3.4. Colour spaces

Colour spaces usually either model the human vision system or describe device dependent colour appearances. Although there exist many different colour spaces for human vision, those standardized by the CIE (i.e. XYZ, CIE Lab and CIE Luv, see for example Wyszecki & Stiles 2000) have gained the greatest popularity. These colour spaces are device independent and should produce colour constancy, at least in principle. Among device dependent colour spaces are HSI, NCC rgbI and YIQ (see Appendix 1 for formulae). The different versions of HS-spaces (HSI, HSV, Fleck HS and HSB) are related to the human vision system; they describe the colours in a way that is intuitive to humans.

Usually the output from CCD element is expressed as RGB values or corresponding values. This can be understood as a basic colour space from which the values are converted to the other device colour spaces. The RGB values are redundant and intensity dependent. Therefore, in many device colour spaces the intensity is separated from the chrominances. Use of only chrominance values offers robustness against changes in illumination intensity both in the time and spatial domains. A disadvantage is the loss of information related to different intensity levels of the same chrominance; in other words, for example black, grey and white cannot be separated by using only chromaticity values. It is interesting to note while the intensity may be the most significant feature in segmentation (Ohta et al. 1980), it is also the most sensitive component to changes in practical imaging conditions.

The values of device colour spaces can be converted to the corresponding values of a human colour space. For example, this transformation can be made by first selecting representative samples and calculating the transform matrix from them or by using the samples to train a neural network. The transform can be non-linear. The inputs (i.e. RGB) do not necessary have to be a 3x3 matrix; their values can be also obtained using polynomials with different degrees of polynomial. However, the created transform function depends heavily on the illumination conditions under which it was made. Therefore, the transform to human colour space still does not solve the colour constancy problem but alleviates the device dependency problem.