Very interesting workaround but the values required for Shift X/Y don’t seem to be very intuitive, any tips for matching the original frame? edit: Oh, I see, looking through the camera view makes it more obvious, it’s going to deal with the aspect ratio.
edit2: It seems more complicated than just changing the camera Shift. You also need to change the focal length to adjust the frustum for the 1/3rd size(assuming 3x3) but then Shift values are back to not being obvious. I’m sure there’s some math, just need to find it.
I mentioned the crop because I did take into account the idea that by default rendering a region still includes the entire camera view, so that could be a GPU memory thing but I thought for sure checking Crop would cut that memory down. Guess not.