hatcher@INGRES.BERKELEY.EDU.UUCP (03/12/87)
In article <505@ubu.warwick.UUCP> rolf@warwick.UUCP (Rolf Howarth) writes: >I thought I'd ask if anyone could explain to me how the human brain >perceives colour. >red + green light "gives you yellow", where "yellow" is also what you see >at a particular position in the spectrum (when you shine white light >through a prism). As far as I understand it, these two "yellows" are different >spectroscopically , yet the eye perceives them to be the same colour. The human eye has three kinds of color receptors; each one is sensitive to a range of colors but has a peak near the colors red, green, and blue, respectively. These are more accurately called long, medium, and short wavelength receptors (since there is less implication that they respond to only a single pure color). If a red receptor receives exactly two monochromatic frequencies (say from a controlled source like a laser) that cause it to produce a 1 picovolt signal, then the brain has no way of distinguishing that signal from the same red receptor stimulated with a single wavelength that is by itself strong enough to also produce a 1 picovolt signal. That particular example assumes that the wavelengths in question fall outside of the spectrum that the blue and green receptors respond to, to simplify the issue. In general, if a receptor receives a mix of frequencies that cause it to output a signal of strength N, then there will be an infinite number of other mixes of similar frequencies that also produce a signal of strength N. That's because it only gives output depending on overall signal strength, there is no way for it to figure out exactly which frequencies caused the stimulation. The cones that are used in human night vision behave the same way, but are receptive to a broader range of colors. Still, they only give out a signal that shows relative intensity. Thus colors fade as things get darker, and when it gets too dark to stimulate our color sensitive rods, we depend on our cones for vision. Since there's only one kind of cone, this gives us black and white images. They are more sensitive to dim light than rods because they respond to any visible color. So you get increased sensitivity at the expense of color resolution. Back to rods: each rod behaves like a cone, but only over a narrow range of frequencies. Thus the brain receives signals from the rods that indicate the relative intensity of light, as broken up into the categories red, green, and blue. The three types of rods are distributed around the retina somewhat the way that a color tv has triads of red, green, and blue phosphors. (The distribution of rods in the eye is not quite so geometrically perfect as a tv, for interesting reasons, but that's a different story) Imagine that a yellow image falls on the retina. It will stimulate both the green and red rods (since they are both somewhat sensitive to this wavelength, even though not as much as they are to green or red). Both types of rods will give an output signal to the brain in response. Now imagine that an image that is a mixture of green and red falls on the retina. The green stimulates the green-sensitive rod, and the red stimulates the red-sensitive rod. So again, both types of rods give an output signal. If the intensity of the green and red lights are at the right level, then these output signals will have exactly the same intensity that they did when the yellow light was shining on the retina. Given the same signal to the brain, the brain cannot tell the difference between these two scenarios. That's why a mixture of green and red looks yellow to a human being, despite the fact that a spectrometer will easily show the difference. You might think that we are missing out on a lot of interesting color perceptions because of this. After all, the ear's ability to hear chords of music is one of the things that makes music so interesting. A single voice melody is not as richly interesting as a whole orchestra. But while this seems logical in the abstract, it turns out that, in terms of what is available to be perceived in the natural environment, we aren't missing much. This is because of the nature of the types of reflective surfaces in the world. Any flower that reflects yellow light is pretty likely to reflect red and green, too. So even if we saw "chords" of color, the flower would still look yellow. If you do a 3D histogram of all the colors that appear in a digitized image of a natural scene, you will find that they all tend to clump around a plane running from one diagonal to the other, in a fairly even distribution. In order for "chords" of colors to really make a difference, you would need a distribution that had the same sharp boundaries and clusters as the notes in a symphonic score, and this just doesn't happen. Natural surfaces don't tend to behave that way. Thus our color vision is well suited to our natural environment. See: Human Color Vision, R.M. Boynton (1979) Visual Perception, T.N. Cornsweet (1970) Handbook of Perception, ed. J. Thomas (1986) Color Science, G. Wyszecki & W.S. Stiles (1982) References from Prof. Brian Wandell, Stanford Psych. dept. who was one of a panel of speakers on color perception at a recent SIGGRAPH local chapter meeting. As for your question about how a TV image might appear to someone with a color defect, yes, there are sometimes differences in perception. However, they are almost never as dramatic as the scenario you offer, because the human brain is extremely good at adapting to whatever input it has. After all, a banana still looks like a banana on a black and white TV (usually!). But some of the simpler tests for color blindness do involve multicolored images with controlled grey scale equivalences for the different colors. Anyone lacking a particular receptor will not be able to distinguish the corresponding color from the background. He will still SEE it, but it won't look different than the carefully-chosen background. It is uncommon to find natural images with this property, so people deficient in only one type of rod are often not really aware that their vision is different until they are tested, because their brain adapts so well. Doug Merritt
