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Determining Color Difference Using the CIELAB Model?
Colour Blind asks: "I am working for a not-for-profit organization developing a website for kids. I am attempting to develop a method for testing if two colours (as defined by R, G, and B values [0-255]) are adequately different to be visible on top of each other. So far I have tried many things but this is the one that, by all accounts, should work: I have converted from RGB to (CIE)XYZ using a 3x3 matrix transformation. From here I have used three more equations to convert to CIELAB colour. I have then calculated the distance between the two colours in question in CIELAB colour space. The results are not correct: there are pairs of colours that are quite far from visible that yield the same difference as colours that are plainly acceptable for visibility. Any suggestions?"
A friend of mine wrote a paper on this topic:
Limitations of Colour Management.
In addition, it sounds like you're hoping to test whether things are sufficiently perceptually different on people's monitors. The sad news is that the variation between different monitors, between LCDs and CRTs, between different brightness and contrast settings, between different phosphor technologies, differences in how long the monitor has been warmed up, and differences in the aging of the phosphors mean that no two monitors will actually produce the same color from the same R, G and B equivalents, and you'll get different distinguishabilities for different colors on different monitors.
As a nature photographer, I have to jump through hoops, including hardware sensors for detecting the output of my monitor, to get anything like reproducable color out of my own equipment. It's just a difficult problem, I'm afraid.
I'm a nature photographer.
- The CIE color space: A pretty decent introduction to what the CIE color space is
- Color FAQ: I haven't read through this, but it seems to be a more extensive coverage of color and how it's much more than RGB, HSV, or CYMK.
The short version is that all the different primary color systems--RGB (red-green-blue), CYMK (cyan-yellow-magenta-black), HSV (hue-saturation-value)--can represent some, but not all, of the colors visible to the human eye. Even specifying colors by the wavelength of the light emitted or reflected covers only a small subset of colors--in fact an even smaller subset than any of the primary color representations. The CIE system identifies colors by an XYZ coordinate system, where X, Y, and Z are artificial primary colors that span the full range of colors visible to the human eye."It take 9 months to bear a child, no matter how many women you assign to the job."
I have to agree with this. I am the author of Text Mode Doom and I hit a problem along these lines during development: the RGB value itself is not particularly useful when trying to find a numerical value for a particular "colour" that the human eye perceives. In text mode doom I was faced with the problem of mapping the particular colour value to a corresponding text colour, of which I was limited to a small value (under 16). I solved the problem by converting the RGB values into HSV and then assigning text colours to the real colours based on their hue. The results I found with this were quite impressive
were you expecting to see a sig here? perhaps you'd rather see the inside of an ambulance!
Hear hear!
This is a complex problem and I think a lot of the answers I'm reading on this topic center around a misconception on a lot of people's part about the difference in luminance perception and chrominance perception. The key issue is:
The eye is most sensitive to luminance changes in GREEN.
The eye is most sensitive to chrominance changes in BLUE.
Thus if you are trying to determine at what point the eye will say "hey there is a block of a different color on top of that one", blue is going to be an important part of your color model since while your eye isn't so good at picking up how bright a blue is, it is very sensitive to the relative shade around the blue wavelength something is.
So if you are trying to do a 3-space transform and distance and finding that colors which are linearly close to each other in CIELAB space are perceptually very different, it is very likely because it is a color in a space where the human eye is more perceptually sensitive. You are going to get "dead areas" of the model where the eye is not so good at viewing differences, and active areas where in a small space there seem to be quite a few different colors because the eye is perceptually sensitive in that area.
This is the reason you see odd patterns in color changes in a rainbow. You are seeing the relative sensitivities of your eye to pick up colors. Color perception is decidely non-linear and doesn't even fit a nice equation.
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