Sure, I'll give it a shot. The idea is you need to be comparing depth values in the same space. When you generated your shadow map, you used a certain model, view, and projection matrix for a piece of geometry. When you render the scene from the camera's point of view, you have a model, view and projection matrix as well, but this results in a value that's in a different space. When you render from the point of view of the camera (the second pass), you must also transform by the lights model, view and projection matrix. This will give you a value that's in the range [-1, 1] for X, Y and Z. Texture coordinates are in the range [0, 1] though, so you must scale the value by .5 (note for OpenGL you must also negate the Y texture coordinate). After doing this you'll have texture coordinates to look into the shadow map and see what depth value the light "saw". You also have your pixel that you transformed into the same space, so you know what its depth value is. If the depth value you have for the pixel you are drawing is less than the depth value in the shadow map, that means the pixel you are drawing is closer to the light than what the light saw. In other words, it's not occluded and thus should not be in shadow.
Edited by tool_2046, 21 April 2013 - 04:08 PM.