What is Wide Colour Gamut (WCG)?

Discussion in 'General TV Discussions Forum' started by Steve Withers, Aug 20, 2016.


    1. Steve Withers

      Steve Withers
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    2. Trollslayer

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      Thank you, this explains so much and so clearly.
      I even understood it shortly after getting out of bed!
       
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    3. MikeTVMikeTV

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      Thanks for the effort in not only making the article but trying to explain in "laymans" terms, i still a little overwhelmed as my geek speak isnt as good as others.

      Maybe do a video with Margot Robbie in a bath to explain it for us poor folk.
       
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    4. simonblue

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      Thanks will have a good read later :)
       
    5. Manni01

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      Hi Steve,

      Great intro. A couple of things that you might want to check. AFAIK DCI-P3 doesn't define a fixed white point. It can use D65, but also D55 and other values.

      Re your colour volume graph, I know you use BT2020 and BT709 as a shortcut, but it would be clearer (less potentially confusing) to label these BT2020 HDR and Rec-709 SDR.

      It's perfectly possible to display UHD Bluray in SDR BT2020 (I would even recommend it with projectors that can't reach more than 300nits peak brightness in HDR on the smallest screens and much less on larger screens, and even then have to seriously raise the black levels to achieve such highlights in HDR, unlike OLED which can only reach 500-600nits but keep their black levels). In that case (SDR BT2020), the peak brightness is no more than 100nits (most likely around 50nits if calibrated for a dark room), just like for rec-709 SDR. BT2020 is only the gamut. It's only if you add HDR that the volume increases. Although with Rec-709 we didn't have to make this distinction between SDR and HDR, I think it's going to be quite crucial to make it for BT2020, especially until HDR10 becomes more of a standard (the end user part isn't defined and left to manufacturers, which means it's almost impossible to calibrate an HDR10 display accurately unless the manufacturer provides some form of proprietary autocal or provides the golden reference for the display to the calibration software, like Dolby Vision does).

      By the way all the first UHD Bluray titles are mastered in HDR BT2020, but SDR BT2020 is part of the standard and we might see titles mastered that way as well at some point. So this distinction is not only valid for those who prefer to convert HDR to SDR BT2020 due to their display limitations.

      Finally it would also help to clarify that the 10,000nits is the theoretical maximum for HDR. You do this with BT2020 explaining that most consumer displays can't reach the limits of the container yet and even studios use displays with a native gamut closer to DCI-P3 for grading, but the same applies brightness wise for HDR. Because most consumer displays struggle to reach even 1000nits (on a small window, not on the whole screen), most content is graded today at 1000-4000nits, so there is space to grow there as well.

      If your last graph displayed the actual (not theoretical) color volume used both on the mastering side and on the consumer display side for UHD Bluray currently mastered in HDR BT2020, it wouldn't go higher than the 1000nits mark in practice at the moment (because while some titles are mastered to 4000nits, no consumer UHD display can reproduce this, even the Dolby Pulsar grading monitor that goes that high has to use 1080p panels and water cooling to achieve 4000nits).

      You're much better than I am at explaining things in layman's term, so that's possibly why you left these details out. :)
       
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      Last edited: Aug 30, 2016
    6. Steve Withers

      Steve Withers
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      Thanks for the comments Manni but I think you're over-complicating the issue, the article is just to give people an idea of what is meant by wider colour gamut and the graphs are only supposed to be a rough guide. Fair point about 10,000 nits only being theoretical at this point, I'll make that clear.
       
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    7. Manni01

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      I have no doubt I'm overcomplicating the issue. As I said, you're much better than I am at putting things in layman's terms :)

      Still, I believe BT2020 SDR will play a big part in the next couple of years at least, for many people especially people using projectors, so as you're educating your readers there is nothing wrong in establishing clearly that BT2020 can be mastered and used in both SDR and HDR, and label your graph to make it clearer that it's HDR that causes the volume to increase vertically with brightness, and BT2020 that widens the gamut (as you explain in the article).

      I don't think that the average reader of AV Forums is unable to grasp this, especially as you explain everything else very well in the article, but you know your audience better than I do. :)
       
      Last edited: Aug 20, 2016
    8. Goldorak

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      @Steve
      Very good article and I appreciated in particular the 3D representation of the color including the nits

      It made me understand the comment from a Philips engineer stating that light is very important for color...your 3D representation makes this reference to volume very clear...

      It is just crazy how things changed and fast. feels to me that Oled have to reach quickly 800-1000 nits to stay in the game?
       
    9. Final Flash

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      Great article Steve!:thumbsup:
       
    10. Greg Hook

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      Great article and explained it in perfect layman's terms.

