@Christopher_Crowell (Christopher Crowell) posted a new topic, subject "Light catching in animation". It read:

   Say, hypothetically, I wanted to make a scene, but my computer can only handle one character at a time without the special effects, due to all of the particles. I want to render everything one or two layers. First the character, then the scene. How can I render the character and add all of the lighting from the explosions and whatnot? Kind of like a shadow catcher, except with lighting instead of with shadows.

Then @sundialsvc4 (sundialsvc4) said

    This, broadly stated, is what compositing is all about. For instance, you might create one “layer” of the base scene, another of the particles, another of one-or-more explosions. You record these to MultiLayer OpenEXR files, and each one contains only the effect: every other area of the frame is “Alpha=0” as in “there is nothing there.”
    
    If something needs to “cast shadows upon” something else, you can design that particular render so that it records the shadows that are cast upon some dummy object, but not the object itself. The “MultiLayer” file format will record this shadow information – this layer – separately from every other layer in the file.
    
    Admittedly, “compositing is an art,” and it takes advance planning. But nearly everything that you see in real-world CG is done this way. In preparing each component, the computer needs to focus on only one rendering task. The process of compositing the various elements together becomes a later, human task. You can “tweak” and “fine-tune” this process in nearly real time since the “rendering” is complete.

Then @Christopher_Crowell (Christopher Crowell) said

    …ok… I know what compositing is. My question was, if I render a layer with the model… but not the explosion, how can I cast the light from the explosion on to the model. I already know how to cast shadow. It's casting the light that I'm asking about


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