Asymetric Screen Shot output?

I am attempting to perform high-accuracy rendering & screen shotting with UE 4.19, using the HRSS capability. Configuration is a flat plane centered at (0,0,0), with “BackgroundMaterial” texture, Camera actor & directional light both at (0,0,large value). Given the physics, I’d expect the image taken from the camera to be symmetrical, with row & column values matching. However, I get fairly large differences in row/column values:

241440-delta-row-col.png

241441-delta-mirrors.png

this happens with both orthographic & perspective cameras. Camera aspect ratio is 1, so the image is square.

Given the mismatch in row/col values, I think this more than an off-by-one (odd/even image size) error