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Re: Discuss HDR at CSS WG next week ?

From: Mark Watson <watsonm@netflix.com>
Date: Thu, 15 Sep 2016 11:01:23 -0700
Message-ID: <CAEnTvdBFoE5j1mjfb9G4NB7Aa5oNFZGcv6vmt+_S-3Zki+7fBg@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 10:41 AM, Rik Cabanier <cabanier@gmail.com> wrote:

>
>
> On Thu, Sep 15, 2016 at 10:21 AM, Mark Watson <watsonm@netflix.com> wrote:
>
>>
>>
>> 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.
>>
>
> Yes, you can by either providing an image with an embedded profile that
> has a higher gamut than sRGB, or by specifying a CSS color [1] with an HDR
> profile.
>

​Could you give an example of how you would specify an HDR profile with CSS
?​ For example, if I wanted some white text that was twice as bright as
sRGB(255,255,255) text ? I don't see that in the css-color specification.

Note that I am not talking about a wider gamut - that seems to be well
covered - but a different dynamic range (i.e. a larger color volume, not
just larger color gamut).


> Those colors are then mapped to the "device" color which usually is the
> monitor's colorspace. If you have an HDR monitor and specify an HDR image,
> you will get colors that are brighter than sRGB white.
>
> Safari is already doing this for images so you can try this for yourself
> if you have a capable device.
>
>
>>
>>> 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.
>>
>
> I think we need to specify what is done with video. It seems that this
> information needs to be used so the browser knows how to manage the video's
> colors.
> what happens today with video?
>

​Well, this is all rather new, so I doubt there is any consistency. On the
various devices we have seen which composite sRGB graphics with HDR video
it's highly variable. I expect when we see this on devices with
desktop-class browsers, the operating system will have made some decision
as to what luminance to map sRGB to, relative to the HDR images. I've heard
80 nits. I've also heard of people planning to map sRGB peak white to panel
peak white.​



>
>
>> 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.
>>
>
> Yes, extract the transfer function and other information from the video
> stream and use it to construct a profile. Then embed that profile in your
> still image.
>

​What about actually specifying one of those colors in CSS ? So that I can
have text or graphics matching ​the colors in the image ?



>
>
>>
>>>
>>>> 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
>>>
>>>
>>
> 1: https://drafts.csswg.org/css-color/#icc-colors
>
>
Received on Thursday, 15 September 2016 18:01:53 UTC

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