I had always been creating a scene with Film:Transparent, then using Alpha Over to place it on top of a background image. That is simple and intuitive, but no longer seems to be the most efficient method.
A slightly awkward alternative seems to avoid the difficulties.
- Create scene with Film:Transparent
- Put the Scene/View Layer into the TOP input of Mix (this sets “output size”, per the documentation)
- Invert the View Layer’s Alpha – use it as the Mix factor.
- Put the background image into the BOTTOM input of Mix
- Uncheck “Use Alpha” on Viewer/Composite
Now the Viewer/backdrop shows the scene/render bounds on the result. The background image can be translated/scaled as needed, and the Viewer/backdrop shows a preview of what will be rendered. Background does not need to be cropped separately.
Sometimes an old dog must learn new tricks.
