Shifting background images during render, is it possible? (2.79)

My movie project is getting a bit demanding, and rather than having my rig render for three days straight I decided to render the demanding background scenery first, then use the still images as background when rendering the foreground characters. But because I hav shifting camera angles, I need shifting background images. Is there a way to have the background image shift in mid render? I use 2.79…

Are you doing motion shots or still-cut-still at different angle?

Still-cut-still only, in this case.

I may have found the solution, but I am not sure, so anyone who feels there is a glaring hole in this, let me know:

Step 1: Render all the various camera backgrounds as images.
Step 2: Take the foreground scene and under the Render tab and Shading, set Alpha to “Transparent”.
Step 3: Create a new scene, go into the video sequencer, add the background images in as needed in one channel, then the add the foreground scene in a channel above that.
Step 4: Mark the foreground scene first(!!!) and then an image and add an “Over Drop” effect strip. Do this for every image.
Step 5: Render.
This seems to work, but with only evry limited testing. If anyone spots flaws, pleasetell me!!

I imagine you could save the step of creating a new scene and using the sequencer by just using the compositor with an Image Texture input node and an Alpha Over node. It may create the exact same output, but it would save you from having to create a new scene each time you need to composite a new shot.

If you need a more detailed walkthrough, let me know.

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I’d love a walkthrough! There seems to be comparatively little of the sort for th compositor, and I am VERY green with it!
Edit: What the compositor refused on my first attempt was swapping the background images when matching new camera angles. Can that be done with your method??

I might need a bit more clarification on exactly what you want to achieve - you can make Blender do just about anything with enough coaxing. I think the Switch Node (or a combination of switch nodes) might get you what you want:

How many background images/camera angles are you trying to switch between?

I am doing a scene of two people walking to a bench in a park and sitting down to talk, but it’s a bit long and will likly have about 12 camera angles, some possibly identical. The peeople and foreground render quick, but the background park renders sloooowly, so I want camera angle backgrounds to just be a still image of a single reder each.
I’ll read up on that link, thanks!

And how are you switching your camera angles for each of these shots? I came across a method using markers to switch active camera’s here:

If you use that method, you could get away with a series of 11 Switch Nodes (or n - 1 nodes where n is the number of unique camera angles) where they cascade through each other’s top socket. When you want to switch to a different angle, you make sure you are on the same frame your active camera changes, and then you keyframe in the Switch checkbox for the angle you want.

That is a VERY interesting method, thank you!! Never even heard of markers before…
I’ll have to try this out, it seems much better than my animated cameras (constant interpolation).
Also, I found a node named “Switch” under Layout, but how do I set it to switch at a certain frame?