IRC log of hdr on 2015-10-28

Timestamps are in UTC.

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Meeting: High Dynamic Range breakout, TPAC 2015
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scribe: nigel
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chair: Mark Watson
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present+ wdh
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present+ markw
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present+ nigel
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07:09:57 [nigel]
markw: This seems like a topic that hasn't been discussed much in W3C.
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... There may be people here who are interested but not understand it.
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07:10:26 [nigel]
... Is there anyone here who is not familiar with High Dynamic Range (HDR) and
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... Wide Color Gamut (WCG)?
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group: [would like the introduction]
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markw: Presents slides
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... [We've had more resolution and framerate, now looking for 'better pixels']
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... HDR is about luminance, WCG is about colour range.
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... [slide with london bus] This isn't the real colour of a London Bus.
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... [Luminance slide] Luminance is measured in candela / metres squared or nits
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... range of perception is much wider than what SDR TV and HDR TV cover.
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... HDR is much bigger than SDR - the brights are brighter and the darks are darker and
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... more detail can be viewed in the dark and light regions.
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... [Slide showing colour gamut] We take 3 points on the colour diagram and all the
07:14:15 [nigel]
... colours we can represent are inside the triangle that they define. Shows Rec709 as
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... the smallest triangle, Rec2020 as the biggest, some in between. There aren't many
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... displays that can show the whole 2020 colour space. We can signal and encode
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... colours in that space but the device will shrink those down. Lasers are needed right now
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... to show the whole gamut.
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... [Slide showing 3d picture of WCG and HDR] We talk about colour volume now,
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... representing the entire range of colours and brightness that we can encode.
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... [Slide, example from Dolby with picture of flowers] All of the red points are those that
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... fit in the color volume, and the blue ones are outside it. An ordinary picture has
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... colours in it that are way outside the range that we can represent today.
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Bill: There's a lovely example of why this matters. Two NY Giants jerseys. A 30 year old
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... one and a 10 year old one. Part of their brand identity is the exact colour that they
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... use. If you're limited to RGB or Rec709 you cannot accurately and reliably capture the
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... colour that is part of the brand identity. So there's a commercial gain to doing this
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... more accurately.
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XXX: Cinema displays can show a bigger volume than TVs. When you go through the
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... process of making a film for cinema and TV you force the colour grader to make
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... limiting choices.
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markw: For video and contribution display the introduction of HDR and WCG affects all
07:19:01 [wdh]
07:19:21 [nigel]
... the steps in the chain. Video cameras can capture a lot. On the production side
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... lights need to be considered too. In Post production color grading and reference monitors
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... need to be changed. They need to learn how to use the new tools.
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... In Encoding and Delivery you need more bit depth (10 bit), a different transfer function
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... (that maps from a 10 bit number to luminance value), and colour space.
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... In displays there's lots of new technology in consumer displays - LEDs with local
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... backlights and dimming, OLEDs etc. "Tone mapping" is important - the fact that a
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... signal that you've encoded has been created for a reference master display. No
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... consumer displays have the same brightness as the master display. Tone mapping
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... is far from straightforward, to map from one to the other.
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... [Web Platform Issues]. Questions for the Graphics plane and the Video Plane.
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... There will be devices that support HDR for video but not for graphics. In both cases
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... we have both an encoding problem and a discovery problem, to discover display
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... capabilities. There's also the question of image formats for HDR images.
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... There's video capture on the web platform too.
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YYY: for webcam if you have strong backlight, e.g. for video conferencing, it is very
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... difficult to get a good picture.
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wdh: Consumer capture is really interesting.
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markw: Codec and display capabilities are two independent things. Our current APIs
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... are quite good at codec discovery - MIME types with codecs, levels, profiles etc.
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... But just before I have an HDR codec does not mean I have an HDR display.
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nigel: There are also questions around compositing graphics on video. Plus there was
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... a liaison from ATSC to TTWG asking about how to deal with HDR and WCG given that
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... timed text formats only support sRGB.
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markw: Is there a requirement around compositing?
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wdh: It feels like an implementation thing.
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bill: Don't you need to specify an EOTF.
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pierre: If you composite SDR over HDR then there are circumstances when you might want
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... to boost or darken the graphics compared to the image. Using an 80 nit max for
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... sRGB might be too dark or too light depending on the image.
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markw: So you want the application to have a brightness control?
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pierre: Give the author the ability to specify that.
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markw: That's what I meant.
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bill: Having a brightness knob on the entire plane might be an interim solution.
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pierre: You don't want to have to backport HDR into CSS, SVG, TTML etc. Also if you have
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... an existing library of SDR content does it make sense to make an HDR version of the
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... same content?
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bill: Sometimes yes, sometimes no, depending on the commercial motivation.
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nigel: One thing that's really easy to specify is that sRGB values must not be interpreted
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... as HDR values, because that would result in full intensity values being painfully bright.
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... (accepting that the relative brightness might need to shift up and down).
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dsinger: Actually displays are already a bit brighter than sRGB peak white (80 nits).
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markw: You might have to set the brightness control right.
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wdh: You've identified these are interesting problems. Are there others?
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YYY: Do you mean the graphics plane as encrypted?
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markw: I'm talking about the display. Televisions have this problem particularly.
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... They have two separate pipeline processes for video vs graphics.
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bill: What we've seen is that they do some mapping.
07:34:37 [nigel]
dsinger: There's another issue. In the world of audio we've had to manage range to
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... avoid hurting people's ears. In the pictures we may need to do something similar.
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wdh: In the UK there are requirements about that.
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group: discusses the calm act and the need to make content accessible.
