I have an application where I want to render a 3D scene in multiple fragments due to reasons of complexity and then stitch them together in a second pass. I can do that essentially in two blender projects where the first one just renders all the fragments and the second project is a compositor-only one and stitches the fragments together (would be also possible with any video editor).
For reasons of compactness I'd like to do this in a single Blender project where a script is walking through all frames, renders the fragments for each frame and then uses some compositor node setup to assemble the final image. The latter requires that the script does a re-routing in the compositor such that the output node receives the image not from the "Render Layers" node but from the nodes that are stitching the fragments together.
So while this is essentially working, Blender does re-calculate the 3D scene although it is not being used resp. not connected. This is nonsense, of course. Is there some way to suppress this 3D rendering?