Quirks in Blue

Tenth in a series of blog entries about color theory with live help from the ColorTheory (Step 10) .  First_post,  Prev_post, Next_post

One of the annoying things I discovered as I explored the various color options (several of which are in the Color Theory application), was that the color “blue” always kept popping up as a problem.

For example, in trying to map the hues in the color wheel according to the CAM02 model, the math started to fail right around hue angle = 240, pure blue in the sRGB model. In the color wheel you can see a definite strong edge right at that point.

CAM02 hue as a function of sRGB Hue

We can look directly at the hue angles in the figure to the left- the RGB hue angle is across the bottom, and we see that the CAM02 hue angle not just stops, it reverse as we approach RGB blue.

Rim of RGB gamut in CAM02 Oppoenent Processes

We can plot this in terms of the red/green, yellow/blue opponent processes as used by CAM02 as well. We can plot the rim of the RGB color wheel (which is better described as a hexagon). As we see in the figure to the left, there is a definite “horn” right at that corner of the RGB.

Of course, CAM02 is not the only uniform perception model, and maybe this is just a “quirk” in that specific model. So let’s look at another unform color space, the Uniform Color System developed by the Optical Society of America (OSA-UCS). This is an interesting model in that it was specifically developed for the purpose of determining a perceptually uniform distribution of the colors. Unfortunately for my use here, it’s got two quirks that make it more difficult to use that the CAM02 model- it uses a different set of fundamental color observers (the 1961 10 deg CIS observer models) so that it can’t be computed from the XYZ or the RGB color models without reference to the actual paint chips involved, and it is difficult mathematically to invert it. But it makes for good test set to evaluate more standard models.

Rim of RGB gamut in extrapolated OSA UCS

At any rate, there is data on the RGB values for the colors used in the OCA UCS model, and between that and a least square fit to approximately map it to RGB space, we can look at the RGB colorwheel rim in its coordinates. And there we discover an even bigger “horn” than in the CAM02 model.

OSA UCS opponent color range

(This isn’t a completely fair graph- the actual colors used to define the OSA-UCS weren’t extended to the complete RGB gamut as seen in the following graph.)

Sharpman-Stock Cone Fundamentals

We can look at the fundamental cone response curves for an explanation. At “perfect” blue in the RGB model, the green and red “lamps” are completely off. As the hue shifts to either side, we turn on either the green or the red lamp. But it is a misnomer to consider these as pure red or green in the cone responses- the red and green responses are very close to each other. So the red light triggers green cone response, and the green light triggers some red cone response. The blue response is far from each. So the eye can detect that there is something other than blue being added, but it can’t tell if the added light is red, green, or grey.

palette, label, x, y, clr, note, ...
quirk, palette1pat1, 132, 41, #0401fc, "center", ...
quirk, palette1pat2, 132, 77, #2e2ee6, "below", ...
quirk, palette1pat3, 132, 173, #ecf005, "yell, center", ...
quirk, palette1pat4, 196, 77, #2b01fc, "right", ...
quirk, palette1pat5, 68, 77, #012bfc, "left", ...
quirk, palette1pat6, 68, 209, #fcd201, "yell left", ...
quirk, palette1pat7, 132, 209, #e6e62e, "yell below", ...
quirk, palette1pat8, 196, 209, #c9f005, "yell, right", ...
quirk, palette1pat9, 260, 77, #5501fc, "far right", ...
quirk, palette1pat10, 4, 77, #0155fc, "far left", ...
quirk, palette1pat11, 132, 113, #5252cd, "far below", ...

The bottom shows a pure yellow, followed by a trio of yellow plus red, yellow plus white, and yellow plus green. The top shows the same for blue, but it also show the next step out as well, because it remains very difficult to tell which side is adding green, and which side is adding red.

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