Path: utzoo!mnetor!uunet!husc6!mailrus!nrl-cmf!ames!pacbell!att-ih!ihnp4!inuxc!iuvax!pur-ee!uiucdcs!uxc.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.graphics
Subject: Re: Naive Question About Primary Colors
Message-ID: <46900013@uxe.cso.uiuc.edu>
Date: 28 Mar 88 15:19:00 GMT
References: <7871@oberon.USC.EDU>
Lines: 46
Nf-ID: #R:oberon.USC.EDU:7871:uxe.cso.uiuc.edu:46900013:000:2004
Nf-From: uxe.cso.uiuc.edu!mcdonald    Mar 28 09:19:00 1988



>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