[FFmpeg-user] Best way to scale interlaced video
battistel at gmail.com
Fri Mar 7 12:29:44 CET 2014
2014-03-06 21:15 GMT+01:00 Andy Furniss <adf.lists at gmail.com>:
> Mark Himsley wrote:
> Now, I need to tell you that your scale from 1920x1080 down to
>> 720x432 will leave your video about 2.5% wrong in aspect-ratio.
>> That's because you've fallen in to the usual trap of thinking that
>> the 720 pixels across an SD frame are all the "active" pixels. You
>> should actually be scaling the video down to 702x432 and padding that
>> all back up to 720x576 to preserve the aspect ratio correctly.
>> Unfortunately, padding 9 pixels each side of the 702 up to 720 is not
>> possible with sub-sampled chroma, so in the BBC we have calculated
>> that we get much more accurate scaling if we pad the source video up
>> and then scale down rather than scaling down and padding up.
>> Therefore I suggest this is the correct scale (within 0.06%) for HD
>> down to 4:3 SD.
thank you very much for valuable informations!
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