarpo’s webiste for more info on relief mapping. I especially recommend the following: Relief Mapping Paper – here Fabio talks about shadows in relief mapping, at page 4. But you should read the whole paper, in order to understand why relief mapping has so many in common to your design.

First, notice that it’s strict top-down. And the main thing to look at is the shadow that is being cast on the stone from the character. The point is that stone pixels will be higher in the height map, and so when character’s shadow is drawn on the stone – it is, mm, shifted.

The point is that I seek for universal solution: where the whole height map render target is filled and then the shadows render target is filled, so when the shadow-drawing pixel shader is called, it doesn’t know if the current pixel is over the terrain or stone – all he knows is the shadow texture, light vector and the current pixel height.

It would be really helpful to know this, since it may affect the question in various ways. Depending on how your characters look / move in the world, the problem may or may not be solvable. So a screenshot would really help me visualize the problem.

The next step is harder. We fill height map for the whole scene, so it will have normals both for terrain and character. The next thing that adds complexity is that character sprite can be rotated. And the final thing is that the shadow sprite offset depends on the vector light->character, and so all these things must be taken in count in the pixel shader that will do shadow distortion.

The whole thing become pretty complex and I wonder if it’s solvable at all.