RE: High Dynamic Range imaging and ICC colour management

Hi Joe,

I guess this may be unfamiliarity on my part and I apologise if this is obvious.

If you create an image and tell me only the peak luminance, how should I display such an image? In the general ‘broadcast model’ I was presenting, there are two independent elements: an image or content creator and a display controller. The content creator produces content intended to be displayed on any conforming display and so knows nothing about the actual display’s peak white or the level that the display controller has chosen to display ‘diffuse white’.

It seems that at least for cases where the display has a lower peak luminance than the content, some mapping must be done to present the image on the display without significant loss of quality. Traditionally this has been achieved by applying tone compression to values greater than diffuse white and to do this effectively, it seems that the display controller needs to know both. Similar issues arise for displays with power-limiting circuits where the metadata associated with some (most?) PQ systems can be used by the display controller to present the image optimally on the display.

How about another (slightly contrived) case where a content creator wishes to make two images of a scene which has a small, very bright LED. On the first day, the LED is off and the second day it is on. Let’s assume that the LED does not change the average picture light level significantly. Let’s also assume that on the first day, the creator does not know about the LED. How should the content creator select the peak luminance for both cases to ensure that the display does the right thing?

What am I missing?

Best regards,
_Craig

From: Joe Drago <jdrago@netflix.com>
Sent: 05 March 2019 15:59
To: Craig Revie <Craig.Revie@ffei.co.uk>
Cc: Mark Watson <watsonm@netflix.com>; public-colorweb@w3.org
Subject: Re: High Dynamic Range imaging and ICC colour management

It depends on your goals, and who your "masters" are.

Top-of-head, I see a handful of possibilities for these luminance values:

* None specified at all. In this case, we implicitly consider peak white == diffuse white == our global diffuse white. This is how we treat all SDR content currently.

* Only peak white specified. This is when your master is "creative intent", and if the author of the image wants the white in that image to be really drab or really fierce, "I said what I said", and you must honor it. In this case, diffuse white be damned, try your best to honor the exact luminance offered in the file.

* Both diffuse and peak white are implicit in the transfer function (such as HLG). This buys you the scene-referred environment I believe you're hinting at.

* A diffuse white and either a unorm scale factor or "value of diffuse white" (between [0-1]) are provided (which is different than offering peak white). The latter would be a way to simulate overranging, as all of the file formats Netflix uses are internally unorm. Either way, diffuse white becomes the master here.

Perhaps to your point, the value of diffuse white should be consistent, and it is really only known at runtime. For example, MS Windows offers a slider which lets you pick how bright diffuse white is in their display settings. That said, if the image author wants authoritative control over how their pixels are rendered (diffuse white be damned) and wants to preserve perfect creative intent, the only value necessary is what their image's range of unorm values represent in luminance (peak white). On the other hand, if the author wants to guarantee that their diffuse whites look correct and are comfortable with *all other pixel values in their image* changing in luminance when someone manipulates diffuse white's value (such as an enduser tweaking that slider), that can be offered as well. This type of metadata would make sense (say) for sprite atlases full of UI affordances.

Right now, Netflix's pipeline and engines support the first two bullets, and I could imagine us wanting at least one of the other two for specific UI reasons, but right now the UI is happy with bullet #1 for those types, and the creatives are happy with #2 because they can dial in exactly whatever pixels they want.

As a final aside, one other possible abuse for the lumi tag would be to instead leverage the X and Z values (which are required in the spec to be 0) instead, and legislate one of the above bulleted scenarios (amongst others I haven't thought of) depending on which are nonzero. This might be a safer way to maintain backwards compatibility.


On Tue, Mar 5, 2019 at 7:18 AM Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>> wrote:
Thanks. Probably for interoperability both peak and diffuse white should be included?

