APPENDIX. A computer model of the Land Mondrian retinex experiment.

In the Land Mondrian retinex experiment (Land, 1974) a multicoloured abstract scene consisting of rectangular patches is illuminated using three projectors. Each projector is fitted with a coloured filter which passes either predominantly longwave, medium-wave or short-wave light. The intensity of light from each projector may be independently varied. Constructing a computerised version of this experiment requires two steps; firstly, the colour of each patch must be calculated given the intensities of the three projectors and secondly that colour must be emulated on the graphics output device of the computer (cathode ray tube (CRT) or liquid crystal display (LCD) projector). XYZ values for each of the three illuminators (long, medium and short-wave) can be calculated separately and then added together to give a combined XYZ for the patch. Using this method the XYZ tristimulus values for any patch can be calculated for any combination of intensities of the three illuminators.