[Matroska-devel] New Matroska field: HDR

Joseph Ashwood ashwood at msn.com
Thu Sep 24 00:38:48 CEST 2015

> Date: Wed, 23 Sep 2015 17:50:56 +0200
> Subject: [Matroska-devel] New Matroska field: HDR
> For those following the news in television sets, the new thing this
> year is HDR (High Dynamic Range). When coupled with 4K videos it gives
> a bigger range of light to make more realistic pictures.

>From the inside it is looking to me like those will all collapse, much like 3D did. What we are seeing instead moving forward is that REC-2020 will be the dominant force in HDR for at least a few years, being displaced by ACES.  The other color space/gamut combination vying for the position is CIE XYZ. 
2020 is already supported, and available in both H.265 and VP9. We are a few years from 100% 2020 coverage.
CIE XYZ matters already in movies, but nothing comes even close to covering the space. This is the space that maps to the human eye, attempting just about anything in it today results in learning new curse words to yell at your system.
ACES is to me the leading option for HDR. There are no widely available codecs that handle ACES. ACES uses 16-bit floating point colors from -65504 to +65504, with {1,1,1} being white and {0,0,0} being black. We have a long road to support ACES at even movie theaters, new codecs need to be developed.
I don't see where adding a new side channel for HDR will help. 
So that's how they are stored during editing.
During recording there are really two methods.
Red/Sony are going down the single sample path. Each sense point is measured with enormous dynamic range (16-bit fixed point), and with gamut curves that allow for pushing the image around a lot.
Arri/Canon are doing double sampling. Each sense point connects to two different ADCs, one for high range, the other for low range. Today these two channels are combined in camera. I do expect that as storage becomes even cheaper that the different channels will be available to the color correction suite.
In all cases though the intermediary content is stored in a different color space, today DCI-P3 on the high end, tomorrow REC-2020, and we are seeing ACES start to make an impact.
If we simply support the high dynamic range spaces and gamuts then whether to send it to the display as 1 stream of 50 becomes an implementation detail that can freely change between devices.
For delivery, historically Matroska's strong suit, REC-2020 is all that needs support.
For professional usage, I actually do see where there is plenty of opportunity to make use of a high range/low range image split. This is a temporary solution. This would double the video channels, doubling the data rate. A high range channel and a low range channel, allowing for storing the actual raw data that should come out of the ARRI and Canon devices within a few years. Provide default max/min points for each channel. 		 	   		  
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