[x264-devel] Re: [patch] VUI extended corrected

[Christian Heine]

Hi,

first of all, thanks for your input.

Loren Merritt wrote:
 > [...]
> under "Sample Aspect Ratio":
> You've already said that it may be called "Pixel Aspect Ratio", so don't 
> then use PAR to mean something else. Change "Picture Aspect Ratio" to 
> "Display Aspect Ratio".
Display Aspect Ratio may be misinterpreted as the aspect ratio of the 
displaying device which is a whole other story. I changed it to Image 
Aspect Ratio (IAR).

> under "Overscan":
> I'd put a stronger statement against using it. Overscan is useful only 
> if you're encoding for a device that can't display the whole frame.
Or if the image has a different IAR because of the overscan. (see below)

> If the decoder/hardware is capable of displaying the whole frame, 
> cropping off the borders is always better than leaving them black.
I totaly agree. However it is not always possible. PAL with 702 active 
pels is fine (crop to 704 pels), but NTSC has an active line of 711 
pels. In this case cropping won't work, because width should be mod16.

> SAR has nothing to do with overscan, unless you mean crop and scale to 
> mod16, in which case you should say so. [...]
Overscan has to do with IAR, in a sence that an image is always shown 
with a certain IAR, regardless if it has overscan cropped or not. That 
means that an image that has a cropped overscan has a different IAR 
before cropping. Using the formulas in Section 1 this also results in 
different SARs. That's the theory. In practice: I have countless DVDs 
where the SAR for overscan cropped/ not cropped differ. H264 spec also 
creates the same link between SAR and overscan. See Table E-1 in Annex E.

However I removed some of the nasty details in vui.txt, as not to scare 
off the users with this topic.

Regards,
Christian Heine

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