[FFmpeg-user] Automatic Aspect Ratio Conversion

Dan Flett dflett at bigpond.net.au
Tue Jul 26 14:56:21 CEST 2011


-----Original Message-----
Behalf Of Stefano Sabatini
Subject: Re: [FFmpeg-user] Automatic Aspect Ratio Conversion

On date Tuesday 2011-07-26 14:51:39 +1000, Dan Flett encoded:
> However I found that if the input video is anamorphic (i.e. non-square 
> pixel), the output is also anamorphic, and the DAR / SAR information 
> is lost.

Yes this is a bug which needs to be fixed.
Currently a=in_w/in_h, which is not what the docs say.

Possible solutions:
1. keep the current semantics for a, and fix the docs 2. change semantics
for a to match the docs

I can't say which of 1. or 2. is better, they both have problems, what do
you suggest to do?

And of course we'll have to add the sar/dar variables.

Under the "Pad" heading, it does list:
`dar, a'
    input display aspect ratio, same as iw / ih 
Did someone just change this? :)

Perhaps, to be consistent with the ffmpeg/ffprobe output, sar should be par
(or par and sar be the same).

I've noticed that when using "scale", using -1 for width or height maintains
the display aspect ratio, but throws away the sar, leading to possibly
anamorphic output.  

I find that in general, I like to start with the "scale=iw*sar:ih" to get
everything as square pixels, then do my transformations as square pixels,
then scale it to anamorphic at the end if I need to (and if it's IMX-D10,
pad 32 lines to the top after that).

I also can't find much in the way of documentation for the functions such as
"max()" - max() itself is fairly obvious, but I'd like to know what other
functions I can use, and how they work.  I checked out the libavfilter code
and couldn't find them there either.

More information about the ffmpeg-user mailing list