      Sorry Manni, but your post above made my eyes gloss over. :)
       
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    11. SonOfSJ

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      Another in your 4K, HDR, 50fps stream (see what I did there?) of reference articles that are perfect for the interested but uninformed layman and should be required reading by all, or certainly most. Excellent stuff, Mr Withers! And I'm sure that Manni01 in his post #5 meant well, but .....

      Just one specific question. On the graphs, what are the quantities x and y on the axes, and what are their physical units (voltage, cd/m2, etc.) please?
       
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    12. kosch

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      Great article, thank you!
       
    13. meansizzler

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    14. wongataa

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      Adobe RGB is similar to DCI-P3 but has more greens. Google to see the comparison. Bit depth has nothing to do with colour space. The colour space of a screen is a physical property and is related to the backlight (with LCD) properties and the properties of the colour filters.

      The bit depth is just how many gradations you have. A useful analogy is this: Think of a staircase. The overall height of the staircase is the colorspace. The number of steps is the bit depth. From this you can see that the number of steps (bit depth) has no effect on the overall height of the staircase (colourspace). The bit depth just defines how many individual colours in a colourspace and screen can show.
       
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    15. littletee

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      I think i did my isf training with you, and i only just followed this. Very useful article. I think someone needs to do a simpler layman's version, but it may need diagrams or animation. I should try to find some time to look at it. Particularly the brightness volume part that manufacturers ignore.
      Unless this stuff starts to become automated, people will continue to mess up colour, and usually not care. I have never, in twenty years, been in a design studio where anyone had any idea about display calibration or colour management. This article helps with its step by step summary of tv standards. Nobody seems to focus enough on biology though. Our eyes are basically 2.2 gamma in bright lit rooms, and the displays were designed to match this. The extra brightness in hdr, i find, tends to actually reduce shadow detail, as my iris closes down. It's going to take years before colourists can manage it well, and that gets translated into the living room.
      Basically still not time to sell the kuro!
       
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    16. Steve Withers

      Steve Withers
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      That's a great analogy for bit-depth, thanks.
       
    17. littletee

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      Staircase analogy is brilliant!
      The new LG oleds are my main contender for a grading level display, but then I saw Dado show off the Panasonic and it seems very accurate and very like the reference monitor next to it, out of the box. Obviously i am wary of presentations, but these are now neck and neck. But for now, a 1080 lg oled and a LUT box is perfection enough for now.
      And i still haven't calibrated the kuro...
      Light illusions articles are very good as well.
      I played with the Dolby monitor when it came out, and aces was just arriving. It does look like at some point a tv will know it's own profile properly, like computer monitors supposedly do now, and interpret the wider signal as best they can... Oh hang on, that's what they do now! Ok. Calibrators needed more, not less!
      So i just got back from the lg sponsored colour exhibition at natural history museum. There are more colours in the world than we can see. So we are like a tv looking at a 2020 world through almost 709 eyes!
       
    18. Toon Army

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      Great article Steve - This will be my " go for article " as my knowledge and understanding develop. Articles like this make the website stand out from others.
       
    19. Steve Withers

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      In answer to your question Ken, the x and y axes on the graphs are hue (sometimes called tint) and saturation (sometimes called colour), whilst the third axis in a three dimensional colour volume graph is luminance (sometimes called brightness). You will undoubtedly have seen saturation/colour and hue/tint as controls on a TV and a good colour management system will have separate controls for saturation/colour, hue/tint and luminance/brightness.
       
    20. Steve Withers

      Steve Withers
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      I have simplified the article in places to make it easier for a layperson to understand.
       
    21. Manni01

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      Given the reactions in the thread, that was clearly the right thing to do. Apologies for misunderstanding the AV Forums audience and trying to pull you in the other direction, lesson learnt :)
       
    22. charlesaH

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      Very nice article. I have a few questions.

      1) My understanding is that current tv (eg HD, SDR, 709 et al) programs are graded to 100 nits. The color info is sent as three an 8 bit numbers with something close to 0 intended to be interpreted as min and some number close to 255 intended to be interpreted as max. It is up to the TV to decide what to do with the color data. The TV usually assigns the min value to black (no light) and the max value to maximum light (typically 200-300 nits) and uses a gamma curve for all the values in between.

      Is my understanding correct?

      2) In UHD, each color is transmitted as a 10 bit number (correct?). A PQ curve is used to map this 10 bit number to each color's light output by the TV. (The PQ mapping gives us more bits in the low end to avoid banding vs just the old gamma mapping.) But ~0 still means black and ~1024 means .. what? 1000 nits? The brightest the TV can produce (could be 600 nits or 2000 nits)?

