This all also works for images

Getting information about the video (resolution, fps etc.): ffprobe file.mp4

Basic usage (more work needed for sound): ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4

If the files are of different heights: ffmpeg -i In.mp4 -vf scale=-1:<height> Out.mp4

If the resulting width is not an even number: ffmpeg -i Out.mp4 -vf scale=iw-1:ih Out.mp4

OR

change the -1 to -2 in the earlier command

~ffmpeg -i <InFile> -pix_fmt yuv420p <OutFile>

ffmpeg -framerate <framerate> -i /path/to/%04d.png -r <framerate> -pix_fmt yuv420p Out.mp4 Replace -4 with however many leading zeros. Can also use e.g. m%06d.png for e.g. m000123.png from MuMax3.

If you want to specify a background colour (e.g. white), add e.g. -filter_complex "[0]split=2[bg][fg];[bg]drawbox=c=white@1:replace=1:t=fill[bg]; [bg][fg]overlay=format=auto"

If this does not work (e.g. gives segmentation fault), can use ImageMagick to convert the images. E.g. make a directory called "WhiteBG", then for i in *.png; do echo $i; convert $i -background white -alpha remove WhiteBG/$i; done (where of course the echo $i is just to track the progress).

ffmpeg -i <Input> -filter:v "setpts=<Factor>*PTS" <OutName> where a smaller factor results in a higher speedup, i.e. Factor=0.1 results in the video being 0.1* the original length