What is Wide Colour Gamut (WCG)?
Rec. 709, DCI-P3 and Rec. 2020, what do they all mean?
You’ve probably read a lot recently about wider colour gamuts (WCG), DCI-P3 and Rec. 2020 but what are they and why are they important?
If you’ve looked at the marketing for your new Ultra HD TV or read the specifications for your Ultra HD Blu-ray player or even scanned the cover art for that Ultra HD Blu-ray disc that you’ve just bought, you will undoubtedly have seen the phrase ‘wide colour gamut’ somewhere. In fact, along with 4K resolution and High Dynamic Range (HDR), WCG is one of the main benefits of Ultra HD. However before we explain why a wider colour gamut is important, perhaps we should explain exactly what we mean by a colour gamut.What is a colour gamut?
In it's simplest terms, a colour gamut represents all the colours that a TV or projector can display. So the bigger the colour gamut, the more colours a TV or projector can show and the better the picture. A colour gamut is sometimes referred to as a colour space but don't worry, they're basically the same thing; although to avoid confusion we'll only use the term colour gamut in this article.
The graph above is the CIE 1931 colour chart and that horseshoe shape represent all the colours the human eye can see, which is called the visible spectrum. If a TV or projector could display that entire horseshoe shape, it would be capable of reproducing every colour that is visible to humans. Unfortunately display technology hasn't reached that point yet, so instead a TV or projector will only be able to show a percentage of that horseshoe shape, which is the colour gamut.
All colours are composed of the three primary colours of red, green and blue. So if you pick a specific point for each of those three primary colours within the visible spectrum you've got a triangle. That triangle is a colour gamut. The bigger the triangle, the greater the percentage of the visible spectrum that it covers and the more colours it can show. If you look at the image at the top of this article you will see a series of triangles within the horseshoe shape and each of those triangles is a colour gamut.
OK, so a colour gamut is a specified percentage of the visible spectrum but why do we even need one? The simple reason is so that images can be accurately and consistently reproduced. Without some form of agreed standard how would anyone know what a specific colour was supposed to look like? The creation of colour standards goes back to the 19th century and in the early 20th century there was pioneering research conducted on the wavelengths of colours, the visible spectrum and human colour perception.
The CIE 1931 colour chart was the first defined standard that showed the link between the three primary colours of red, green and blue and the way that those colours are actually seen by our eyes. This standard was developed by the International Commission on Illumination (CIE) and the resulting chart shown above was the first graphical representation of all the colours a normal human eye can see and thus provided an overall range from which various colour gamuts can be created.
In terms of video the advent of colour television resulted in a need for engineers, broadcasters and professional bodies to agree a standardised colour gamut for the production, broadcast, reception and display of colour images. The reason was quite simple, without an agreed standard how was a programme maker, a broadcaster or a TV manufacturer supposed to know what colours to use? This adherence to colour standards is vital in ensuring that what is being produced, broadcast and displayed is correct and retains the content creator's original intentions.
The first video colour gamuts were developed for the early colour TV systems – specifically NTSC (National Television System Committee) and PAL (Phase Alternating Line). The advent of NTSC colour television in 1953 resulted in an agreed colour gamut using coordinates within the CIE 1931 colour chart. This colour gamut was later revised and standardised when the Society of Motion Picture and Television Engineers (SMPTE) adopted SMPTE-C in 1987, although in Japan they continued to use the original 1953 colour gamut for NTSC. PAL, which was introduced as a colour TV system in the UK and Europe in the early 1960s, has a colour gamut that also uses the CIE 1931 colour space but is larger than the one used for SMPTE-C.
What is Rec. 601?
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard originally issued in 1982 by the International Telecommunication Union (ITU) for encoding interlaced analogue video signals in digital video form. It covers methods of encoding NTSC and PAL signals and includes the specification of the Rec. 601 colour gamut, which is based on the original PAL colour gamut, as well as the CIE standard illuminant of D65 for the colour temperature of white.
What is D65?
In it's simplest terms D65 is the agreed standard for the colour of white because just like with the primary colours, content creators and display manufacturers need to have a specific point for white. In more technical terms, D65 is a commonly used standard for illumination, as defined by the CIE and is part of the D series of illuminants that try to portray lighting conditions in the open-air in different parts of the world. D65 corresponds roughly to the average midday light in Europe, which is why it is often also called a daylight illuminant, and is the standard used for the colour of white in both Europe and North America. So essentially you can think of D65 as the colour of a piece white paper as seen in daylight at midday.
