Apart from that, I presume differences are down to however Avidemux re-encodes cropped videos when it saves files. However, it's unlikely that I made them exactly identical. The aspect ratio of the two recordings, and their absolute widths and heights, would have been roughly the same, depending on how I sized the page being recorded. So why does cropping in OBS behave differently from cropping in Avidemux? More specifically, what information is YouTube using to decide the displayed width and height of uploaded videos, and why do the two cropping methods not deliver the same information to it? In contrast, the Avidemux-cropped version had no such spaces. When I'd uploaded it to YouTube and made it public, it was smaller than I'd expected, with big black spaces round the outside. So I used OBS's ALT-Drag to resize the red box so it just fit round my animation, then recorded the result. So I tried that instead, as it would save me time. But yesterday, I found out how to crop in OBS by using ALT-Drag.

I've been making YouTube videos of these by using Display Capture in OBS, then cropping the resulting videos in Avidemux using its Crop Filter.

I have an animation engine that runs in my web browser and animates cartoons.

Crop video with avidemux