Color space

The color range between specified references. Typically three reference points are quoted as in television is could be RGB, Y R-Y B-Y, or Hue, Saturation and Luminance (HSL). These are all color spaces. In print, typically Cyan, Magenta, Yellow and Black (CMYK) are used. Film is RGB while digital cinema uses X´Y´Z´. Pictures can be moved between these color spaces but it requires careful attention to the accuracy of the involved processing. Operating across the media – in print, film and TV, as well as between computers and television – requires color-space conversions to display the correct colors everywhere.

Electronic light sensors detect red, blue and green (RGB) light but TV signals are usually changed into Y, R-Y and B-Y components as, or very soon after they enter the electronic realm via camera, scanner or telecine. There is some discussion about which color space is best for post production – the most critical operation being keying. However, with most video storage and infrastructure being component-based, the full RGB signal is usually not available, so any of its advantages can be hard to realize for television-based productions. However, in the Digital Intermediate (DI) process, where movie footage undergoes ‘post production’, RGB color space predominates.

With the increasing use of disk storage, networking able to carry RGB and digital cameras with RGB outputs, RGB infrastructure and operations are more widely used. Even so, RGB takes up 50 percent more storage and, for most productions, its benefits over component working are rarely noticed. One area that is fixed on RGB use is in 2K and 4K digital film (digital intermediates). Modern digital techniques allow the use of both RGB and Y R-Y B-Y to best suit the requirements of production as well as those of distribution and delivery.

More recently the world has become more complex for color, with a wide diversity of camera, display systems and methodologies for moving from one to the other. The CRT display set the colorimetric standard that still holds today even though CRTs are no longer used and other display technologies, LCD, plasma, micro-mirror and OLED, for example, each have their own characteristics for color and light output transforms.

In the television broadest description Color Space defines how all parts of the imaging chain respond to light or electrical drive. Cameras separate out the red, green and blue light using filters. The spectral response, in particular the passable bandwidth, of these filters control the margins of the range of colors that can be represented. These are usually represented in a diagram of the type shown below.

Color Space

The axes of this diagram are the xy of the color notation xyY where the Y component would represent the intensity. The human visible area is infilled with color and, on more detailed depictions of this diagram, show the wavelengths of light representing each color. Saturation increases towards the edge of the locus.

The corners of the triangles forming the ACES, DCI P3 and Rec.709 are at the coordinates of the primary colors used in these displays. Only colors inside the respective triangle can be reproduced by that display. You can see that no display technology covers the full human range for color and that, for example, full green on a Rec.709 monitor is not the same color as full green on the P3 display. Some colors in nature cannot be represented at their proper saturation using today’s imaging technology.

See also: 2K, Keying