hatcher@INGRES.BERKELEY.EDU.UUCP (03/13/87)
In my posting explaining the basics of color perception, I consistently made the error of switching the terms "rod" and "cone" (my upcoming article on low-level brain damage will explain why). If you switch these terms throughout the article, it will be improved, and perhaps even correct. So: cones come in three color-sensitive flavors, while there is only one kind of rod which is used in dim lighting. Doug
michael@crlt.UUCP (Michael McClary) (03/16/87)
In article <505@ubu.warwick.UUCP> rolf@warwick.UUCP (Rolf Howarth) writes: >I thought I'd ask if anyone could explain to me how the human brain >perceives colour. >red + green light "gives you yellow", where "yellow" is also what you see >at a particular position in the spectrum (when you shine white light >through a prism). As far as I understand it, these two "yellows" are different >spectroscopically , yet the eye perceives them to be the same colour. In article <8703121101.AA09386@ingres.Berkeley.EDU>, hatcher@INGRES.BERKELEY.EDU (Doug Merritt) replies with a lengthly discussion of the mechanism of color perception. The main point is that the color sensation is derived from data collected by three groups of narrow-band light-sensing cells in the retina, called "cones", which have peaks of spectral sensitivity in the red, blue, and green. It might be possible to draw incorrect answers to Rolf's questions from Doug's entry, however. (I have only Doug's reply available, unfortunately - we've been having trouble with our news software lately. Please exscuse me if I'm off in the ozone because I made a bad inferrence about what Rolf asked.) Pure colors of different frequencies stimulate the three types of cones in amounts that are a function of the product of their intensity and how much they are absorbed by a sensitive chemical (called a "pigment") in the cone. Different types of cones have different pigments with different curves of light-absorption versus frequency. The sensitivity curves of the various pigments overlap, preventing "holes" in the response. Thus green light stimulates the green cones a lot and the blue and red cones very little, while yellow light stimulates both the blue and green conse considerably and the red cones very little. The brain 'thinks' "Buncha blue, buncha green, tiny bit of red. Aha! It's yellow!" For most colors of light (which are mixes of various colors already), you can approximate the result in the cones by mixing appropriate amounts of three colors of light, tuned to the responses of the three types of cones. For very pure spectral colors located between the various spectral peaks of the cone responses, you can't quite do it, because the approximating colors stimulate the third receptor more than the pure spectral light you're trying to model does. (I believe this is most apparent with yellow, and is why you can't get a very good pure yellow from a color TV.) There is an additional effect, caused by the approximation not quite stimulating the rods (the very sensitive black-and-white broadband receptors) correctly, but this is minor because the visual processing gives priority to the cones when light levels are sufficient to operate them. As to color-blind people, color-blindness comes in many forms. Some are just missing one or more visual pigment. The remaining color pigments are stimulated in the same way as their counterparts in the normally-sighted, so the TV image they see is about as good a match as that seen by the normally-sighted. (They might have more participation from the rods in the color sensation, however, which could distort things a little bit.) In others, one or more of the cones have a mutated pigment - either another copy of one from a different type of cone, or one which has an abnormal sensitivity curve. If they have, say, a copy of the green pigment in the blue receptors, they still get about the same image off a TV as they get from the real-world. If they have something with a sensitivity peak far from TV's standard colors, though, (a yellow-sensitve pigment in the blue cones, for instance) they get serious color distortion. It would probably be possible to design a special television system for these people, with phospors and camera color filters that match their eyes, but it would be dreadfully expensive to do so, and would yeild similarly bad color distortion when viewed by the rest of us. "I've got code in my node." | UUCP: ...!ihnp4!itivax!node!michael | AUDIO: (313) 973-8787 Michael McClary | SNAIL: 2091 Chalmers, Ann Arbor MI 48104 Above opinions are the official position of McClary Associates. Customers may have opinions of their own, which are given all the attention paid for.
malley@utah-gr.UUCP (Thomas J. V. Malley) (03/16/87)
Some interesting reading on this subject can be found in:
Faugeras, O.D., "Digital Color Image Processing Within the
Framework of a Human Visual Model", IEEE Trans. on Acoustics,
Speech, and Signal Processing, vol. ASSP-27, no. 4, August 1979
or in Faugeras' dissertation (6/76, Univ. of Utah).
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