re colour transform for red green colour blindness in article geoffw extro ucc su oz au geoffw extro ucc su oz au geoffrey warren hicks writes posted on behalf of craig southeren who does not have access to a net account can anyone provide me with a colour space transform that would emulate the effects of red green colour blindness i have had a quick look in the computer graphics literature with no results perhaps someone who has access to medical information could help certainly reference follows bear in mind that there are at least two conditions which may be called red green colour blind one protanopia is caused by a lack or major dysfunction of the l cones those that respond best to long wavelengths this gives much reduced red green and red violet discrimination and also means that reds look dimmer than than they do to people with normal vision the other deuteranopia is due to missing or dysfunctional m cones although it can also be caused by a lack of the l m cone difference signals in the retina it also gives reduced red green discrimination but red violet is unaffected unlike protanopia reds are not dimmer than normal on a uniform chromaticity diagram the horseshoe shaped one you see in computer graphics books there is a single confusion point for each type of colour deficiency colours which are confused lie on straight lines radiating from this point for protanopia the point is at u v very close to the far red corner for deuteranopia the point is at u v note that different experimental investigations in the literature give slightly different values for these depending on the precise details of the experimental setup random errors and so on the values quoted are typical so if all colours on a line are seen as the same which colour is actually seen this problem has now been solved tests on people with one normal eye and one affected eye have shown that there is an axis for each type of disorder onto which all the colours collapse for protanopes this joins up nm and nm on the spectral locus the edge of the horseshoe for deuteranopes the line is very similar joining nm and nm so to convert colours from normal vision to a simulated protanopia or duuteranopia plot the colour on a uniform chromaticity diagram construct a line from this point to the appropriate confusion point find the intersection of this line with the appropriate axis line this is the new chromaticity the best reference for this is meyer g w greenberg d p colour defective vision and computer graphics displays ieee computer graphics and applications you will also need a decent reference on basic colour science if you are not familiar with the cie xyz colour model and the uniform chromaticity scale diagram i would like to write a program probably a ppm filter to allow previewing of images to check for sufficient contract for colour blind users not being colour blind myself this is a bit difficult yes this is certainly possible you will need the chromaticities of the red green and blue phosphors of the monitor you intend to display the images on and the chromaticity of the white point this information can be measured or obtained from the manufacturer i posted a list of some monitor chromaticities a couple of weeks ago the procedure for each pixel is as follows some lookup tables might be a help here a convert rgb to cie xyz this is a simple matrix multiplication once you have the monitor data b retaining the y component for later convert xyz to chromaticity coordinates u v using u x x y z v y x y z c find the equation of the line as in step above d find the intersection as in step e convert back from u v to xyz using the y value from step a f ensure that this new colour can be displayed on your monitor if not move it along a line in xyz space towards neutral grey ie half way between black and white until it is displayable please email replies i will summarise and repost if there is sufficient interest done also posted for the rest of us chris lilley technical author itti computer graphics and visualisation training project computer graphics unit manchester computing centre oxford road manchester uk m pl internet c c lilley mcc ac uk voice fax janet c c lilley uk ac mcc
