Thread: The Reference Colour Space

So that accurate image colour relationships can be maintained as image data moves from capture (input) to an edited image ready for reproduction (output), image editing software uses an internal colour space as a reference for colour space translation calculations. This intermediate reference space is used because this minimises the number of device profiles and transforms required to translate from any input to any output.

Gradually over the years reference colour spaces and colour conversion transforms have moved towards better representations of perceptual linearity and towards greater accu¬racy. Perceptual linearity means that the indi¬vidual coordinates that represent each of the measured colour shades the average human can discriminate are equi-distantly spaced within the colour space volume. As with all digital representations colour volumes are quantised, in this case into a number of points on a 3D grid. The spacing of the grid is deter¬mined by the bit depth used to represent it. For example if eight bit values are used for the height width and depth of a cube the cube will have 256 divisions on each axis and contain 16,777,216 equi-distant defined points. If the colour coordinates of all of the discernible shades coincided exactly with these grid points, 8 bit coordinates would be sufficiently accurate. However depending on the colour space used they may not be.



As described in a previous feature the earli¬est colour model in use is the CIE x.YZ (1931) and this is based directly on measurements. The original measurements were made using a sample group of only seventeen people, although the results have since been confirmed using larger sample groups. Within the x.YZ colour space some areas of colour discrimination of the human visual system are packed closely together and other areas are widely spaced. For example the greens occupy a large portion of the space while yellows occupy only a small portion. Based on the original x.YZ measurements, CIE L'a'b' (1976) is one attempt to create a more perceptually linear colour space. Adobe products use CIE L'a'b' as their reference space while CorelDRAW Graphics Suite X4 uses sRGB.

The embedded colour management in Microsoft Windows from Windows 95 up to XP also uses the sRGB colour space for reference. This colour space was originally created by Microsoft and HP as a reference space for colour images on the world wide web. It is based on the typical device colour space for a CRT monitor. In this sense it is an idealised device dependant colour space and has quite a small gamut.