I have a man in a room, but would like to render the man on a separate layer so that the sunlight for the World Tab environment shines on the man. I also need to get rid of any shadows. I have the man, ceiling and walls on separate layers. How can I do this with the compositor?
Use Render Layers to separate your elements. For example, create a new render layer for your man, then tell it to use the scene layer your man is currently in.
For shadows, create a render layer for your environment and tell it to exclude the layer with your man. Here is a great resource if you need more info on render layers: http://www.blenderguru.com/articles/how-to-render-a-complex-scene-without-crashing/.
Also, if you don’t need or want shadows, don’t waste computer-time generating them. Either use a source-of-light that does not make shadows, or turn-off shadow generation.
(If you do need shadows, be sure to request a shadow-only layer, so that you have access to “just the shadows, ma’am.”)
When you render, render to a MultiLayer OpenEXR file format, which will capture all of the RenderLayers separately and make them all available to you by the names you have given them. (The files will be large, but they are “loss-less.”)
To begin with, I would render exactly one frame. Take the resulting file and, in a new blend-file that you will use for the compositing stage, look at just what it contains. Now, construct the compositing “noodle” that produces what you want. You will find that there are several ways to do it.