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The RGB Color System - Additive Color

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The RGB Color System is an "additive" system based on three primary colors - Red, Green and Blue. By combining these three primary colors in different proportions, it is possible to create the majority of different hues that we recognize. This is possible because the RGB System has similarities with the way our brain processes signals from our eyes to create the experience of color vision. When light strikes the retina of the human eye, it stimulates light-sensitive cells called cones. There are three types of cone, each sensitive to different wavelengths of light. These wavelengths approximate the red, green and blue parts of the color spectrum. It is the combined proportional signal from the three types of cone that we experience as color vision.

In the RGB Color System we describe a color using three separate values - one value for each of the red, green and blue components. (eg. Red=100, Green=200, Blue=50.) Each component can have a value between zero and a maximum possible value. that is the same for all three components. RGB values are very often encoded as "8 bit integers" which consist of the range of 256 values between 0 and 255 inclusive. For the rest of this tutorial we wont worry about the different ways of encoding RGB values. We will work only with RGB Values that can be described by three separate 8 bit integers. (eg. {100,200,50} ).


Look at the applet to the right. The box on the left displays the color corresponding to the RGB values displayed at the bottom right. The three sliders on the right labelled R, G and B can be used to adjust the Red, Green and Blue values. The values of R, G and B are all 8 bit integers. Try this simple exercise: Hold a object with a distinct color - like a paint sample or book cover - next to your monitor. Try to match the color on your object by adjusting the sliders on the applet. Do you find working with the RGB values intuitive? If you do not, do not worry - you are not alone! Later we will look at ways of manipulating color that feel more natural to most photographers.

You should be able to use the applet to satisfy yourself that some of the points we have made are in fact true. For example, you should be able to adjust the sliders to display the majority of the simple hues you can imagine (although not in all their possible subtle shades that you can imagine). You can certainly create hues that are definitely not "red", "green" or "blue". That should give you a feel for what we mean when we say the RGB System is an "additive" system. For example: When all the values are zero {0,0,0}, the color is black. By increasing the value of R to 255 giving {255,0,0},you are adding red. So far, not very exciting. Now when you have {255,0,0} and you increase the value of B from 0 to 255 you are adding blue, The color displayed {255,0,255} is an equal mixture of red and blue, a color technically known as "magenta". We can always produce this color by adding the maximum values red and blue in equal proportion. Some common colors can be formed using the RGB values in the table to the right.

In addition to being the standard way of encoding color in digital form, RGB is also used in the design of display hardware. In your digital photography workflow, this means your CRT or LCD monitor. On your monitor each pixel is made up of areas of red, green and blue light of varying intensities. The more intense the light, the higher the numerical value, the brighter it appears. On an LCD, pixel that appears yellow to the viewer is emitting red and green, on and CRT the pixel is transmitting red and green light that has been emitted from the base of the tube, through the glass.

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