How DCP Colour Space Works – Part 1 of 3

The camera is an artist painting a picture into its memory of what it sees through its lens. And the colour space is its paint pallet.

In this diagram, the big colourful horseshoe represents all the colours the human eye can see. And the triangles in the middle represent different colour spaces used by today’s cameras. As you can see, we still have some way to go to be able to record all the colours our eyes are capable of seeing… Or our brains… Or our consciousness…. Or maybe our soul? What is a colour anyway? Okay, we’ll save that for another discussion. 

Anyway, the most common colour space we see used today is our faithful friend Rec.709. With more high-end productions using P3 and even Rec2020.

But, just to confuse things, there is also this term “colour gamut”. And it shouldn’t be confused with colour space. They’re the same thing in different contexts. Colour space is what we call the pallet when we record colours. And colour gamut is what we call the pallet when we display colours. So, whilst a film may have been recorded in Rec.2020, the projector in the cinema will have a P3 gamut. Or a TV at home may have a Rec.709 gamut. And your computer monitor may have an SRGB gamut. That’s why it’s really tricky making sure films are displayed correctly on different devices. And it’s why you need a good encoder to make sense of it all!

The vast majority of display devices we have are yet to progress beyond P3. So it’s kinda weird that cameras are so far ahead of displays. There are cameras that can record in Rec.2020, but, at the time or writing this, the only technology I know of that can reproduce Rec.2020 is the Christie RGB Pure Laser projector. And these are still rare birds indeed. Still, when we’re encoding, we have to consider the future. Just because most displays are limited to P3 gamuts today, we don’t want to have to come up with a new distribution format every time a new colour space is proposed. And DCPs, awesomely, are completely future proof. And it’s all down to a colour space from the 1930s called “CIE 1931 XYZ” that DCP colour information is stored as.

XYZ is the king of colour spaces because all other colour spaces can fit within it. With it, you can plot any colour in 3D space. This is useful for complex mathematical reasons that are beyond me. But it’s also super useful because it’s infinite. The graph has no boundaries. Which means you can describe any colour that exists.  Which means, since all properly made DCPs store colour information as XYZ, you can have a REC2020 DCP. And the DCP format will accommodate any and all future colour spaces. And this is why DCP is considered one of the most future proof of all the distribution formats.

I should mention that today, there are no Rec.2020 DCPs on general release. They are purely experimental. But hopefully that will change soon.

So, as you can imagine, Encoders have a tricky time handling colour space. We have to make sure a REC709 DCP will display correctly on a projector with a P3 gamut. We have to consider lots of other things like the gamma curve, whether it’s legal or full range, the white point, whether the colour information is stored as RGB or YUV. And then we have to express this in CIE 1931 XYZ. Cheap DCP tools give little to no control over these parameters and that’s why the results are often found wanting. And it’s why a good encoder with good kit (I’m a big fan of Clipster) is essential to make sure a film is reproduced correctly on the big screen.

Still to come, we’re going to talk about video signals, bit depth and more. Stay tuned.

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