[FFmpeg-user] Automatically scaling videos and adding padding

[Stefano Sabatini]

On date Sunday 2011-06-12 13:53:28 +0200, Francesco Turco encoded:
> Hello,
> 
> I need to convert some random videos to MPEG files with a 720x576
> resolution in order for my TV decoder to play them. That is, I usually
> need to scale movies and add padding because source files are almost
> never 720x576 already. For now, I use the following command:
> 
> ffmpeg -i input.avi -b 1536k -ab 192k -vf
> scale='gte(iw/ih\,720/576)*720+lt(iw/ih\,720/576)*((576*iw)/ih):lte(iw/ih\,720/576)*576+gt(iw/ih\,720/576)*((720*ih)/iw)',pad='720:576:(720-gte(iw/ih\,720/576)*720-lt(iw/ih\,720/576)*((576*iw)/ih))/2:(576-lte(iw/ih\,720/576)*576-gt(iw/ih\,720/576)*((720*ih)/iw))/2:black'
> -acodec mp2 -vcodec mpeg2video -f mpeg -r 25 -ar 48k output.mpeg
> 
> As you can see, the main problem is the -vf option, which is by far too
> complex. In plain English, it does the following things:
> 
> - It automatically scales the source video in order for it to be put
> inside a 720x576 rectangle without altering its original aspect ratio
> (at this point the video is less than or equal to 720 pixels in width
> and less than or equal to 576 pixels in height)
> - It automatically adds padding around the scaled video, either at
> top/bottom or at left/right, because the output file should be 720x576
> exactly
> 
> The basic idea is that iw/ih is the aspect ratio of the source file,
> which is compared to 720/576, the desired aspect ratio. There are three
> possibilities: iw/ih is less than, equal to or greater than 720/576.
> Each case is treated separately, both when scaling the video and when
> adding padding.
> 
> My question is: there is some other way to accomplish this? A better
> formula perhaps?

gte(iw/ih\,720/576)*720+lt(iw/ih\,720/576)*((576*iw)/ih)

iw/ih = a
gte(a\,720/576)*720+lt(a\,720/576)*(576*a)

this could be better expressed as:
if(gte(a,720/576)\, 720\, 576*a)

unfortunately we don't support if(cond,expr1,expr2), patches are welcome ;-).
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