[gsmollin]

No, computer geeks get blamed for a lot that's not their doing. The additive primary colors are red, green and blue. Look on your computer monitor with a magnifying glass if you don't believe me.

The subtractive color primaries are also cyan, magenta, and yellow. Red looks similar to magenta, and blue looks similar to cyan, but they are not the same.

There are a whole set of relationships between the additive primary colors, and the subtractive primary colors, but you can research that for yourself, if you are interested.

[Ultima]

roberth primary colours for pigment and light are different.

Pigment: yellow, cyan, magenta. Cyan being light blue, and magenta being a purple. Look at a colour newspaper under a magnifying glass to see.

Light: Red, Green, Blue. Check out the pixels of a TV or monitor with a magnifying glass to see this one.

Found a good google'r to illustrate:

http://homepage.mac.com/dtrapp/physics.f/ColorVision.html

wOw the world spins?

[roberth]

Geez, I know I'm getting old, but when I was younger, the primary colours were red, blue and yellow. I guess computers have really changed the way we look at things. When I colour in the sky (blue or is that azure or cyan) with my colouring in pencils and put a yellow sun on there, it still comes out green, so not everything has changed.
Thanks for the insight, guys.

[gsmollin]

quote:Originally posted by gsmollin

I think I'm understanding McQueen's question now, and it is indeed a good one. Let me re-state it, as I understand it.

We can form colors on a screen or page by mixing primary colors. The tri-tone primary colors are red, green, and blue for additive color mixing. So on a TV screen, we can look with a magnifying glass and see red green and blue dots or stripes on the screen being used to make up all the colors. There is quite a lot of information available on the web about this process, if we search on "NTSC color standard".

The tri-tone primary colors are cyan, magenta, and yellow for subtractive color mixing. So we can mix these colors of paint or ink to get "con-tone" images, such as on a photographic print, or an artist's painting. Another application would be "spot" color from a printing press.

Now we come to the question: Half-tone images are constructed of dots, much like the screen of a TV set or computer monitor. Why are the dots colored cyan, magenta, and yellow instead of red, green, and blue? Think before you answer! Each dot is separate from its neighbor, and the reflected light from each and every dot travels an independent path. So there is no color pigment or ink mixing, such as in a painted image or a photographic print. In addition, the dot sizes are very large, say .007 inches (.2 mm), so interference cannot be a significant issue. Why are the primary colors different, CMY, instead of RGB?

[McQueen]

Gsmollin

I almost missed your post. Thanks for re-posting. I think that the mistake we are making is in trying to put labels like “subtractive” and “additive “ to the RGB and CMY colour systems. The reason  I say this is that for the CMY system to be a truly  subtractive system , colours would have to be  put down in layers , so that certain colours are absorbed by certain layers of colour  allowing  other colours to be reflected. In practice this is not what occurs. Paints are colloidal substances , that is they consist of a suspension of pigment particles in a medium. When a painter wants to obtain a certain colour , he doesn’t put first one layer of colour on and then another , what he does is select certain amounts of each colour and mixes them on his palate , what finally reaches the canvas is a mixture of the two colours or spots of pigment of the two colours  which gives rise to  a third colour. Similarly , and this is easy to see with a magnifying glass , Colour photographic prints , are also made up of dots in this case Red , Green and Blue dots. So this discussion has shown that in every case exactly the same system is at work. We have dots of either the primary or the secondary  colours , which get mixed together to give rise to other colours .  This process must necessarily be due to the reflection and interference from the different dots before they reach the retina and not due to any “subtractive”  process ,  although in a sense the “additive “ process is still viable because interference might be considered as being a form of addition of light of different colour.
As to the question of why two systems of colour exist , RGB and CMY .  I think you have pointed out the answer yourself when you had talked about the relative coarseness of the grains found in half-tone images. Since RGB are clearly defined colours , the reproduction using the coarse half tone size of grains would not result in good reproduction. By using secondary colours which are  milder almost pastel shades , good reproduction is achieved in spite of the relatively large grain size.