D65 also has a correlated colour temperature of 6500K, which is why it 's sometimes referred to as the colour temperature of white. Since white is made up of red, green and blue, D65 can also be represented as a specific point within the triangle that makes up any particular colour gamut. In the graph below you will see D65 represented as a dot roughly in the middle of the triangle. When setting up your TV you should select the colour temperature closest to D65 (some manufacturers helpfully call the setting D65 but most use Warm or Warm2) and when a professional calibrator is calibrating the greyscale using the white balance or colour temperature control, they are also calibrating the colour of white as closely as possible to D65.
What is Rec. 709?
ITU-R Recommendation BT.709, more commonly known by the abbreviations Rec. 709 or BT.709, standardises the format of high definition television with a 16:9 (widescreen) aspect ratio. The first edition of the standard was approved in 1990 and covers the resolution, the frame rate, D65 and the Rec. 709 colour gamut, which is shown above. This colour gamut is almost identical to the previously mentioned Rec. 601 and is the current standard for High Definition TV (HDTV), streaming video and Blu-ray. That means all high definition TV programmes are made and broadcast or streamed in Rec. 709 and Blu-ray discs encode films re-graded using the Rec. 709 colour space.
This is why it is important that you set up your TV correctly using our Picture Perfect guide or hire a professional calibrator to ensure that your display is reproducing Rec. 709 as precisely as possible. That way you can be sure that what you are seeing is exactly what the content creators want you to see because they will have been using a professional monitor that is also calibrated to Rec. 709. Unfortunately anyone who tells you to use the widest colour gamut possible when setting up your TV or that you should adjust the colours until they ‘look right’ is demonstrating a fundamental ignorance of exactly how modern video systems are designed and implemented.
The Rec. 709 colour gamut has been the industry standard for broadcast TV, video streaming and Blu-ray since the advent of high definition and it will continue to be important for the foreseeable future. It should be pointed out that if you are watching standard definition PAL broadcasts or playing a PAL DVD then the colour space will actually be Rec. 601 but that's almost identical to Rec. 709 so there would be no appreciable difference. The Rec. 709 colour gamut actually covers 35.9% of the CIE 1931 colour chart and for years TVs and projectors struggled to actually fully reproduce this colour gamut. However recent advances in quantum dot technology, OLED panels and laser projection means that colour spaces wider than Rec. 709 are possible and so new standards have been developed and adopted.What is DCI-P3?
Recently you will probably have seen TV manufacturers promoting the fact that their new TVs can reach a certain percentage of DCI-P3 or, in the case of the Ultra HD Alliance, that a TV needs to be able to display at least 90% of DCI-P3 to be certified Ultra HD Premium. But what is DCI-P3?
DCI-P3 is a specialised colour gamut developed for digital movie projection in professional cinemas and is wider than Rec. 709, covering 45.5% of the CIE 1931 colour chart. This colour gamut was defined by the Digital Cinema Initiatives (DCI) group and published by SMPTE, so when films are graded for digital distribution in the cinema, the colourists creating the grades are using the DCI-P3 colour gamut.
Although domestic TV and projector manufacturers and even the Ultra HD Alliance have been using the term DCI-P3 to describe the maximum colour gamuts that domestic displays are capable of, this is actually rather misleading. DCI-P3 is a professional standard that was developed purely for commercial cinemas, there is no domestic equivalent. So whilst it is true that the content may have been created using the DCI-P3 colour gamut that isn't how the content will be delivered domestically.
So why have domestic TV and projector manufacturers and the Ultra HD Alliance been using DCI-P3 to describe the capabilities of their Ultra HD displays? The simple reason is that modern displays can get very close to the DCI-P3 colour gamut but still have a long way to go until they can achieve the new domestic standard of Rec. 2020. So being able to say that your TV or projector can achieve 98% of DCI-P3 looks better in your marketing than saying it can achieve 70% of Rec. 2020. Which brings us on to the big question, what is Rec. 2020?
What is Rec.2020?
ITU-R Recommendation BT.2020 is a set of standards developed for Ultra HD TV and it covers display resolution, frame rate, chroma subsampling, bit depth and colour gamut. As with Rec. 601 and Rec. 709, it is frequently abbreviated to BT.2020 or Rec. 2020 and as with the other two standards, Rec. 2020 is also used to describe the colour gamut that is specified within the standards. The overall standards developed for Rec. 2020 were intended to be future proof, so some of them are currently unattainable in domestic displays.
For example when it comes to resolution, the definition of Ultra HD is either 4K (3840 x 2160 pixels) or 8K (7680 x 4320 pixels) but at the moment only 4K is really being used in any meaningful way. The frame rate specifications can reach a maximum of 120p but currently displays can only handle a maximum of 60p. The standard allows for a bit-depth of up to 12-bits but currently displays and content use 10-bit and the chroma subsampling can go up to 4:4:4 but Ultra HD Blu-ray is still limited to 4:2:0.
