What is a Gamut or Color Space and why do I need to know about it?

workshops-275 What is a Gamut or Color Space and why do I need to know about it?

Well I have set myself quite a challenge here as this is a tough one to describe and explain. Not so much perhaps because it’s difficult, but just because it’s hard to visualise, as you will see.

First of all the dictionary definition of Gamut is “The complete range or scope of something”.

In video terms what it means is normally the full range of colours and brightness that can be either captured or displayed.

I’m sure you have probably heard of the specification REC-709 before. Well REC-709, short for ITU-R Recommendation, Broadcast Television, number 709. This recommendation sets out the display of colours and brightness that a television set or monitor should be able to display. Note that it is a recommendation for display devices, not for cameras, it is a “display reference” and you might hear me talking about when things are “display referenced” ie meeting these display standards or “scene referenced” which would me shooting the light and colours in a scene as they really are, rather than what they will look like on a display.

Anyway…. Perhaps you have seen a chart or diagram that looks like the one below before.

gamuts-269x300 What is a Gamut or Color Space and why do I need to know about it?
Sony colour gamuts.

Now this shows several things. The big outer oval shape is what is considered to be the equivalent to what we can see with our own eyes. Within that range are triangles that represent the boundaries of different colour gamuts or colour ranges. The grey coloured triangle for example is REC-709.

Something useful to know is that the 3 corners of each of the triangles are whats referred to as the “primaries”. You will hear this term a lot when people talk about colour spaces because if you know where the primaries (corners) are, by joining them together you can find the size of the colour space or Gamut and what the colour response will be.

Look closely at the chart. Look at the shades of red, green or blue shown at the primaries for the REC-709 triangle. Now compare these with the shades shown at the primaries for the much larger F65 and F55 primaries. Is there much difference? Well no, not really. Can you figure out why there is so little difference?

Think about it for a moment, what type of display device are you looking at this chart on? It’s most likely a computer display of some kind and the Gamut of most computer displays is the same size as that of REC-709. So given that the display device your looking at the chart on can’t actually show any of the extended colours outside of the grey triangle anyway, is it really any surprise that you can’t see much of a difference between the 709 primaries and the F65 and F55 primaries. That’s the problems with charts like this, they don’t really tell you everything  that’s going on. It does however tell us some things. Lets have a look at another chart:

Gamuts-1024x632 What is a Gamut or Color Space and why do I need to know about it?
SGamuts Compared.

This chart is similar to the first one we looked at, but without the pretty colours. Blue is bottom left, Red is to the right and green top left.

What we are interested in here is the relationship between the different colour space triangles.  Using the REC-709 triangle as our reference (as that’s the type of display most TV and video productions will be shown on) look at how S-Gamut and S-Gamut3 is much larger than 709. So S-Gamut will be able to record deeper, richer colours than 709 can ever hope to show. In addition, also note how S-Gamut isn’t just a bigger triangle, but it’s also twisted and distorted relative to 709. This is really important.

You may also want to refer to the top diagram as well as I do my best to explain this. The center of the overall gamut is white. As you draw a line out from the center towards the colour spaces primary the colour becomes more saturated (vivid). The position of the primary determines the exact hue or tone represented. Lets just consider green for the moment and lets pretend we are shooting a shot with 3 green apples. These apples have different amounts of green. The most vivid of the 3 apples has 8/10ths of what we can possibly see, the middle one 6/10ths and the least colourful one 4/10ths. The image below represents what the apples would look like to us if we saw them with our eyes.

apples1 What is a Gamut or Color Space and why do I need to know about it?
The apples as we would see them with our own eyes.

