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The additive system

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The additive system: RGB system


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”.

Why these numbers? It’s very simple: in electronics, one works with bits. With eight bits, you can have 256 numbers, ranging from 0 to 255. For 3 colors, 3 × 8 bits make 24 bits and 16,777,216 possibilities, i.e. nearly 17 millions colors.

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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

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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

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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

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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!)

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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

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