I've been doing some tests with 3d tracking objects into live action footage, and I noticed that when I set up my compositor nodes for a render, the render doesn't seem to work when using RPR. I used Cycles for the test with the exact same settings and node setup as I had for RPR, and that worked.
After messing around with the nodes for a bit I think the issue has to do with the "Alpha Over" Node and/or RPR's ability to render a transparent background, because when I change the factor of the "Alpha Over" to .5 and render a test image, I see my live action footage and the object overlayed on top of each other but with the world environment light of the object still showing. Below are some screenshots and test renders.
Picture 1 - This is the node setup that I used for both Cycles and RPR. In Cycles, this node setup worked and resulted in Picture 2 (below). In RPR, the render accounted for the factor of 1 of the "Alpha Over" and only rendered my object with its world environment light (picture 3).
Picture 2 - this is the cycles render using above node setup, working correctly.
Picture 3 - this is the RPR render using above node setup
Picture 4 - this is the RPR render with the world environment light turned off and the factor of the "alpha over" node turned to 0.5
Picture 5 - this is the RPR render with the world environment light back on, and the factor of the "alpha over" node turned to 0.5
If anyone has any insight as to what's going wrong here, that would be really helpful! Or even if there's a different node setup/workaround that I have to use for RPR, knowing that would be good to know!