This future proofing also applies to the Rec. 2020 colour gamut which is massive, measuring 75.8% of the CIE 1931 colour chart, as you can see in the graph above. As with all the other standards, Rec. 2020 still uses D65 but otherwise the colour gamut is very different from what has been used before. In fact it's so wide that not only is it significantly bigger than the DCI-P3 standard used in Digital Cinema (although professional laser projectors can reach Rec. 2020) but it's actually beyond the capabilities of any current domestic display.
So why choose a colour gamut that is so wide no content is made in it and almost nothing can reach it? Well aside from the aforementioned future-proofing, the best way to look at Rec. 2020 is as a container that is big enough to put any other colour gamut inside. If you look at the image at the top of this article, you will see that Rec. 709 fits inside DCI-P3, which in turn fits inside Rec. 2020. At the moment many Ultra HD TV productions and all films are being shot and colour graded using the DCI-P3 colour gamut but in order for this to be delivered domestically that content must be encoded within the Rec. 2020 colour gamut.
Although the cinema industry currently has no plans to move to a wider colour gamut, if they did at some point in the future then that could also be delivered within Rec. 2020. The same is true for TV production as they move away from Rec. 709 and towards a wider colour gamut that is nearer to Rec. 2020. Which brings us to the most recent development, the introduction of High Dynamic Range (HDR).
What is Rec. 2100?
ITU-R Recommendation BT.2100, which again is more commonly known by the abbreviations Rec. 2100 or BT.2100, defines the new HDR formats. These HDR formats are the Dolby Perceptual Quantiser (PQ) which was standardised as SMPTE ST 2084 and Hybrid Log-Gamma (HLG) which was standardized as ARIB STD-B67. The current HDR10 and Dolby Vision formats both use PQ, a 10-bit video depth (up to 12-bit in the case of Dolby Vision) and the Rec. 2020 colour gamut; with the former used by streaming services and Ultra HD Blu-ray and the latter currently being adopted by streaming services. The HLG10 format is designed for broadcast television and uses HLG, along with 10-bit video depth and the Rec. 2020 colour gamut.
Why is HDR important? Well it fundamentally changes the way that content is created and delivered, resulting in a much wider dynamic range – that's the difference between the darkest blacks and the brightest whites – with more visible detail in the shadows and highlights. Until the advent of HDR, domestic content was graded using Rec. 709 and a peak brightness of 100 nits. Now content can theoretically be graded using Rec. 2020 and a peak brightness of 10,000 nits, which brings us on to our last topic – colour volume.
What is Colour Volume?
The various graphs that we have shown you so far in this article have all been in two dimensions, which means you can see the hue and saturation axes that make up a particular colour gamut. The wider the colour gamut, the more colours available and the better and more realistic the colours can look. However there is a third element to colour and that is the luminance or brightness, which would be represented on the CIE 1931 colour chart as an additional axis, creating a three-dimensional graph. This three dimensional space composed of the colour gamut and the overall luminance is what is often referred to as the colour volume. HDR increases the overall brightness of the image, making this three-dimensional graph or colour volume bigger.
The graph above shows you this three-dimensional aspect of colour and, although it's an extreme example, you can see that the difference in colour volume between Rec. 709/100 nits and Rec. 2020/10,000 nits is huge. At present 10,000 nits, which is a measure of luminance, is just a theoretical target because current grading monitors can only reach a peak brightness of 4,000 nits but the graph above gives a clear example of the potential impact of an increased colour volume on the viewing experience. This is why wide colour gamuts are so important and, when combined with a higher dynamic range, offer a viewing experience that is more immersive, more impactful and more realistic than ever before.
So can I just use a wider colour gamut all the time?
When we talk about wider colour gamuts and higher dynamic ranges we are referring to content that has been created using the new standards. A TV can’t re-create or reproduce information that is not present in the original content, so if the colour and dynamic range are not present, then post-processing can never recover them. Too much enhancement of the brightness and colours results in washed-out or over-saturated images, because the colourist had to work within the Rec. 709 standard during the creative process. By increasing the brightness of the original source material and using a higher dynamic range and a wider colour gamut in the content-mastering process, HDR presents more realistic images with levels of detail that post-processing technologies in TVs just aren’t able to produce.
A good analogy would be a photo file, where you start with a large image and make it smaller, maintaining the detail and quality of the image as you go. On the other hand if you start with a small image and make it larger, it becomes distorted or pixelated at a certain point. By increasing the dynamic range and colour gamut of content and maintaining that in the distribution process, you allow the content to be mapped down to the TV instead of being stretched out from the older standard, which would destroy the artist’s original creative intent. So you should always match the settings on your TV to the content you are watching. For most content it will still be Rec. 709 but if you're watching an Ultra HD Blu-ray or HDR content on a video streaming service, then you can take full advantage of the wider colour gamut and higher dynamic range.
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