how to render background images

[Jim Scott]

I’m sorry, but it’s not clear what you’ve done so far. If you provide some more details about your workflow someone will probably be able to help.

[Steve Bentley]

I think what Mike is after is using a background object to hold the original movie that he 3D tracked and have it render behind the C4D generated object so that C4D does the compositing for him.
You can add a background object (create/environment/background) and then make a new material and add a the movie as a texture in the luminance channel (that way you don’t have light the “background” for the movie to show up). As long as the movie and the c4d render size are the same the movie will fit the frame perfectly. Drag that material to the background object.

Depending on your version of C4D and your viewport settings you may or may not see the movie show up or advance frame by frame in the viewport but it will show up when you render.

If the movie in the texture is not lining up to the track or needs to be moved backwards or forwards in time, just click on the name of the file in the luma channel of the material, then select the animation tab in that dialogue. From there you can select what frame of the movie to start on, what frame of the c4D animation timeline that start frame should be attached to and how long the movie should run, and whether you want to assure that its a perfect frame for frame match between movie frame and timeline frame.

[Mike Poliskey]

This is what my question was about. In a meantime I have found that rendering without background footage (transparent) also fits to my current needs, but I will keep this tip for the future, as an another piece of practical technique.
Thank you Steve for another great expalanaition (as always! ☺
Mike