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Re: Discuss HDR at CSS WG next week ?

From: Mark Watson <watsonm@netflix.com>
Date: Thu, 15 Sep 2016 10:21:09 -0700
Message-ID: <CAEnTvdBt-H95+q5NvS3kQruA70D_+cT2UwiQ_XTSDxR0QJuWBg@mail.gmail.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: Rossen Atanassov <Rossen.Atanassov@microsoft.com>, "www-style@w3.org" <www-style@w3.org>
On Thu, Sep 15, 2016 at 9:58 AM, Rik Cabanier <cabanier@gmail.com> wrote:

> On Thu, Sep 15, 2016 at 7:35 AM, Mark Watson <watsonm@netflix.com> wrote:
>> > On Sep 14, 2016, at 10:05 PM, Rossen Atanassov <
>> Rossen.Atanassov@microsoft.com> wrote:
>> >
>> >> On Wed, Sep 14, 2016 at 16:53:53, Mark Watson wrote:
>> >>
>> >> I have some use-cases and issues / questions related to High Dynamic
>> Range
>> >> graphics / images and how they could be supported in CSS. I wondered
>> if this
>> >> topic was or could be on the agenda for next week ?
>> >
>> > Are you referring to our TPAC agenda? If so, we don't have the topic
>> scheduled and looking through everything we have it is doubtful we could
>> get to it.
>> >
>> > Could you summarize your proposal? Is this something proposed at WICG
>> already?
>> Yes, TPAC. I've raised the issue of HDR a few times on the CSS list,
>> but with no real response. We had a breakout at TPAC last year, but no
>> one from CSS attended. I imagine that is because people do not yet
>> have real hardware / platform APIs with which to play with this
>> functionality. Those will soon be available, so it seemed good timing
>> to raise it again and explain the various problems.
>> I only have problems, not a proposal. The basic problem (as I
>> understand it) is that with HDR displays, users are unlikely to want
>> the peak white for regular sRGB to map to the peak luminance of the
>> display. That would make desktops blindingly bright. So there needs to
>> be a way for pages to signal when they are providing data in the sRGB
>> luminance space (where peak white is what the user has set as a
>> comfortable luminance for their desktop according to current ambient
>> light) and when they are providing data in a different luminance
>> space, where peak white is brighter (exactly how bright is one of he
>> questions to be answered.)
> This is defined in the CSS color spec [1]. If no profile is provided, peak
> white is peak sRGB white.
> The fact that you get blindingly white is a browser bug. (I believe only
> Safari does this right today)

​I think there is a mis-understanding here and that is why I think the
issue deserves discussion.

As far as I can tell, [1] is entirely about color gamut. Whilst there are
many ways to identify colors, the specification does not seem to treat
dynamic ​range. The section you reference regarding profiles, says "Others
are more human-friendly to write and understand, *and are converted to an
sRGB color by CSS*".

sRGB peak white will appear at the brightness the user prefers for their
environment (because most of the graphics they see are sRGB and they will
adjust the brightness control on their display).

My point is that for capable displays, it should be possible to make
graphics brighter than that.

> If you do provide a profile, it will be used to map to the display.
> Is there a way to embed a profile in a video stream?

​Here I am not talking about video. But, yes, video metadata specifies not
only color gamut (primaries) but a transfer function which is used to map
between encoded luminance and actual luminance and is different for HDR and
SDR video.

To obtain CSS use-cases, imagine first a still image extracted from an HDR
video, encoded in some still image format which carries HDR metadata and
which is supported by the browser.

It should be possible to specify in CSS any color+luminance that appears in
that image. If the video or still image pixels are constrained to be no
brighter than the sRGB peak white (the luminance the user prefers for their
desktop), then you will not get any improvement in appearance: for example
specular highlights in the video / image would be no brighter than for an
SDR video / image, even though the display may be capable of that.

>> If the WICG is the appropriate place to raise these problems, I can do
>> that, but the experts are in CSS WG so I wondered if there was
>> interest in learning about these issues.
>> Thanks ... Mark
> 1: https://drafts.csswg.org/css-color/#color-type
Received on Thursday, 15 September 2016 17:21:42 UTC

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