[gsmollin]

Actually, I was posting an answer to my question, but it never happened. Here is what I meant to say:

R+G+B=white, so these are the additive primary colors. This is true if they are truly mixed, say using prisms, mirrors, or lenses. It is also physiologically true if they are mixed by showing dots too small to be resolved by the eye at a distance. The dot pitch on your computer monitor, for instance is about 0.26 mm. With a magnifying glass you can see the red, green, and blue dots. The use of additive mixing in CRT screens is a natural for the way a CRT works. When the electron beam is off, the tube looks black. The surface of the tube has a black coating on it to make it look black. The RGB guns light up the RGB phosphors, and we see colors.

The opposite is true of the photo, painting, or half-tone-printed page. The paper is white, so to see colors, we must subtract the colors we don't want to see. This is obvious to anyone who has colored or painted. The problem I was having was with half-tone printing. It is a mosaic of CMY dots and other shapes. (Many times special colors are used, however, so you must be sure you are looking at a CMY-half-tone-printed image) But these dots reflect light individually, so their contributions are added- giving us additive color mixing. That was my point of confusion. The apparent answer to this question is that the subtractive primary dots are combined by our visual process, and appear to us as if they had been mixed together like paints.

I imagine the visual process works something like this:
The light from the C, M, and Y dots all falls on 1 cone cell, since the dots are too close together to be resolved individually. The cone cell responds as if the C, M, and Y inks had been mixed together, since it sees them as one. The result is subtractive color mixing. The same visual process is happening for additive color mixing.

OBTW, if also found out that "k" is used for black, since "b" was already used for blue. Duh.

[McQueen]

An interesting corollary to the theory which I have put forward  is that all light is due to absorption and re-emission of photons of  a particular colour. The proof of this is very simple , just look in a mirror !!  A mirror as we all know is made up of a glass  sheet generally  of from 3 – 5 mm thickness at the back of which is a silvered layer. Now , it is common knowledge that light travels through substances like glass by being absorbed and re-emitted by electrons in the glass , therefore in order to see the colours of an image reflected in a mirror , the photons of those particular colours must first journey through the glass being in turn absorbed and then emitted by electrons in the atoms making up the glass , till they reach the silvered surface where they are absorbed and re-emitted one final time before making the reverse journey back through the glass from atom to atom and from there to the retina. The question of why the silvered layer reflects light back in the direction from which it has come is due to the properties of metals but it seems that this property must be shared at least partially by all substances , although it is obvious that with metals all light is emitted and absorbed while other substances selectively absorb and emit light.  Thus it is plain to see that all colours are due to a process of absorption and emission of photons of the requisite wave length.  Now supposing we have an object which is pure blue under white light what is happening ? We have to assume that the object is made up of atoms whose electrons are particularly susceptible to photons with the energy and wave length  of blue light (around 550 nm ) and practically ignore all other wave-lengths and energies. We know that something very much like this must happen from our study of spectroscopy , where certain substances only absorb and emit light of certain colours , for instance sodium will only emit and absorb yellow light , ignoring all other wave lengths and energies. No-one would suggest that sodium is absorbing all the photons of other colours and reflecting only the yellow light , in fact it is simply ignoring the other photons and selectively absorbing and emitting yellow photons ! Returning to the case in question of the blue object , we must assume that when white light shines on this surface , the electrons of the atoms making up this object absorb and re-emit only the blue light so that the  object looks blue. This is an intrinsic property of the  object , thus it retains this property even when there is no light. However if light of a different frequency , (as for instance ultraviolet light ) falls on the surface , it activates different electrons with the result that photons of a different wave-length are absorbed and emitted , while the blue photons which were predominant under white light are suppressed , and the object changes colour. The most interesting part of this discussion so far as I am concerned lies in coming to an understanding of just how fast and how continuously electrons emit and absorb photons under the influence of radiation even by  white light . The emission and absorption of photons by electrons under the influence of radiation  is seen to be a practically   continuous and unremitting process. It is difficult to imagine while looking into a mirror or through a window ( this for those who  still have reservations  about my ideas )  the number and speed at which these emissions and absorptions of photons  must  take place in order to form the continuous  images which we perceive.