Recently, I have been working on a project where a spaceship zooms to a CG earth, and then in the next scene, you can see it slowly landing onto the ground. I am using real footage for the second scene, and I already have it camera tracked. The only problem is that the hover engines of the CG ship are emitting blue lights, and I want the light shining from them to show on the ground of the real motion tracked video. In case if I'm not being clear enough imagine I modeled a flashlight that actually emitted light, and I wanted to CGI it into a video where I am shining an imaginary flashlight onto the ground. With a little bit of masking and object tracking, I would be able to put the flashlight into my hand, but how would I have the light shining from the CG flashlight look like it is shining onto the ground of the video?
Create the models that represent real ground and other environment objects and add a white diffuse material to them. Then light them with the lights that your spaceship model is using and add (Mix node, Add blending mode) the resulting render to your footage. Put spaceshit to a layer that is not visible, but is not excluded, you only want to see the environment proxy objects but still need the interaction with the spaceship.
The light that your spaceship emits is additional light in the scene, so you need to literally add light to filmed stuff. Make sure that the environment proxy objects are lit with ONLY the lights that are related to spaceship. Disable all environment lighting, ambient occlusions, sun lamps and other stuff that already is present in the actual filmed footage.