From: Joe Drago <jdrago@netflix.com<mailto:jdrago@netflix.com>>
Sent: 05 March 2019 15:16
To: Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>>
Cc: Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>>; public-colorweb@w3.org<mailto:public-colorweb@w3.org>
Subject: Re: High Dynamic Range imaging and ICC colour management

Peak white; Diffuse white is chosen globally at app startup based on a handful of factors, such as output mode (HDR10, DVLL, etc).


On Tue, Mar 5, 2019, 6:43 AM Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>> wrote:
Hi Joe,

Thanks for your reply and a good overview of the work being done at Netflix. It is not my intent to offer any solutions at this stage in the discussion, but it seems that you are in a similar place to others working in this area, some of whom are members of this group, in that you have adopted ICC and (for perfectly good reason) given it a proprietary flavour. It seems to me that there may be scope for a suitable group of stakeholders in this area to work with the ICC to develop an open framework providing interoperability.

As well as Netflix, I am aware of work in this area by a number of other companies and it would seem that working together as a group may produce a much better solution for all concerned.

One question about your use of the luminance tag. Are you using this for peak white or diffuse white? Either way, it seems that at least for PQ image encoding both values are needed.

Best regards,
_Craig

From: Joe Drago <jdrago@netflix.com<mailto:jdrago@netflix.com>>
Sent: 05 March 2019 00:38
To: Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>>
Cc: Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>>; public-colorweb@w3.org<mailto:public-colorweb@w3.org>
Subject: Re: High Dynamic Range imaging and ICC colour management

Hello Craig (and all other colorweb folks) --

In the meantime, please let me know if this is something you would like to work on or if you are already aware of other solutions or proposed solutions to this problem.

I figured I'd chime in here as I was responsible for the implementation of HDR image packaging here at Netflix, and my approach leverages (if not abuses!) ICC profiles. Here are some jump-off points which I'll reference in my explanation:

https://medium.com/netflix-techblog/enhancing-the-netflix-ui-experience-with-hdr-1e7506ad3e8

https://joedrago.github.io/colorist/


Just as a clarification for these links: The first link is our Netflix Blog post where we discuss the strategy we employed, and the second link is the open source tool (colorist) I created to implement that strategy. Seeing as you're all experts in ICC and I'm squarely in the newcomer group, I'll spare any explanations on ICC's guts and skip to my approach to the situation and various learnings/conclusions I came to while tackling this problem. I'll recap my (ab)use of the ICC profile from the colorist site's Overview block in this email below.

My mandate here at Netflix was to come up with a way to pack an HDR image coming from a source video that was HDR10 (BT.2020 PQ, 10000 nits display-referred) into any file format I deemed capable, and then see where we could leverage preexisting standards as much as possible. Seeing as both our source data and our output sink both operated in absolute luminance (display-referred with a range of [0-10000] nits), I based my solution around that.

I learned early on that ICC profiles could perfectly describe our color primaries, and after being disappointed that there wasn't a parametricCurveType for PQ, I realized if I was willing to use more bits per channel (16 instead of 10), that such a severe curve as PQ wasn't necessary for the transfer function, and I could instead stick with a more friendly curve that maintains enduser blending expectations (BT.1886 or even just a plain 2.2 gamma). This solved the insufficient precision in the curv LUTs cited in your slide deck, as (for example) a Type1 'para' with a value of 2.2 is quite friendly and packs nice and tiny for image file embedding. However, to fully realize a display-referred image with a standard ICC profile, I needed one more piece of information: the absolute luminance range.

I noticed that ICC profiles have had a 'lumi' tag, but despite it being populated in some very common ICC profile chunks out there (including on color.org<http://color.org>), it was merely informative and modern image editing software / CMMs appeared to ignore the value completely. However, if I were to honor & leverage this value during image creation and conversion, I could maintain a proper display-referred image file, which I could then convert on the fly when rendering on game consoles, and thus maintain creative intent exactly.

In summary: I'm storing the "correct" value in the lumi tag based on the spec, but I'm actually *using* it to derive a luminance scale when converting and rendering, thus making the ICC profile display-referred.

