[Ffmpeg-devel] DAR and SAR FFmpeg option confusion

Roman Shaposhnik rvs
Tue Mar 13 19:55:40 CET 2007


Hi,

On Tue, 2007-03-13 at 18:36 +0000, Robert Swain wrote:
> >    Which numbers and why do you think they are inconsistent ? Once
> > again, please go read: http://www.mir.com/DMG/aspect.html and if 
> > you find yourself in disagreement with what's written there then we
> > can talk about it meaningfully. The numbers Robert and I quoted are
> > very consistent with what's written as part of the resource mentioned
> > above.
> 
> Not to put words in his mouth but I think what M?ns is getting at is that the 
> DAR should be 15:11, not 4:3 as they claim. The video stream is 720x480 and not 
> 704x480. If you have to set a DAR of 4:3 in the stream but the DAR should 
> actually be 15:11, this is the lie/hack.
> 
> Anything using DV has to be aware of this nuance of the format if it analyses 
> the stream, finds a DAR of 4:3 and frame dimensions of 720x480, the only way 
> it's going to display the stream as intended is with a workaround for DV.

  I don't think its just DV, but that aside -- it seems that we've got
three ways of solving this issue:
  
    * as you and M?ns pointed out admitting that the DAR is 15:11 will
      do the trick. I guess it'll mean lots of FAQing, but that's ok

    * somehow make libavcodec aware that there's a smaller DISPLAY area
      embedded inside the coded frame. And for that area the SAR is 
      (obviously!) the same, but the DAR is what everybody expects it
      to be

    * Don't futz with DAR at all, but use SAR instead. The SAR has an
      additional benefit of not depending on coded vs. displayed 
      geometry and lets everybody calculate DARs for the areas they
      really need. 

  Now, let me take a step back and mention that this very discussion has
started with your post asking how one can set SAR explicitly, to which
Michael replied that the propper way of doing it is specifying geometry
and DAR.

  Now, given the amount of confusion it takes to figure out what the
real DAR of the DV (and couple of other formats) frame is -- it seems
to me that your original question still has merit. We might just need
that way of specifying SAR explicitly after all. Don't you think ? 

Thanks,
Roman.





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