Apple Aperture Digital Photography Fundamentals - Manual (Page 34)

34 Chapter 2 How Digital Images Are Displayed
Understanding Color Gamut
In 1931, a group of scientists and intellectuals who called themselves the Commission
Internationale de l’Eclairage (CIE) had the goal of defining standards for color. Using as
much objectivity as is possible with this highly subjective topic, they developed a
coordinate system for categorizing the world of colors. According to this system, every
hue the eye can see can be described in terms of x and y coordinates. Taking it one step
further, every device that reproduces colors can have its RGB color primaries described
by the CIE x and y values. This provides the basis for color-management systems such as
ColorSync. The total number of colors described by the two-dimensional plot of these x
and y coordinates is often referred to as the device’s color gamut. In other words, a
system’s color gamut refers to the total set of possible colors that system is capable of
displaying. In addition to this two-dimensional color description, color gamut has a third
dimension: its brightness. Unfortunately, the color gamut of displays does not
correspond exactly to the subtractive color of print. For example, certain colors that
appear onscreen cannot be exactly reproduced in print, and vice versa.
Displaying Images Onscreen
As mentioned earlier, when working with images on your computer screen, you are
working with additive light. The display converts electricity into light and the pixels on
the screen produce an image by using an RGB color space model. (Color space refers
to the limits, or parameters, of a given visible spectrum. Common color spaces are
sRGB and Apple RGB.) This process begins when the image file on the computer’s hard
disk is processed and then sent to the graphics card for further processing and
temporary storage in memory. The graphics card processes the image, preparing to
display it in the specific resolution and color profile of the display or displays
connected to the computer. (A color profile is a compilation of data on a specific
device’s color information, including its gamut, color space, and modes of operation.)
Processing the image may take some time, depending on the size and bit depth of the
image file, the size and number of displays in the system, and the resolution of the
displays. Whether an image was scanned or downloaded directly from a camera, the
image was recorded digitally in an RGB color space.
The essence of RGB is the combination of red, green, and blue colors emitted from a
light source to form a wide variety of additional colors. On color displays, three colored
elements (one red, one green, and one blue) combine to form a pixel. When red, green,
and blue are combined at their maximum intensities, the color white is created. When
there is an absence of light in all three colored elements, the color black is inferred.