07:36:37 [nigel]
wdh: The brightness issue is valid - just dealing with ambient light feels like an implementation
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... as opposed to a standards thing.
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bill: The tone mapping is outside the standards space.
07:37:01 [nigel]
markw: A possible technical area that might require work is: You have a site with HDR
07:37:23 [nigel]
... graphics and some adverts. Should the adverts be able to constrain the page peak brightness?
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bill: It's not just peak brightness - the appearance isn't just defined by the peak.
07:38:02 [nigel]
nigel: Are you talking about dynamic adjustment of the whole composited plane?
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markw: I'm talking about content in an iframe changing the appearance of the main content.
07:38:57 [nigel]
dsinger: Like a dark goth page that's spoiled by a bright pink advert.
07:39:08 [nigel]
nigel: Isn't that a content provider issue?
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markw: Yes you could request that only adverts conforming to a particular profile are served.
07:39:38 [nigel]
... If the web platform could in some way cap or constrain the brightness of content
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... that could be useful.
07:39:54 [nigel]
bill: An argument in favour is the health safety.
07:40:06 [nigel]
markw: That would argue for some kind of transformations in the CSS realm.
07:40:12 [nigel]
wdh: That's interesting.
07:40:25 [nigel]
markw: The immediate thing that's of most concern is video display capability discovery.
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... I thought at some point that CSS was the right place to discover this, but in realising
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... that the graphics capability and the display capability might be different I think that
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... they're separate.
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nigel: So if you discovered you had a sRGB display or a 2020 display what would you do?
07:42:28 [nigel]
markw: You'd choose which content to download and show.
07:42:40 [nigel]
nigel: Why wouldn't you apply the transformation locally to the same content?
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dsinger: Some high value content is carefully graded to be optimal for a particular display.
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wdh: So if you signal a particular codec and you're casting it to the TV, that doesn't
07:44:24 [nigel]
... allow enough information about what exactly to put on the page. Plus the video window
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... might be on a monitor or dragged over to a TV.
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dsinger: You can ask 'do you have a 10 bit codec' but that doesn't tell you anything about
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... the color space. We've traditionally thought of the display capabilities as being
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... independent of the codec.
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markw: I've even wondered if you might have a situation where you have an HDR display
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... but somehow the content that you fetch is SDR.
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YYY: You might have a problem where the display falls back to the lowest common denominator space.
07:46:43 [nigel]
markw: You mean if you were compositing graphics on video?
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YYY: Yes.
07:47:02 [nigel]
markw: So if we thought that compositing SDR graphics on HDR video would cause the
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... whole thing to drop down to SDR we wouldn't do that.
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markw: From our company perspective everything has to all work together and look
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... right for the product. A question in this group would be, for the web platform, if
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... we are going to introduce HDR, should we place the same requirements on it.
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... You're not allowed to advertise HDR capability unless you can successfully composite
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... these things together.
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wdh: Set top boxes have a hard problem - multiple in, multiple out. It's even harder than
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... for TVs. We're doing it but it's ugly.
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dsinger: I've seen an example where negative primary values can be used to go outside
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... the color gamut.
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markw: +1 to just specifying a new color gamut.
07:50:23 [nigel]
dsinger: Practically, a media query in CSS needs to come up with a boolean answer.
07:50:40 [nigel]
... I'm wondering if asking "What %age of the sRGB or 2020 triangle do you claim to be
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... able to render?" then maybe that would work given that the triangle is normally
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... reasonably well centred, so you could get close enough that way.
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pierre: The challenge is that there are lots of other parameters, not just color volume,
07:51:36 [nigel]
... but also max and min luminance. What about querying specific formats from the display.
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... rec709, BT2020 etc.
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markw: I think that you can assume that if a display can understand the color values in
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... 10 bits then it can display them.
07:53:31 [markw]
bill: boolean for colorspace is probably ok
07:55:09 [markw]
billh: seeing that luminance threshold depends on device : OLED vs LCD etc.
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... 400nits OLED may be ok, but 700 for LCD
07:56:07 [markw]
dsinger: displays will improve over time, unlike the static situation we believed we were in before
07:57:17 [nigel]
markw: We're near the end of the session. We should try to input this to the CSS WG
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... as a starting point to generate some more discussions. I don't have any bright ideas
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... about indicating the display capability.
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markw: Is that a category - do still images go with the video capability or the graphics capability?
07:58:43 [nigel]
dsinger: Is the TV rendering it all?
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pierre: Captions - the television composites them on the video.
07:59:04 [nigel]
... What's the process in CSS WG?
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markw: The process is first find a browser who's interested and then take it to the group!
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dsinger: I'm going to talk to my guys about this and do some experiments.
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pierre: Do you think they'd be interested to specify colours differently or assume that
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... CSS will continue to render to sRGB or both?
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dsinger: CSS will need to be able to represent colours outside the sRGB plane.
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nigel: Then you have the reverse problem - what do you do if a colour outside sRGB
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... needs to be rendered on an sRGB display?
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dsinger: yes.
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dsinger: Just to frighten you, what if someone just offers full ICC colour profile?
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pierre: ICC has the concept of diffuse white, which PQ doesn't have.
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markw: What is diffuse white?
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pierre: When you define an ICC colour profile you have to define diffuse white. Today
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... most implementations match that to their maximum white luminance. An example
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... is a HDR PNG with an embedded ICC profile that specifies diffuse white at 100 nits.
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... Most applications just clip everything above 100.
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Action: markw to forward these minutes to the CSS group!
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scribe: markw
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I have made the request to generate nigel
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