[Matroska-devel] New Matroska field: chroma range/pixel format

Joseph Ashwood ashwood at msn.com
Thu Sep 24 00:03:56 CEST 2015

> Date: Wed, 23 Sep 2015 21:27:16 +0200
> From: nfxjfg at googlemail.com
> Subject: Re: [Matroska-devel] New Matroska field: chroma range/pixel format

> On Wed, 23 Sep 2015 18:00:58 +0200
> Steve Lhomme <slhomme at matroska.org> wrote:

> > We currently have a ColourSpace field 
> > When storing raw video, the pixels are stored in a certain way and
> > decoding the pixels need some conversion from this encoding to
> > whatever your screen is using. Right now we don't have all the fields
> > needed to describe these raw data.

Actually I think it is of significant value beyond just raw video.
Certain codecs only support certain color spaces. I see no reason that Matroska cannot store a field that labels the correct color space/gamut/etc. for the output. The fields will need to be very flexible because the list of color spaces/gamuts just keeps growing. We are now seeing even near consumer cameras that support enough variability for tens of thousands of different space and gamut combinations. As an example the Sony a7s mark 2 has one hundred different combinations just for the standard gamuts, with the ability to configure more. Being able to store those accurately, without any space/gamut conversion will lead to better picture. 
> > It may also be useful to tell easily if a H264 or VP9 video is in 8> > bits per pixels, 10 or 12 bits, without having to inspect codec
> > private data or even Blocks.
> > 
> > I think a field for Chroma Subsampling (I420, I422, Y410, etc) and one
> > for Chroma Range (BT.608, BT.709, BT.2020) should be sufficient. The
> > Chroma Subsampling generally described how each "plane" is layed out
> > in the raw data.
> That kind of redundant info just seems to make everything harder than
> it is...

Having a way to allow a codec to store a different color space, like DCI-P3, or even something like {Sony A7S2, Color:ITU709, Gamma:Movie} becomes important in using Matroska professionally.
To be perfectly clear, such detail should be optional. I don't expect the consumer playback devices to have access to the endless list of color spaces and gamuts that are in cameras today. It is professional work that needs that information. For consumers simply knowing sRGB, 709, or 2020, ACES is more than enough information.

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