You can composite most elements using additive composition.
The main exceptions are the background element, atmospheres, and shadows:
- Background: The background is not trimmed against geometry, the background should be composited under the other elements.
- Atmosphere: The atmosphere element should be composited over all other elements.
- Black-and-white shadows: Black-and-white shadows should be composited over the rest of the image (aside from the atmosphere), to dim color in the shadowed areas. This technique does not take colored lighting into account.
In other words, the layers when you composite using black-and-white shadows appear like this:
Top: Atmosphere
Second from top: Shadow element
Middle: Diffuse + Specular + ... (other elements)
Bottom: Background
"Screen" Compositing for Specular and Reflection Elements
The other exception to additive composition is when specular or reflection elements have been generated by certain material shaders. These shaders generate specular and reflection elements you must composite differently:
- Anisotropic
- Multi-Layer
- Oren-Nayar-Blinn
Shaders are assigned on a per-material basis, in the Material Editor. If you render specular or reflection elements in a scene that uses these shaders, then composite them with the diffuse and other foreground components (aside from colored shadows, as described above), by overlaying them using an operation called "Screen" in some compositing programs.
Screen compositing uses this formula to combine elements:
Background * (1 - Foreground) + Foreground
The background is multiplied by the inverse of the foreground color, and then the foreground color is added to the result.
For more information, see the documentation for the compositing program you use.