Twilight Render User Forum

Start by reading this about placing a background image using Alpha Mask.
I will be creating a tutorial especially for Twilight, but for now there is one old tutorial I made that relate to this topic perfectly.

pg8 of the Twilight User Manual 'Advanced Settings' section wrote:

1. Select an object.
2. Choose “Alpha Mask” as your render method. (Camera Tab: Be sure exposure and gamma = 1.0)
3. Start the rendering.
4. When asked to render only selected objects choose “yes”.
5. A black and white image will be created of the object.
6. Save the rendered mask image.

a PDF version step-by-step what to do with the alpha mask once you have it