smolrtsp VS avidemux2

I agree; I would especially not recommend abusing macros throughout an application codebase too much. Conceptually, Metalang99 is more of a (sub)language than a library, which also adds some entry barrier. Ideally, I see the application of Metalang99 being used "behind the scenes", e.g., encapsulated in separate code files/libraries such as Datatype99 and Interface99. This is what I (mostly) do in SmolRTSP.

Well since others have mentioned their favorite alternatives to plain ffmpeg for this task, I'll just mention Avidemux. It has a crop filter that lets you pick any frame you want and crop to that, but it also has a VERY good auto-detect button that almost always detects the edges correctly (the exception is very low light scenes, but you can use a slider to pick a brighter frame). Alternately, there is also a quadrature filter in Avidemux that lets you expand (or reduce) the size of the video without changing the size of the output video, so you could essentially blow up the "good" area to push the black bars off the screen (although that will undoubtedly cause some loss of resolution, though there is also a sharpen filter that might compensate for some of that). Avidemux is cross-platform so there are Windows, MacOS, and Linux versions, so you might want to give it a look. See https://en.wikipedia.org/wiki/Avidemux for a better description or download from here: https://github.com/mean00/avidemux2/releases/tag/2.8.0 (or if you want the latest nightly or need an older version then https://www.avidemux.org/nightly/).

Btw i dont see any proprietary building commands in avidemux source: https://github.com/mean00/avidemux2/blob/master/bootStrap.bash