20 Nov 2020

Developing Wayland Color Management and High Dynamic Range

(This post was first published with Collabora on Nov 19, 2020.)

Wayland (the protocol and architecture) is still lacking proper consideration for color management. Wayland also lacks support for high dynamic range (HDR) imagery which has been around in movie and broadcasting industry for a while now (e.g. Netflix HDR UI).

While there are well established tools and workflows for how to do color management on X11, even X11 has not gained support for HDR. There were plans for it (Alex GoinsDeepColor Visuals), but as far as I know nothing really materialized from them.  Right now, the only way to watch HDR content on a HDR monitor in Linux is to use the DRM KMS API directly, in other words, not use any window system, which means not using any desktop environment. Kodi is one of the very few applications that can do this at all.

This is a story about starting the efforts to fix the situation on Wayland.

History and People

Color management for Wayland has been talked about on and off for many years by dozens of people. To me it was obvious from the start that color management architecture on Wayland must be fundamentally different from X11. I thought the display server must be part of the color management stack instead of an untrusted, unknown entity that must be bypassed and overridden by applications that fight each other for who gets to configure the display. This opinion was wildly controversial and it took a long time to get my point across, but over the years some color management experts started to open up to new ideas and other people joined in the opinion as well.  Whether these new ideas are actually better than the ways of old remains to be seen, though. I think the promise of getting everything and more to work better is far too great to not try it out.

The discussions started several times over the years, but they always dried out mostly without any tangible progress. Color management is a wide, deep and difficult topic, and the required skills, knowledge, interest, and available time did not come together until fairly recently. People did write draft protocol extensions, but I would claim that it was not really until Sebastian Wick started building on top of them that things started moving forward. But one person cannot push such a huge effort alone even for the simple reason that there must be at least one reviewer before anything can be merged upstream. I was very lucky that since summer 2020 I have been able to work on Wayland color management and HDR for improving ChromeOS, letting me support Sebastian's efforts on a daily basis. Vitaly Prosyak joined the effort this year as well, researching how to combine the two seemingly different worlds of ICC and HDR, and how tone-mapping could be implemented.

I must also note the past efforts of Harish Krupo, who submitted a major Weston merge request, but unfortunately at the time reviewers in Weston upstream were not much available. Even before that, there were experiments by Ville Syrjälä. All these are now mostly superseded by the on-going work.

Currently the active people around the topic are me (Collabora), Vitaly Prosyak (AMD), and Naveen Kumar (Intel). Sebastian Wick (unaffilated) is still around as well. None of us is a color management or HDR expert by trade, so we are all learning things as we go.

Design

The foundation for the color management protocol are ICC profile files for describing both output and content color spaces. The aim is for ICCv4, also allowing ICCv2, as these are known and supported well in general. Adding iccMAX support or anything else will be possible any time in the future.

As color management is all about color spaces and gamuts, and high dynamic range (HDR) is also very much about color spaces and gamuts plus extended luminance range, Sebastian and I decided that Wayland color management extension should cater for both from the beginning. Combining traditional color management and HDR is a fairly new thing as far as I know, and I'm not sure we have much prior art to base upon, so this is an interesting research journey as well. There is a lot of prior art on HDR and color management separately, but they tend to have fundamental differences that makes the combination not obvious.

To help us keep focused and explain to the community about what we actually intend with Wayland color management and HDR support, I wrote the section "Wayland Color Management and HDR Design Goals" in color.rst (draft). I very much recommend you to read it so that you get a picture what we (or I, at least) want to aim for.

Elle Stone explains in their article how color management should work on X11. As I wanted to avoid repeating the massive email threads that were had on the wayland-devel mailing list, I wrote the section "Color Pipeline Overview" in color.rst (draft) more or less as a response to her article, trying to explain in what ways Wayland will be different from X11. I think that understanding that section is paramount before anyone makes any comment on our efforts with the Wayland protocol extension.

HDR brings even more reasons to put color space conversions in the display server than just the idea that all applications should be color managed if not explicitly then implicitly.  Most of the desktop applications (well, literally all right now) are using Standard Dynamic Range (SDR).  SDR is a fuzzy concept referring to all traditional, non-HDR image content.  Therefore, your desktop is usually 100% SDR. You run your fancy new HDR monitor in SDR mode, which means it looks just like any old monitor with nothing fancy.  What if you want to watch a HDR video? The monitor won't display HDR in SDR mode.  If you simply switch the monitor to HDR mode, you will be blinded by all the over-bright SDR applications.  Switching monitor modes may also cause flicker and take a bit of time. That would be a pretty bad user experience, right?

A solution is to run your monitor in HDR mode all the time, and have the window system compositor convert all SDR application windows appropriately to the HDR luminance, so that they look normal in spite of the HDR mode. There will always be applications that will never support HDR at all, so the compositor doing the conversion is practically the only way.

For the protocol, we are currently exploring the use of relative luminance.  The reason is that people look at monitors in wildly varying viewing environments, under standard office lighting for example. The environment and personal preferences affect what monitor brightness you want. Also monitors themselves can be wildly different in their capabilities. Most prior art on HDR uses absolute luminance, but absolute luminance has the problem that it assumes a specific viewing environment, usually a dark room, similar to a movie theatre.  If a display server would show a movie with the absolute luminance it was mastered for, in most cases it would be far too dark to see. Whether using relative luminance at the protocol level turns out to be a good idea or not, we shall see.

Development

The Wayland color management and HDR protocol extension proposal is known as wayland/wayland-protocols!14 (MR14). Because it is a very long running merge request (the bar for landing a new protocol into wayland-protocols is high) and there are several people working on it, we started using sub-merge-requests to modify the proposal. You can find the sub-MRs in Sebastian's fork. If you have a change to propose, that is how to do it.

Obviously using sub-MRs also splits the review discussions into multiple places, but in this case I think it is a good thing, because the discussion threads in Gitlab are already massive.

There are several big and small open questions we haven't had the time to tackle yet even among the active group; questions that I feel we should have some tentative answers before asking for wider community comments. There is also no set schedule, so don't hold your breath. This work is likely to take months still before there is a complete tentative protocol, and probably years until these features are available in your favourite Wayland desktop environments.

If you are an expert on the topics of color management or HDR displays and content, you are warmly welcome to join the development.

If you are an interested developer or an end user looking to try out things, sorry, there is nothing really for you yet.

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