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Re: Discuss HDR at CSS WG next week ?

From: Rik Cabanier <cabanier@gmail.com>
Date: Thu, 15 Sep 2016 11:25:46 -0700
Message-ID: <CAGN7qDCawt+H0e9WLSutKGCeZMh2Tz-yZUyGinbzOxgdahNJHQ@mail.gmail.com>
To: Mark Watson <watsonm@netflix.com>
Cc: Rossen Atanassov <Rossen.Atanassov@microsoft.com>, "www-style@w3.org" <www-style@w3.org>
On Thu, Sep 15, 2016 at 11:01 AM, Mark Watson <watsonm@netflix.com> wrote:

>
>
> 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.
>

Dean wrote a blog about this:
https://webkit.org/blog/6682/improving-color-on-the-web/
I don't know what profile is twice as bright as sRGB, but color(rec2020 255
255 255) would be brighter than rgb(255, 255, 255)


> 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).
>

What do you mean by "color volume". Are you talking about higher bit depths
for compositing?


> 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 ?
>

You can place the profile on your web server,load it with css using
@color-profile and use the name with the css color property.


>
>>
>>>
>>>>
>>>>> 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
>>
>>
>
1: https://drafts.csswg.org/css-color/#at-profile
Received on Thursday, 15 September 2016 18:26:16 UTC

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