drapeau@alheka.usc.edu (George Drapeau) (03/25/88)
I apologize in advance for being obtuse about this, but if you would please indulge me... 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? If you look at a color wheel, red and green are at opposite ends, as opposed to forming two parts of the primary triangle. Thanks in advance, George _____________________________________________________________________________ George D. Drapeau Internet:drapeau@zaurak.usc.edu University Computing Services, UUCP:...!oberon!zaurak!drapeau University of Southern California
grady@Apple.COM (Grady Ward) (03/26/88)
The question is not so naive. While frequency of light is a continuum, the eye believe 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, beacuse, like a TV set, we have three sensing rod varieties. 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). Paint pigments combine in a subtractive way, though, and red blue yellow are usually chosen by people using pigments (paints) rather than light becasue they, too, require the least addition of white to make all the rest of the non-primary colors. A succinct primer on color physiology is to be found in Feynmann's work in beginning physics (1962). Grady Ward
turk@mit-amt.MEDIA.MIT.EDU (Matthew Turk) (03/26/88)
In article <7779@apple.Apple.Com>, grady@Apple.COM (Grady Ward) writes: > > ... 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). Matthew
cfchiesa@bsu-cs.UUCP (Christopher Chiesa) (03/27/88)
I read with interest the original not-so-naive question on choice of primary
colors; wasn't going to post a response but after reading the preceding respon-
ses reaching this node so far, I thought I'd add two cents' worth of hopefully
helpful "layman's terms" information...
RED, GREEN, BLUE are "ADDITIVE" primaries: they are used in situations where
light is being emitted; i.e. where "if you were to do NOTHING, you'd have
BLACK." Examples: three separate beams of light (one each R,G,B) superimposed
on a white screen; color television.
Conversely, MAGENTA, YELLOW, CYAN are "SUBTRACTIVE" primaries: they are used
in situations where one light is being filtered to obtain a desired resultant
color, or where a NEGATIVE color image will be produced. Examples: color film
(transparencies; negative-image printing); color filters; four-color halftone
printing process.
I realize that this still doesn't explain the use of RED, YELLOW, BLUE as
primaries, although I seem to recall from childhood (when I asked Dad, a
professional-level photographer and Kodak researcher) that these are SUBTRAC-
TIVE primaries... I usually see MAGENTA, YELLOW, and CYAN in commercial
color PRINTED materials, but usually RED, YELLOW, and BLUE when dealing with
PAINTING and other non-photographic art. Do any of you "I work with this
every day" types have any clarification? I'd be interested in hearing about
it.
UUCP: <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!cfchiesa
cfchiesa@bsu-cs.UUCP
--
UUCP: <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!cfchiesa
cfchiesa@bsu-cs.UUCP jcl@bdrc.COM (John C. Lusth) (03/28/88)
In article <7779@apple.Apple.Com> grady@apple.UUCP (Grady Ward) writes: > > 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). A small point. Given primary colors A, B, and C, and target color X, it is not necessarily true that some combination of the primaries will match color X. A + B + C matches X *** not necessarily true *** However, it is true that for any X that some combination of X and one of the primaries can be matched be some combination of the other two primaries. A + B matches X + C *** always true *** John C. Lusth Becton Dickinson Research Center Research Triangle Park, NC 27709 ...!decvax!mcnc!bdrc!jcl
mcdonald@uxe.cso.uiuc.edu (03/28/88)
>I apologize in advance for being obtuse about this, but if you would please >indulge me... >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? >If you look at a color wheel, red and green are at opposite ends, as opposed >to forming two parts of the primary triangle. Pardon me if there is a real answer already posted, but the two to reach me don't answer the question. The (non-color-blind) human eye has three forms of light sensors used in reasonably bright light. One detects blue. The other two are rather broad in response, put are peaked in the red and green. (Please note that pure colors, as in white light split by a prism or grating, range in color like this (roughly to scale on a linear energy plot): violet blue green yellow orange red bbbbbbbbbbbbbbbbbbbbbbbbb b b g g g ggggggggggggggggggggggg g g r r r r r rrrrrrrrrrrrrrrrrrrrrrrrrrr The b g and r show where the different sensors respond. The eye can be tricked into seeing any of the spectral colors, plus purple, by adding together spectrally pure beams of red, green and blue lights. Thus the real (additive) primary colors are red, green and blue. The subtractive primary colors commonly referred to as yellow, red, and blue, actually refer to a system where the yellow primary absorbs the blue light, and hence determines how much blue you see. The "red" one, actually, is more "magenta", and controls how much green you see by subtracting the green part of the spectrum. The "blue", actually blue-green, one controls red by subtracting the red part of white light. If you take a filter transmitting ONLY red and one transmitting cyan (blue-green) and look through them held together, all light is stopped (together they appear black). Likewise for magenta and green and for blue and yellow. Doug McDonald
turk@mit-amt.MEDIA.MIT.EDU (Matthew Turk) (03/29/88)
In article <310@bdrc.UUCP>, jcl@bdrc.COM (John C. Lusth) writes: > > A small point. Given primary colors A, B, and C, and target color X, > it is not necessarily true that some combination of the primaries will > match color X. > > A + B + C matches X *** not necessarily true *** > > However, it is true that for any X that some combination of X and one > of the primaries can be matched be some combination of the other > two primaries. > > A + B matches X + C *** always true *** > Actually, neither are *always* true, but it is usually true that one of them will be true! Think of the primaries A, B, and C as corners of a triangle. Some perceivable colors are inside the triangle and some are outside. Any mixture of A, B, and C will fall somewhere inside the triangle. A mixture of any two will fall on the line between the two (one of the triangle sides). So if X is inside the color triangle, then the first statement is true. If X is outside the triangle the second statement is sometimes true, and the rest of the time yet a third statement will be true: A matches X + B + C (when that is necessary is left as a proof to the reader!). Remember that adding A to one side is conceptually the same as subtracting (B + C) from the other. Of course all this discussion is ignoring the fine print in the "optional" section of the text book -- the part that says it's not really true for all colors.... Matthew
james@sunne.Sun.COM (04/05/88)
On the one hand, having simple questions get asked is undoubtedly boring and offensive to the geniuses out there. On the other hand, these questions (as the one about RGB color) usually get lots of responses, and often (as in the example) bring out some useful information that even those of us that thought they knew the answer can learn from. So, great, you start a new group for the new users. That way us geniuses don't have to be bothered. And this group can fill up with flames of the form: "that was a naive question, you fool. You should have known to post it in *.newuser.* " I'd rather see the questions, than the flames..... James Triplett Sun Microsystems, Lexington, Mass jtriplett@sun.com ...sun!sunne!james