[MPlayer-users] Transcoding from one aspect ratio to another

[Krzysztof Duchnowski]

On 03.11.2010 17:55, JD wrote:

>>> I have a video file that has an aspect ratio of
>>> 2.353 : 1
>>> I would like to transcode it to an aspect ratio
>>> of 1.6 : 1 (like 1280x800).
>>> Is this even possible?
>>>
>> I assume you do not want to skew the picture, just change the 
>> resolution? If so, just -vf scale= will do that for you and figure out 
>> the proper aspect ratio itself.
>>
> Yes - you assume correctly. Thank you Rolf. I will try it.

First of all, dscale is better - you give max resolution and it is
rescaled properly, but virtually. After that you mut scale it with scale
but no guessing of resolution.
I had that problem with script that need to scale DAR(1.33:1)/SAR(1:1),
DAR(1.77:1)/SAR(1:1) and DAR(1.33:1)/SAR(1:1.33) (anamorphic, displayed
DAR=1.77) videos to 1.77:1 screen (MP4 for Android phone) without hand
made optimization every time - example for you:

-vf-add dscale=1280:800:0,scale=0:0,expand=1280:800

This is for maitaining SAR. If you don't want to use bitrate for
encoding black border then it is enough to expand to power of 16:

-vf-add dscale=1280:800:0,scale=0:0,expand=::::::16

If you change DAR of video wihtout maitaining SAR then it will squeze it
verticaly or horizontaly (depend of source material).

So, when changing DAR 2.352:1 video (1920x816) to DAR 1.6:1 video
(1280x800) you must either crop it or expand with black borders to
maintain SAR/PAR (pixel aspect ratio - square pixel, no squeze).

-- 
Krzysztof 'DK75' Duchnowski
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