To be clear, I'm not trying to make the case that this is the one-true-way forward for HDR or anything; I recognize the conveniences that HLG provides and that using absolute luminance signs up various decoders for luminance scaling and tonemapping to interpret the image in the first place. That said, I do think that absolute luminance in image files is the most accurate way to maintain creative intent, and for any BMFF-derived image file (HEIF, MIAF, AVIF) using an 'nclx' type colr box, they already offer a means to specify a PQ transfer_characteristics value which implies absolute luminance anyway, so this is something we need to honor in the future regardless of my ICC lumi tag abuse.

As a wish list for ICC profiles in this very specific, display-referred image file world, here's some things I wish I had:

* I wish there existed a tag (such as the lumi tag) which indicated that the associated pixel data was display-referred, and provided a real luminance range that must be honored. While I'm using the lumi tag currently, there are 30 years of files with embedded ICC profiles that may or may not contain "interesting" lumi tag values, so ideally it'd be a new tag type, or perhaps co-opting a lesser-used tag (viewingConditionsTag?).

* A new official parametricCurveType type value for PQ, so CMMs could do proper precision PQ math without making ICC profile chunks really large.

My disclaimer for the second wish there is that I only think PQ is necessary for HDR when at 10 and 12 bpc. Once you have 16bpc, I think you can get away with murder and just use a simple 2.2 gamma; everything looks fantastic and trivially blends in the 'incorrect' way people are used to. Our final deliverables in production for our HDR images ended up being .JP2 files encoded with 16bpc BT2020 2.2 gamma, display-referred at [0-10000] nits. I still think a PQ para type should exist.

Anyway, this is where I landed on packaging HDR images. You can see my full implementation for conversion and reporting in the colorist repo linked above, including my (new and incomplete) method for detecting special ICC profile combinations that can be repackaged as compliant and ICC-free BT.2100 nclx colr boxes in AVIF files. I'm confident that I'm maintaining the fidelity of these stored images and rendering the correctly, and I'm also confident that I'm ever-so-slightly abusing ICC profiles to achieve it.




On Mon, Mar 4, 2019 at 2:04 PM Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:
Hi Craig,

This is very interesting and something we have been looking at in the context of applications running on Games Consoles and Set Top Boxes where the output is locked to HDR (when the TV supports), to avoid the problems associated with HDR / SDR switching on the HDMI link.

I'd like to introduce my colleague Joe Drago (who joined this list after your post, which is the main reason for this message). He has some comments / questions.

...Mark

On Mon, Mar 4, 2019 at 6:07 AM Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>> wrote:
I have been asked to post a presentation that I made in last month’s ICC meeting to this group. The links below are to my presentation and a recording of the meeting including discussion.
http://www.color.org/groups/displays/20190215_High_Dynamic_Range_imaging_and_Hybrid_Log-Gamma.pdf

http://www.color.org/groups/displays/High_Dynamic_Range_Imaging_and_ICC.mp4


This presentation was the result of discussion with several experts from different areas, some of whom are members of this group. The problem that we are trying to address is that of effective presentation of High Dynamic Range (HDR) and Standard Dynamic Range (SDR) images or video sequences on the same display. This is particularly relevant where the content creator is not able to control the way in which the content is rendered to the display or to broadcast content appropriate to a specific display.

An effective solution to this problem requires an architecture similar to that of the ICC where an input transform maps colour content to a well-defined ‘connection space’ and an output transform maps from this connection space to a device (usually a display or printer). Although the presentation is from an ICC perspective, other models could be used.

This is a very basic outline of one possible solution to this problem and at this stage I am interested to identify others (from this group and elsewhere) who would be interested in working further on this topic. The response from the ICC was very positive and I believe that the ICC would be willing to host a forum for this discussion, especially to explore whether the current ICC model (with minor extensions) would be useful.

I would be happy to present these ideas in more detail if anyone is interested.

In the meantime, please let me know if this is something you would like to work on or if you are already aware of other solutions or proposed solutions to this problem.

Best regards,
Craig Revie
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