[FFmpeg-user] Quicktime reports diiferent width than ffmpeg

[Dave Bevan]

>> >Anyone know why QuickTime would report a different width for an .mov
>> than ffmpeg does?
>> 
>> Check aspect ratio flags (DAR/PAR etc). If I remember correctly, QT reports,
> for example, 1024x576 for anamorphic 720x576.
>Stream #0.0(eng): Video: svq3, yuvj420p, 648x486, 1577 kb/s, SAR 43185:32768
>DAR 14395:8192, 29.91 fps, 29.95 tbr, 29954 tbn, 29954 tbc
>the wxh shows ~4:3 SAR is also ~4:3 yet DAR is ~16:9 
>the actual video plays at ~16:9 or 854x486
>the only dimension reported here evenly div by 4 is 648 which makes me think
>ffmpeg isn't so much reading the dimensions as it is calculating them?
>Still learning

So your DAR (display aspect ratio) is 14395:8192. Take that with your HEIGHT, and you get:

(14395 / 8192) * 486 = 854.000244....

So, it's working exactly as expected, with QT doing exactly what's asked of it - to take 648x486 video and display using the DAR defined in the wrapper, giving you 854x486.

Basically, video is stored in square pixels - in your case 648x486 - so 314928 pixels per frame. Pixels can have DAR applied and it's down to display devices to STRETCH the original video WIDTH at the DAR ratio to give the final output.

In European TV land, all [digital] video of Standard Definition is transported down video pipes that are 720x576 in size. Further, there is padding applied which actually means the "active picture" is contained in 702x576 pixels, so 9px padding on left/right. When WIDESCREEN was introduced, none of the physical equipment changed. Simplistically, all that occurred was flagging of video with a 16:9 flag. It meant that when a camera captured video, it took in a "viewport" that was 16:9 aspect ratio and scaled/squashed the horizontal pixels seen into the 702x576 box. Then the video travelled around all it's pipes, vision mixers etc, eventually to TV transmitters into your house and into your TV, all squashed into the 702x576 box. A side-band flag tells your TV to stretch the video picture back to be shown to you as widescreen. That "display" 4:3, 16:9 button on your TV remote is simply overriding the signal from the TV station saying the picture should be shown in widescreen, vs the old 4:3 aspect ratio.

Many people thought that the introduction of 16:9 meant better pixels, but in fact it means less quality, because the width of what you see is being stretched more, rather than less.

In HD, things are different of course - the signal [usually] transported to your TV is 1920x1080, though it can be 1440x1080, 1280x720, or even 960x720.

--Dave.



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