If we were shooting with a camera designed to match the 709 display specification, which is often a good idea as we want the colours to look right on the TV, the the greenest, deepest green we can capture is the 709 green primary. lets consider the 709 green primary to be 6/10ths with 10/10ths  being the greenest thing a human being can see. 6/10ths green will be recorded at our peak green recording level so that when we play back on a 709 TV it will display the greenest the most intense green that the display panel is capable of.  So if we shoot the apples with a 709 compatible camera, 6/10ths green will be recorded at 100% as this is the richest green we can record (these are not real levels, I’m just using them to illustrate the principles involved) and this below is what the apples would look like on the TV screen.

apples3 What is a Gamut or Color Space and why do I need to know about it?
6/10ths Green and above recorded at 100% (our imaginary rec-709)

So that’s rec-709, our 6/10ths green apple recorded at 100%. Everything above 6/10 will also be 100% so the 8/10th and 6/10ths green apples will look more or less the same.

What happens then if we record with a bigger Gamut. Lets say that the green primary for S-Gamut is 8/10ths of visible green. Now when recording this more vibrant 8/10ths green in S-Gamut it will be recorded at 100% because this is the most vibrant green that S-Gamut can record and everything less than 8/10 will be recorded at a lower percentage.

But what happens if we play back S-Gamut on a 709 display? Well when the 709 display sees that 100% signal it will show 6/10ths green, a paler less vibrant shade of green than the 8/10ths shade the camera captured because 6/10ths is the most vibrant green the display is capable of. All of our colours will be paler and less rich than they should be.

apples4 What is a Gamut or Color Space and why do I need to know about it?
The apples recorded using a big gamut but displayed using 709 gamut.

So that’s the first issue when shooting with a larger colour Gamut than the Gamut of the display device, the saturation will be incorrect, a dark green apple will be pale green. OK, that doesn’t sound like too big a problem, why don’t we just boost the saturation of the image in post production? Well if the display is already showing our 100% green S-Gamut signal at the maximum it can show (6/10ths for Rec-709) then boosting the saturation won’t help colours that are already at the limit of what the display can show simply because it isn’t capable of showing them any greener than they already look. Boosting the saturation will make those colours not at the limit of the display technology richer, but those already at the limit won’t get any more colourful. So as we boost the saturation any pale green apples become greener while the deep green apples stay the same so we loose colour contrast between the pale and deep green apples. The end result is an image that doesn’t really look any different that it would have done if shot in Rec-709.

apples31 What is a Gamut or Color Space and why do I need to know about it?
Saturation boosted S-Gamut looks little different to 709 original.
gamuts-269x300 What is a Gamut or Color Space and why do I need to know about it?
Sony colour gamuts.

But, it’s even worse that just a difference to the saturation. Look at the triangles again  and compare 709 with S-Gamut. Look at how much more green there is within the S-Gamut colour soace than the 709 colour space compared to red or blue.  So what do you think will happen if we try to take that S-Gamut range and squeeze it in to the 709 range? Well there will be a distinct colour shift towards green as we have a greater percentage of green in S-Gamut than we should have in Rec-709 and that will generate a noticeable colour shift and the skewing of colours.

apples5 What is a Gamut or Color Space and why do I need to know about it?
Squeezing S-Gamut into 709 will result in a colour shift.

This is where Sony have been very clever with S-Gamut3. If you do take S-Gamut and squeeze it in to 709 then you will see a colour shift (as well as the saturation shift discussed earlier). But with S-Gamut3 Sony have altered the colour sampling within the colour space so that there is a better match between 709 and S-Gamut3. This means that when you squeeze S-Gamut3 into 709 there is virtually no colour shift. However S-Gamut3 is still a very big colour space so to correctly use it in a 709 environment you really need to use a Look Up Table (LUT) to re-map it into the smaller space without an appreciable saturation loss, mapping the colours in such a way that a dark green apple will still look darker green than a light green apple but keeping within the boundaries of what a 709 display can show.

Taking this one step further, realising that there are very few, if any display devices that can actually show a gamut as large as S-Gamut or S-Gamut3, Sony have developed a smaller Gamut known as S-Gamut3.cine that is a subset of S-Gamut3.

