>>535614976
>last time I used ImageMagick to crop screenshots, it seemed to affect the image brightness or color depth somehow
Damn, you know what? I think it's actually just Linux Mint's image viewer (xviewer) being stupid. I tried cropping one with
>convert -extent 1600x1200 -gravity center whatever.png whatever.crop.png
and yes, the cropped one did appear slightly darker as I flipped between the two images in xviewer, but the cropped and uncropped images look the same to me in GIMP and even in Firefox. Moreover, both images in Firefox look the same as the cropped one looks in xviewer, so darker is correct.
I had noticed xviewer was displaying images differently in this old /lgg/ thread when I was playing Enclave, but by this time I had forgotten about the apparent issue with ImageMagick-cropped images, and didn't think to go back and check if xviewer was just showing the uncropped images as too bright.
I don't know shit about image processing, but I guess raw screenshots are saved with some parameters that make xviewer not display them with exactly the right colors/brightness, and passing through ImageMagick "fixes" this somehow.
So I guess I'll go back to using ImageMagick to crop my letterboxed/pillarboxed images with no worries. I haven't searched for a way to autodetect the black bars though.