Here's the command that'd add pillar- or letterboxing for a fixed output width. It's a tad long, but you'll have to specify the padding some way.
First, in your shell define output width and height:
width=700
height=400
Then run the command:
ffmpeg -i in.mp4 -filter:v "scale=iw*min($width/iw\,$height/ih):ih*min($width/iw\,$height/ih), pad=$width:$height:($width-iw*min($width/iw\,$height/ih))/2:($height-ih*min($width/iw\,$height/ih))/2" out.mp4
This is stripped down to the bare essentials needed to resize and pad—add your other video and audio options as you see fit. Note that the numbers for width and height have to be divisible by 2 in order to work for most codecs.
Here's the explanation of what's going on:
- Scaling:
- First we need to figure out whether to scale by width or height.
- To do this, we divide the output width by the input width, and output height by input height. This will give us the scale factors for each dimension.
- We then check which one is lower (with
min()) and choose only that factor for resizing. - Finally, we multiply both input width and height by that factor (
iw*min(…):ih*min(…)). - Padding:
$width:$heightis the output width and height- To figure out where to place the resulting video, we need to subtract the scaled width from the maximum output width, and the scaled height from the maximum output height.
- The scaled widths and heights are the expressions from the
scalefilter. - We divide the resulting offset by 2 to add borders at both sides.