The benefit of this smaller gamut is that the red green and blue ratios are very close to 709. If you look at the triangles you can see that S-Gamut3.cine is more or less just a larger version of the 709 triangle. This means that colours shifts are almost totally eliminated making this gammut much easier to work with in post production. It’s still a large gamut, bigger than the DCI-P3 specification for digital cinema, so it still has a bigger colour range than we can ever normally hope to see, but as it is better aligned to both P3 and rec-709 colourists will find it much easier to work with. For productions that will end up as DCI-P3 a slight saturation boost is all that will be needed in many cases.

So as you can see, having a huge Gamut may not always be beneficial as often we don’t have any way to show it and simply adding more saturation to a seemingly de-saturated big gamut image may actually reduce the colour contrast as our already fully saturated objects, limited by what a 709 display can show, can’t get any more saturated. In addition a gamut such as S-Gamut that has a very different ratio of R, G and B to that of 709 will introduce colour shifts if it isn’t correctly re-mapped. This is why Sony developed S-Gamut3.cine, a big but not excessively large colour space that lines up well with both DCI-P3 and Rec-709 and is thus easier to handle in post production.

21 thoughts on “What is a Gamut or Color Space and why do I need to know about it?”

  1. Thanks for the comprehensive info.
    I have a question on Gamuts for Slog2:
    Do know any Sony Slog2 LUTS that convert from SLog2 gamma, but Normal/rec709 colorspace to rec709 gamma and colorspace.
    Our cameracrews have shot Slog2 footage but in Normal/rec709 colorspace.
    All Sony provided Luts are from Sgamut-colorspace to rec709. So when I apply these luts in Avid the colors become oversaturated and shifted.

    1. No, there are no LUT’s for this as shooting this way will almost certainly result in colour shifts during grading as you will be making large contrast and level shifts to the log luma while the 709 colour does not need or want these shifts.

      1. Thanx for a great article! Just to be sure. When I shoot slog2 and wish to use a LUT, I need to use the original s-gamut-setting and not the S-gamut.3 or S-Gamut3.cine?
        Thank you
        Søren

  2. Great info, thank you. It explains how mistakes may occur through mapping problems. It does not explain why a bigger gamut is used when the final result will be in a 709 space anyway.
    More possibilities in post…
    Could elaborate on that?

    1. The camera can differentiate between more shades so the colourist can work with those additional shades either together or in isolation.

  3. Great info! I was wondering if you know if the Sony A7s is true S-gamut or just a replication of it? Is it safe to say that if i shoot s log2 with s gamut on the a7s, then bring it into post, balance the rgb, and then apply a VisionColor film lut, that is a good workflow? The film luts are effectively bringing the log profile into rec 709 color space and luminance correct? Sorry for the million questions, any info you have would be greatly appreciated! Thanks!

    1. If the LUT’s are designed for S-Log2 then it should be good. The camera has to use S-Gammut as all S-Log2 LUT’s are normally made specifically for S-Gamut. However I’m not sure that the sensors gammut is large enough to fully fill the recording gammut.

      1. A quote from the Sony a7s help guide page 129 “S-Gamut is a color space unique to Sony that provides a wide color space equivalent to film cameras. However, S-Gamut setting of this camera does not support the whole color space of S-Gamut; it is a setting to realize a color reproduction equivalent to S-Gamut.”

  4. Thank you for great info, now I finally have a little understanding of what all these weird terms actually mean!

    Clearly this is aimed at already at least semi-professional cinematographers and colorists. I’m however completely new to shooting raw footage (or s-log and the like) and a total noob when it comes to grading – but that’s gotta change (hence, I’m reading your post).

    Reading this, although refreshing to finally learn what these things mean, I kind of come to the frustrating conclusion that it’s a dead end street.
    What’s the solution?
    What are we supposed to do with our s-gamut material?

