I'm working on my rendering engine, and I've got all the core features that I'm basing it around working. I've already got dynamic shader generation, const buffer management, texture management, etc. all implemented. Now I'm trying to work on shadow mapping though, and I'm not sure how to combine the results of multiple shadow passes.

I already have a system for rendering shadow maps for each light, but after I've rendered my regular diffuse+specular shading pass, how do I combine the shadow map passes? Are shadow maps generally combined using blend states? It would probably work well for simple cases, but what if I have lights illuminating part of another shadow? Then both shadows will just draw on top of each other as one combined shadow. If I just blend the shadows at half opacity though, then they won't properly block shading in un-shadowed areas.

So the shadow maps should be generated first and the lighting for those lights done at the same time then? What about when I have to run multiple passes? This is just an example, because I probably wouldn't need this many shadow casting lights, but If I light four spot lights with shadows, and then I make a pass for directional lighting, how do I combine those? Since I'm doing shading with the shadows, do I just additively blend the lighting results?

Okay, that makes sense now. Thanks for the help.