How can I fit a video to a certain size, but don't upscale it with FFmpeg?

Question

I need to fit videos to 640x360 (the maximum my phone's player can handle), while also preserving aspect ratio, but I also want the video to be unchanged if it is smaller than 640x360 (no point in up-scaling it after all).

Is there a way to get this behavior using ffmpeg's command line?

Answer 1

First, define the width, height and aspect ratio of your output. This will save us some typing.

width=640; height=360
aspect=$( bc <<< "scale=3; $width / $height") # <= floating point division

Now, let's apply the super complex filter command that Jim Worrall wrote:

ffmpeg -i input.mp4 -vf "scale = min(1\,gt(iw\,$width)+gt(ih\,$height)) * (gte(a\,$aspect)*$width + \
lt(a\,$aspect)*(($height*iw)/ih)) + not(min(1\,gt(iw\,$width)+gt(ih\,$height)))*iw : \
min(1\,gt(iw\,$width)+gt(ih\,$height)) * (lte(a\,$aspect)*$height + \
gt(a\,$aspect)*(($width*ih)/iw)) + not(min(1\,gt(iw\,$width)+gt(ih\,$height)))*ih" \
output.mp4

I won't really go into explaining what this all does, but basically you can feed it any video, and it will only downscale, not upscale. If you're up for it you can dissect the filter into its individual expressions. It might be possible to shorten this, but it works like that as well.