OT: Question about RGB color method

Paul McGuire ptmcg at austin.rr.com
Tue Apr 10 21:53:05 CEST 2007

On Apr 10, 2:32 pm, John Salerno <johnj... at NOSPAMgmail.com> wrote:
> Sorry for this non-Python question, but since it's computer related I
> know you guys will have an answer, and I don't really know where else to
> ask. Mainly I'm just curious anyway.
> I'm wondering, why do computers use a RGB color scheme instead of the
> primary colors? Is there something you can't do with yellow? It seems
> weird that RGB can be combined to make all colors, when that's supposed
> to be the job of the primary colors. I'm sure there some technical
> computer-related reason that it had to be this way.
> Thanks.

See this link: http://www.rgbworld.com/color.html

The "red-yellow-blue" system we used in elementary school art classes
corresponded to the color mixing capabilities available using the
Tempra paint technology at hand.  At the RGBworld URL, the web page
shows how paints and inks follow a "subtractive" color system (called
CMY for cyan-magenta-yellow, instead of blue-red-yellow, but close
enough), in that paint reflects a given color by subtracting out all
frequencies that are not that color.  So mixing two subtractive
filters follows a subtractive color "algebra".

However, computer monitors do not follow a substractive scheme, but
rather an additive one, by adding brightness along red-green-and-blue
(RGB) dimensions, mixing red and green light to get yellow.  This
raises an interesting problem when *printing* a color image.  You
cannot just render the RGB colors from the monitor's additive system
into CMY colors on your inkjet printer's subtractive system.  Google
for "RGB color" and you will be presented with a long list of
references for dealing with this issue, including several competing
standards on how to handle it.  Even the Wikipedia article on RGB
colors is largely devoted to this issue (as opposed to your more
common question).

-- Paul

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