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

[Loren Merritt]

On Wed, 28 Sep 2005, Christian Heine wrote:
> Loren Merritt wrote:
>> OK, but when/if I add non-mod16 support, it will be better to crop. Then 
>> you're telling the codec exactly which pixels won't be displayed, rather 
>> than just say that some pixels might not be displayed.
>
> Really? Even if you can specify a cropping rectange, you can't encode 
> something smaller than a macroblock with h264. You can clear the overscan 
> area, but this won't solve the problmen.

A cropping rectangle is exactly the same as overscan, except that it's 
normative: the decoder is required to crop off the cropping rectangle, 
whereas overscan is just a suggestion.
The encoder is free to fill the invisible region of the edge macroblocks 
with exactly the same data as they would have contained with overscan... 
or it could chose to put in any other data that happens to compress 
better.

> Besides it is quite possible that other standards (like sucessors of the 
> DVD-Video format) use h264 as video coding method, and make aditional 
> constraints on the resolution, making cropping imposible. (like DVD-Video 
> standard does to MPEG2)

OK

>> Sure, the source's SAR depends on whether the source was intended to have 
>> overscan cropped off before scaling to the source's DAR.
>> But that's a property of the source that you can _determine_, not something 
>> you can _decide_ as part of the encoding process. Once you know what the 
>> source's SAR is, the only way to change it is by scaling.
>
> The new version of vui.txt makes this more clear. The encoder doesn't do 
> anything with the overscan or to the SAR. It just passes the information to 
> the bitstream, for later use by the playback equipment. Since playback 
> equipment mostly ignores the information if there is an overscan or not, I 
> proposed an alternative that says: "set overscan off and use a different 
> SAR", which will at least result in a correct SAR/IAR, since playback 
> equpiment generally respects SAR information.

The "use a different SAR" part is still wrong.
No matter how much cropping the encoder or playback does or does not do, 
SAR doesn't change.

--Loren Merritt

-- 
This is the x264-devel mailing-list
To unsubscribe, go to: http://developers.videolan.org/lists.html