The Geometry of Color Perception

Exactly right. It has to be adjacent to both blue and red, because it’s the middle of the gradient mixture of blue and red. We don’t have any other mixture gradients on the color wheel. Do you see the problem yet? What if we did have other mixture gradients?

> He didn’t say it has nothing to do with it

Hmm, that’s not my read, it was stated multiple times and quite clearly and flatly that the left hump is “not” the reason, and that nothing changes if it didn’t exist. The top of the thread declared unambiguous disagreement.

> All these entirely different ways of laying out the colors in the article seem to put purple and red a lot closer than green and red.

These are abstract organizational tools you’re referring to, they do not prove anything about the proximity of purple and red. On the color wheel, red and blue are placed just as far apart as red and green (even though we know they are different perceptual distances). Since purple is a red-blue mix, and therefore in between the two at the midpoint, then it must be closer to red than green is to red, by color wheel definition. So your point about green is therefore tautological and uninteresting, if we’re talking about color wheels.

BTW, there’s a pretty important subtlety you seem to have missed in my argument. I am not claiming that purple and red aren’t somehow close. The actual claim of this thread being debated is that left hump is not involved (splitting hairs between zero and insignificant is irrelevant in my view). I’m asking for a definition of closeness, a metric that proves it, because I believe that whether the left-hump matters may hinge on such a definition. Rob’s argument depends on how you define closeness. He is right within his definition of closeness based on the color wheel, red is close to purple, the problem is the circular logic of this definition, it means the question of whether red is close to purple is a basically meaningless question, another uninteresting tautology that dodges explaining why red and purple might seem close (or more importantly why red and violet might seem close.)

> if the opponent process is relatively accurate for how vision works, then the signal from the blue cones is likely factored in to the “red/green” signal and tilt is it red at very short wavelengths independent from any bump coming from the red cone itself.

Interesting thought! I’ll have to think about it. What if it’s not independent, but two sides of the same effect? Maybe you’re right, and the left hump is caused by the mechanisms people call opponent process. This still begs the question even more acutely of how you would define color proximity. Opponent process color theory points out that we can see completely non-physical colors, colors that don’t exist in nature and cannot be displayed, like red-blue. How far apart is opponent red-blue from the purple that is red+blue mix?

My reading is that Rob’s argument tosses your opponent process theory out as well, right? He already claimed multiple times that non-zero perception of red in blue colors is not the reason that purple is close to red. That’s the problem I have with Rob’s argument, that he ruled out perception of red from blue colors, and just cited the color wheel, without even stopping to ponder the question of whether the color wheel is the way it is because of the left hump.