Q100328: What is a Colorspace?

Peter Coleman

Updated : October 16, 2024 14:43

Created : March 14, 2017 17:53

SUMMARY

This article is a high level overview of what a colorspace is, and the components which make up a colorspace.

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What is a colorspace?

A colorspace is a set of parameters which define the color capabilities of a particular device or stored in a digital file.
According to the ISO 22028-1 Standard, an RGB colourspace is defined by three 3 components:

Primaries - 3 points indicating the chromaticity values for Red, Green and Blue
Whitepoint - a point defining the measurement of "white" in a colorspace, generally where R=G=B
Transfer Functions - a function, or set of functions which related the encoded color values to real world light values

Each monitor and file can display / store a certain range of colors, which is known as its color gamut.

For example if you have two displays with different colorspaces, and you look at the same file on different devices, the output from the displays will be different as one device may not be physically capable to displaying the same colors as another display.

Using specialized hardware, you measure the various aspects which make up a displays output that can be simplified to its red output, its green output, its blue output, known as its color primaries, and its white point which is the point of the purest and lightest white in a colorspace.

However, just having these values doesn't help much as they can all be arbitrary depending on the hardware used to measure them, so standards are used to make all the values comparable.

The most common standard is the CIE 1931 Chromaticity Diagrams and more information about this can be found here: Q100330: Generating chromaticity diagrams

This CIE 1931 Chromaticity standard diagram is the most commonly used standard to compare colorspaces of displays and files for the human eye. Each colorspace will cover a range from this diagram (it's gamut), with some being bigger that the CIE range. However, using colors outside of this range have very little value as the average human eye cannot see those colors.

The image below shows some examples of different colorspaces when compared to the CIE 1931 Chromaticity standard, such as ACES2065-1, ACEScg, DCI-P3Rec, Rec 709 and Rec 2020:

The points of the triangles for these colorspaces are defined by the color primaries and the dot in the middle of the triangle is the white point.

For example, the Rec709 primaries (1.3), white point (1.4) and transfer function (1.2) as defined by the ITU-R Bt.709-6 are:

The source of the table above can be found in the ITU-R documentation.

These colorspaces are then used to calculate how every value of RGB should look like to the average human eye.

Why are there so many colorspaces?

Colorspaces are standards which when they have been defined, cannot be changed. So sRGB, for example, was developed by HP and Microsoft for use on CRT monitors, printing and on the internet, and as such has a range of values that meet what a CRT monitor can output.

However as technologies develop, users need access to a wider range of color values so newer standards are developed to encompass those requirements, such as Rec2020 for HDR content.

Other colorspaces are defined by the physical attributes of the recording devices, so RED for example have a range of colorspaces for their various camera models depending on the capabilities of the cameras hardware.

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