The term ‘color space’ is often thrown around, the most familiar being Rec709, but what is a video color space? Let’s go over the fundamentals.
Rec709 is just one of a few common video color spaces that are important to know.
I’ve been writing a lot lately over at cinema5d.com on color management starting with an explanation of LUTs, What Exactly Is A LUT? and now going onto professional color management and leading on to The ACES Workflow (Academy Color Encoding System).
I want to take a closer look at the most important color spaces and when they should be used.
First of all, we need to take a look at what a color space is.
The CIE 1931 XYZ Color Space
In 1931 the ‘International Commission on Illumination’ (CIE) created the CIE 1931 RGB and CIE 1931 XYZ color spaces which for the first time linked the pure spectrum of color (the actual electromagnetic wavelengths of the visible spectrum) with the human perception of color.
Looking at the chromaticity diagram you can see the wavelengths of light (in nanometers) indicated around the outer boundary. The boundary defines the maximum saturation.
In this diagram you can also see an arc which defines the chromaticity of perfect black body light sources of various temperatures marked in degrees Kelvin.
So simply put the CIE XYZ color space defines the full extent of every color human vision can perceive. That’s all you need to remember.
The video technology we all know and love has always been limited to far less than the full spectrum of visible color.
So, a ‘color space’ as we typically use it refers to an accepted standard defining a limited set of colors that a particular technology is able to describe, with Red, Green and Blue extents (primary chromaticities) that are mapped to sit inside the full CIE XYZ space.
The space within the full CIE XYZ space that a particular color space covers is called its Gamut.
For example here you can see the Rec709 color space mapped to CIE XYZ. Rec709 is the standard for HDTV and is basically the same as sRGB which is the standard for computer displays, printers and the internet.
So a color space has a gamut defining the extents of the colors it can cover, and it also needs a white point. In the Rec709 example, the point marked D65 is a single point that actually lies on the arc in the first diagram, it represents white at a ‘daylight’ color temperature of 6500 degrees Kelvin.
A transfer function describes the relationship between the intensity of the primaries and the actual number stored to describe it. For Rec709 (and sRGB) this curve closely matches the gamma curve of a CRT display. This non-linear curve allows for a more efficient assignment of the values in relation to the perception of human vision.
Please check out Video Color Space | Choosing The Right Color Space for more!