      3) Some UHD makers (e.g. Samsung, Sony) have a mode which claims to expand SDR content into the full range of the TV's 1500+ nit capabilities. How is this different than what is currently done with HD tvs?.

      Is is bad to expand to 4000 nits if the content was graded to 1000 nits? Haven't we been doing something similar for years in expanding 100 nits graded content into a 300 nit capable TV? Real world outdoor scenes are much higher than 1000 nits (>10000). Why isn't this kind of expansion encouraged? Expected?
       
    23. Queens Pawn

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      a very clear explanation - thanks.

      Do all HD Tvs from the past few years fully display Rec709?

      My recent TV (Panasonic 852) has sublime colours compared to my older TVs circa 2008/9 when showing plan old HD...
       
    24. Steve Withers

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      I can't speak for every TV but certainly the majority of the models we've reviewed in the last two years have been capable of delivering 100% of Rec. 709.
       
    25. stevebk

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      Wow....
      This WCG has been so educational.
      With the speed of technology we have today, it does make me wonder what we will have in 10 years with WCG
      A big thanks Steve for putting so much information, into so few easy to read paragraphs.
       
    26. Chester

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      Hey @Steve Withers that was a great article and very informative as others have said. Really filled in some knowledge gaps. Couple of questions if I may:

      You mention Chroma Sub-sampling. I've seen these ratios x:x:x before but have no understanding of it all means. Is this something that is exclusive to Hayley Davidson riders with all that chrome, or is this significant to the rest of us? Or is it best left to one side and embrace matte black!?!

      Call it irrational fear or not, as TVs are getting capable of brighter and brighter images, I wonder if we are going to have our retinas burnt out at some point! Whilst indoors with light levels much lower than the great outdoors in bright sunshine, our irises open to allow more light to travel through our pupils. Does this make our eyes more sensitive and subject to potential damage from HDR levels of strobing or other extremely dynamic effects? I wonder if going for OLED would be a good thing just because they're not capable of going beyond 600 nits yet, so a limitation becomes a good safety feature. What are your thoughts here?
       
    27. geogan

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      Jesus, that's scary. What do they teach people in art/design colleges at all these days! Seems the lecturers are as clueless as the students - most likely hippy painters with no tech knowledge at all. Those design studios must be sending out some awfully badly coloured stuff then - how do clients/printers never complain about colours being all over the place? I knew all about color management and calibration all the way back in the Amiga 1200 days with print management software I bought and included excellent over 100 page manual ***!!

      Also annoying that I understand all that (including Mannis comments perfectly) and you only just followed it, and yet you are supposedly ISF certified and I am not (because it costs shed loads to get the certification)
       
    28. Chester

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      I went on Pier's course that was organised through AVF some years ago. That probably helped me understand this article somewhat. I can understand how it could be difficult for some to understand, this is after all a highly technical area.
       
    29. Steve Withers

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      Like the video bit depth, the chroma sub-sampling is related to the colour gamut but outside the scope of the article. However essentially chroma sub-sampling is a way of compressing colour data by taking advantage of how the human eye works. We are very good at noticing differences in luminance (brightness) but not so good when it comes to chroma (colour). So if a 4:4:4 signal is basically uncompressed with a full luminance channel (the first 4) and two full chroma channels (the second and third 4s) then a 4:2:0 (which is what Blu-ray and Ultra HD Blu-ray use) delivers the full luminance channel, half the first chroma channel and none of the second chroma channel, which reduces the bandwidth by half compared to a 4:4:4 signal. The player can then recover much of the encoded chroma information and output the signal as 4:2:2 or in some cases like the Panasonic UB900 as 4:4:4. It won't have the fidelity of an original signal that was actually encoded at 4:4:4 but the differences shouldn't be perceptible.

      In regards to your query about brightness, it isn't that the overall image is 1,000 nits (or 4,000 or 10,000) that would be uncomfortable, it's just the specular highlights that deliver that peak brightness. So if you think of sunlight reflecting off the chrome on that Harley Davidson you mentioned, that's where the 1,000 nits of peak brightness is, in that small area not in the overall image. The result is a picture that is far more realistic but isn't uncomfortable or damaging to watch.
       
    30. Chester

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      Thanks for that Steve. I remember in my youth that an S-video signal was made up of luminance and chroma (colour). A signal with just luminance is just varying shades of grey from black to white, so I can understand how deciding how to compress chroma information is quite important.

      Regarding highlights in HDR, if that's how the bright part of HDR is being used, I can see that as 'safe'. I guess you've seen plenty of UHD HDR footage now to have a good appreciation of how directors/cameramen/colourists are using it.

      Do excuse any scepticism I have on this technology. I'm used to stuff going wrong! Let's face it, even trying to get HDR data to a display was a mission not so long ago.
       

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