[Ffmpeg-devel] Sample Aspect Ratio & Pan&Scan

[Nico Sabbi]

Steve Lhomme wrote:

> Hi,
>
> So far in DrFFMPEG we used a modified version of mpeg12.c that doesn't 
> take the pan_scan size to set the sample_aspect_ratio. (see around 
> line 2142) But since FFMPEG doesn't seem to have problems to deal with 
> it I think we may do something wrong...
>
> Because of DivX certification constraints we only output square 
> pixels. So we have to translate the source aspect ratio to square pixels.
>
> So far we assumed that the sample_aspect_ratio was a pixel aspect 
> ratio and it worked well for most format. But it seems that it's not 
> good enough in the general case.
>
> For example I have 2 VOB files. They both use pan & scan and have a 
> pixel aspect ratio of 16:9. But for the first one (chems1.vob) we get 
> a sample aspect ratio of 256:135 (1.896) and the other one has a 
> sample aspect ratio of 32/27 (1.18). For chems1.vob the original pixel 
> size is 720x576 and for the other one it's 720x480. While the source 
> dimensions and pixel aspect ratio are similar, the sample aspect ratio 
> is very different. That's because the pan & scan dimensions are very 
> different (for chems1.vob the width is bigger than the height).
>
> Our hack so far was not to take the pan & scan in account and it 
> worked well. So is there any reason we should use the pan&scan value ? 
> From what I understand the pan&scan feature is made to crop the 
> original picture ? On demand ? Maybe it should be optional in FFMPEG ? 
> It seems that encoding with FFMPEG works because both the (pixel) 
> aspect_ratio_info is stored and the sample_aspect_ratio. But the 
> aspect_ratio_info field is MpegEncContext specific... Should it be 
> more general ?
>
> Steve


it's something that left me some doubts, too.
If pan&scan is supposed to work with the intervention of the player (by 
means of cropping)
shouldn't lavf export both the original par(without keeping into account 
the pan&scan) in
AVCodecContext->sample_aspect_ratio and {width, height, pan_scan_par} in 
AVPanScan?