The additive system: RGB system

Introduction

There are three basic colored lights, red, green and blue. Modern chemistry can easily give us 3 transparent colorants for making good color screens. If you look at a PC or TV screen through a powerful magnifying glass, you will only see red, green and blue points. And every colored point is a little light that is ADDED to the light of the next ones. That’s why we call these colors the additive primaries.

Every little light can be more or less powerful. Their energy ranges from 0 to 255, i.e. from “no light” to “full light”.

(Back to top)

Black, white and gray

For example, if a group of three points gives full light (= full power) — it means red 255, blue 255 and green 255 —, the result is pure white. If we have, red 0 blue 0 and green 0, we see no light at all, the result is black. If we have middle values like red 128, blue 128 and green 128, the result is a neutral gray. That’s the RGB system (RGB for Red, Green, Blue).

See here more examples under any: let’s begin with Black, White and Gray.

Red	Green	Blue	Result	Name
255	255	255		White
128	128	128		Gray
0	0	0		Black

(Back to top)

Primaries and secondaries

Now the primary colors: the Additive Primaries. Every color is pure and at its full power, 255.

Red	Green	Blue	Result	Name
255	0	0		Red
0	255	0		Green
0	0	255		Blue

And the secondary colors: the Additive Secondaries. These are mixings of the three primaries by two: 100% of the first and 100% of the second one. Thus red 255 + green 255 = yellow, etc.

Please note the name of these colors: Cyan (mixing of Green and Blue lights) and Magenta (mixing of Red and Blue lights). They will come back later in this talk.

Red	Green	Blue	Result	Name
255	255	0		Yellow
0	255	255		Cyan
255	0	255		Magenta

(Back to top)

Tertiaries

The tertiary colors: the Additive Tertiaries. These are obtained by mixing two primaries in unequal proportion: 100% of the first color and 50% of the second one, or reversely. In a digital system, their energies become 255 (100%) and 128 (50%), respectively.

Red	Green	Blue	Result	Name
255	128	0		Orange
128	255	0		Yellowish Green
0	255	128		Bluish Green
0	128	255		Greenish Blue
128	0	255		Violet
255	0	128		Bluish Red

(Back to top)

Oranges and browns

To complete this information, let’s say, for example, that an Orange can be more Red or more Yellow. The same for every other color. But the Orange example is very interesting, because when a Reddish Orange is less powerful, it becomes a brown.

Red	Green	Blue	Result	Name
255	192	0		Yellowish Orange
255	64	0		Reddish Orange
170	43	0		Marroon
112	28	0		Dark Brown

You certainly have already remarked that these 3 “reddish oranges” above are exactly the same: only their powers differs. They contain indeed the same relative proportion of Green and Red: 64/255 = 25%; 43/170 = 25% and 28/112 = 25%. (There is a tiny arithmetical error — less than 0.3%: 43/170 = 25.29% —, because it’s impossible to cut a bit in two!)

(Back to top)

Colored grays and pastel tints

When you have mixings of the 3 additive primaries, the results are more or less colored grays or pastel tints. Very dark Grayish Oranges are perceived as Brown varieties too.

Red	Green	Blue	Result	Name
204	170	170		Reddish Gray
170	170	136		Yellowish Gray
119	119	170		Bluish Gray
204	255	238		Pastel Green
221	187	255		Pastel Violet
102	51	34		Dark Brown
102	34	34		Dark Reddish Gray

And so, with the 256 possible light powers of each of the three additive primaries (= colored lights), we get more than 16 millions colors (exactly 256 × 256 × 256 = 16,777,216 different colors). That’s the additive system used on the PC monitors. On this manner, these screens can reproduce an enormous variety of colors.

You could imagine systems with more than 256 powers for each primary color, e.g. 512 or 1024. The results would be 134,217,728 or 1,073,741,824 colors respectively, and so on... But such a complication would be absolutely unnecessary, because the 16 millions colors system is already too good for most human eyes, which are not able to see the difference between two nearby colors, for example 204-255-170 and 204-255-171, as shown in the picture below.

204-255-170 204-255-171

Copyright © 2001 Sea Ooh See All Rights Reserved