[x264-devel] I444 support

[Loïc Le Loarer]

Le mardi 27 novembre 2007 à 12:39:39 +0100, Gregor Riepl a écrit:
> hello list
> 
> i'm working on a system to archive old media digitally, and i want to  
> do that with as little data loss as possible.
> 
> as h.264 offers very good compression ratios and can operate in a no- 
> quantisation mode (qp=0), i want to use that, if possible.
> but there is another bottleneck besides quantisation: the colorspace  
> compression.
> luckily, the h.264 specification describes support for 4:4:4 (no  
> chroma subsampling) in the fidelity range extensions - unfortunately  
> x264 does not (yet) support this.

Well, in fact, the 4:4:4 profile of fidelity range extensions (FRExt)
have been removed as obsolet by Amendment 1 (in 06/2006). Fortunately,
a new and better designed 4:4:4 profile has been added again in
Amendment 2 (in 04/2007). You can find the spec here
http://www.itu.int/rec/T-REC-H.264/en, it is freely downloadable using
the 3-FREE offer (lokk at this page:
http://www.itu.int/publications/bookshop/how-to-buy.html).

> in fact, i've been looking for a suitable codec that can do rgb24 or  
> yuv4:4:4 for quite some time.

I'm convinced that this lastest 4:4:4 profile of H.264 is good for RGB
or YUV4:4:4.

> how hard would it be to add this missing feature? does the current  
> implementation rely on a subsampled colorspace? and where should i  
> start if i want to implement it myself?

The new 4:4:4 profile has a mode which uses 3 monochrome planes, one of
each color component (either RGB or YUV), encoded independently, to
support 4:4:4. It shouldn't be really difficult in add support for this
mode in x264 (or any other encoder) as it is the matter to first add
monochrome support (remove chroma compression) and them consider each
picture as 3 monochrome planes instead of only one.

Best regards.
-- 
Loïc

"heaven is not a place, it's a feeling"
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