    -We can’t up the saturation cause the primaries are maxed out and it will only raise the mid-levels (effectively reducing contrast/range – which defeats the whole purpose of shooting in this high DR in the first place!)
    -We can’t squeeze the s-gamut’s range into R709 cause it will skew or push all colors so they turn out wrong

    So what to do?
    I’m not trying to become a proffesional colorist. And I’m not trying to make Michael Bay-movies here. I just want to learn some simple steps to bring out some good images from my a7s. Otherwise I can just go back and shoot video-y videos on my 5d… and shoot myself while I’m at it

    /Sebastian
    grateful and frustrated amateur filmmaker

    1. It’s far from a dead end. By using a LUT you can move from SGamut to 709 adjusting the primaries so that the color shifts are taken care of. You can increase the saturation and while some colors beyond the 709 range may be lost, you will still in most cases end up with a better image than if you capture in 709. Currently 709 does limit what we can display but HDR is just around the corner (amazon have said they will start streaming HDR this year).
      The key thing though is that by shooting with a large range we can then take that in to the grading suite and fine tune exactly what we extract for our limited 709 viewing range.
      It’s no different to taking photographs using raw to capture the best possible “digital negative”, then adjusting that data to give the best possible print. Photographers have been doing this kind of thing for years, but it’s only now just becoming easy to do with video. While SGamut/S-Log is not raw, it does offer most of the same adjustment possibilities when shot correctly.

  5. Thanks for such clear and informative articles, Alister. I think the work you do is so important, especially with so much miss-information flying around the web.

    Despite reading and re-reading your articles, I am still struggling with the S-gamut colour space. Having shot some test clips in S-Log/S-Gamut, applied a small amount of colour correction (the picture was slightly warm due to our off-white walls), then applying a LUT made for S-Log (I’ve tried yours, color grading central, and the default 1D LUT that comes with DaVinci Resolve) but I’m still finding some of the colours to be incorrect. I noticed it on my wife’s turquoise jacket. In the footage it comes out too blue, when it should be turquoise and I have to apply an additional Hue vs. Hue shift to change the colour of the Jacket alone. I kind-of remedied the situation by changing the colour mode to Pro in PP7, but then ran into problems when I tried to push the luma around in post, the colours in Pro just weren’t up to it like you mention here. Do you have any suggestions?

  6. All I can say is thank you very much for your articles. I just came across them and they are fantastic. Question though… I get what you’re saying regarding the S-Gamuts and REC 709 in regards to the greens and color shift. This explains the green tint in all my footage when shooting Cine4/S-Gamut.

    My question is, if I intend to use LUTs to grade my footage, either in Cine4, or SLog2, is there a reason to use anything other than S-Gamut3.cine since the color is improved? Or am I getting this backwards, since LUTs are designed to be used with the S-Gamut color space, does this effectively handle the green tint?

    I’ve done a comparison with SLog2 and a LUT, and while the S-Gamut footage does not look green tinted, the S-Gamut3.cine has slightly more magenta in the image, not necessarily in an unpleasing way, just a bit more magenta.

    I guess what I’ve taken from all of this is that S-Gamut3.cine is superior. Is it a waste of time using any other color space since nearly all monitors and TV essentially display REC709?

    I’m still trying to digest the contents of this article. Thank you again sir.

    1. S-Gamut3.cine is all you need with the F5, FS5 or FS7 as these cameras cannot capture more than this due to sensor limitations. There is zero benefit from trying to record a larger color space as you will just be creating a lot of empty data.

  7. Hi , myself Tany Chako working as a freelance D.I Colorist. I don’t have proper calibrated monitor. I use to work in my HP23Fi monitor . How could I set a proper
    Gama level while working on a cinema for theater release.

    1. You can’t. You need a monitor that supports P3 gamma and colorspace.

      Instead use DaVinci Resolve and use the Resolve color managed project settings. Set the color management to Rec-709 and grade as normal. Once you are finished change the colour management to DCI-P3 and export the file. It will look odd on your monitor but should look correct in the cinema. It’s not the best way to do it, but better than guessing.

  8. So on the A7s, would it be “ok” to shoot in S-Log2 and the Cinema (rec709) colour space?

    Would I then just use a LUT for rec709?

    I’m only now understanding that with this set up, you can shoot in Log but still be in rec709, avoiding the complications of S-gamut.

    Thanks

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