mikec@tekgvs.TEK.COM (Micheal Cranford) (03/28/88)
George Drapeau recently asked: "Why is it that for video, the three primary colors are (usually) red, green, and blue, whereas for painting, the primaries are red, *yellow*, and blue? [1] If you look at a color wheel, red and green are at opposite ends, as opposed to forming two parts of the primary triangle." [2] To which Grady Ward responded: "The question is not so naive. While frequency of light is a continuum, the eye believe [sic] there are primary colors because of its own physiology, not because colors exist external to it. Any set of three colors can be "primary" for non-color blind humans, [3] beacuse [sic], like a TV set, we have three sensing rod varieties. [4] [5] The colors red, blue, green are only one example of primaries (ones which require the least addition of white to make all the rest of the colors). [6] Paint pigments combine in a subtractive way, though, and red blue yellow [1] are usually chosen by people using pigments (paints) rather than light becasue [sic] they, too, require the least addition of white to make all the rest of the non-primary colors." [6] To which Matthew Turk subsequently responds: "> ... Any set of three colors can be "primary" for non- > color blind humans, because, like a TV set, we have three sensing > rod varieties. Well, almost. The three colors must be "perceptually independent", i.e. no mixture of two of them can make the third. Also, in additive color the choice of the three primaries limits the gamut of colors that can be represented (to the inclusive triangle in the CIE chromaticity diagram)." [3] To which I feel hopelessly compelled (ACKK!!!) to correct: [1]. Artists and color scientists do not use the same color naming convention. The most common color primaries for additive color systems (e.g. color CRT) are Red, Green, and Blue. The most common color primaries for subtractive color systems (e.g. paint pigments) are Cyan, Magenta, and Yellow. Artists are usually taught to call Cyan "blue", Magenta "red", and Yellow "yellow". (While one out of three may seem bad -- it could have been worse.) Most of us (including me) were (mis-)taught the subtractive system in grade school. Thus, paint pigment primaries are really Cyan, Magenta, and Yellow. Note that in color hardcopy (ink printing) black is also used as a "primary". [2]. Pathetic teletype graphics of a standard color wheel: \ Red / Additive Subtractive \ / Do not take the chirality Magenta \ / Yellow C=G+B R=M+Y or the rotational phase __________\ /__________ M=R+B G=C+Y of this diagram literally. / \ Y=R+G B=C+M Blue / \ Green / \ / Cyan \ Note that Cyan (artist's Blue) is the complement of Red, Magenta (artist's Red) is the complement of Green, and Yellow is the complement of Blue. It would seem that George Drapeau was referring to artist's Red (Magenta) and Green as being complementary (opposite on the color wheel) colors. A less likely explanation is the NCS (Natural Color System) proposed in Sweden several years ago (where Red and Green are complementary, Yellow and Blue are complementary, and White and Black are complementary) but this is not in common use (generally only found in color perception research). Since Yellow and Blue are complementary (diagonally opposite) colors no additive or subtractive combination can be used to generate a good Green or Cyan. Since Red and Yellow are additive and subtractive primaries, respectively, they make a poor choice, as a pair, for primaries in any color system. A (bizarre) full color system using Red, Yellow, and Blue as primaries would only be viable if both additive and subtractive color processes were used. [3]. Grady Ward's suggestion that `Any set of three colors can be "primary"' is inaccurate. Matthew Turk's explanation clarifies this point quit well. [4]. Color television sets, generally, convert non-visible energy to visible energy. They have no color "sensing" capability. There are exceptions to this in some color laboratories. Grady Ward's analogy is poorly chosen, since a color CRT generally has 3 color emitters, while a human generally has 3 color sensors. Not every human is a trichromat (three primary color sensors). In addition to the obvious color blindness (monochromats and dichromats) a few people are tetrachromats (four primary color sensors). This is a good example of evolution (i.e. change) in action. [5]. Retinal rods are part of the Scotopic visual system and do not take part in the discrimination of color. Retinal cones, being part of the Photopic visual system, do discriminate colors. Grady Ward's claim is "confused". [6]. In any additive color system, adding white decreases the saturation of a color (i.e. decreases the radial distance from the equal energy point on a CIE diagram). Grady Ward's definition of what constitutes a primary color is incorrect while Matthew Turk's definition is correct. Subtractive color systems generally "subtract" light from the equal energy point by absorbing the complementary wavelengths (i.e. Cyan pigment absorbs Red wavelengths). Micheal Lucifer Cranford
eugene@pioneer.arpa (Eugene N. Miya) (03/29/88)
Chris Shaw suggested adding a newuser subgroup. I like this idea and
send him a private note in favor, but it seems that the topics like color
are among of those which we can hash again and again (even if you do go
to the library). [Been going on over a hundred years.]
Micheal Cranford wrote some pretty good points. I also suggest a
Scientific American article of Feb. or March 1987 on color vision, I
can't remember which, but it was shortly after our meeting on color.
I also suggest if you have not had a check up (physical), ask to take a
color blindness test and ask yourself, if you are not color blind, what
a color blind person might see. Think about what they are trying to
discriminate in each of these tests [color wheels are too much of a
continum].
Edwin Land [Polaroid Corp.] is another character who has interesting ideas
[for a person not trained in physicological psych] about color.
Lastly, we are planning a Fall meeting on the limits of human
perception. Should be a fun meeting.
From the Rock of Ages Home for Retired Hackers:
--eugene miya, NASA Ames Research Center, eugene@ames-aurora.ARPA
"You trust the `reply' command with all those different mailers out there?"
"Send mail, avoid follow-ups. If enough, I'll summarize."
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