Understanding how colors are defined in graphics data is important to understanding graphics file formats. In this section, we touch on some of the many factors governing how colors are perceived. This is by no means a comprehensive discussion. We just want to make sure that you have an appreciation of some of the problems that come up when people start to deal with color.
The eye has a finite number of color receptors that, taken together, respond to the full range of light frequencies (about 380 to 770 nanometers). As a result, the eye theoretically supports only the perception of about 10,000 different colors simultaneously (although, as we have mentioned, many more colors than this can be perceived, though not resolved simultaneously).
The eye is also biased to the kind of light it detects. It's most sensitive to green light, followed by red, and then blue. It's also the case that the visual perception system can sense contrasts between adjacent colors more easily than it can sense absolute color differences, particularly if those colors are physically separated in the object being viewed. In addition, the ability to discern colors varies from person to person; it's been estimated that one out of every twelve people has some form of color blindness.
Furthermore, the eye is limited in its ability to resolve the color of tiny objects. The size of a pixel on a typical CRT display screen, for example, is less than a third of a millimeter in diameter. When a large number of pixels are packed together, each one a different color, the eye is unable to resolve where one pixel ends and the next one begins from a normal viewing distance. The brain, however, must do something to bridge the gap between two adjacent differently colored pixels and will integrate, average, ignore the blur, or otherwise adapt to the situation. For these reasons and others, the eye typically perceives many fewer colors than are physically displayed on the output device.
How a color is created also plays an important role in how it is perceived. Normally, we think of colors as being associated with a single wavelength of light. We know, however, that two or more colors can be mixed together to produce a new color. An example of this is mixing green and red light to produce yellow light, or mixing yellow and blue pigments to produce green pigment. This mixing of colors can also occur when an object is illuminated by light. The color of the object will always mix with the color of the light to produce a third color. A blue object illuminated by white light appears blue, while the same object illuminated by red light will appear violet in color.
One implication of this is that the same image rendered to two different devices will look different. Two different color monitors, for example, seldom produce identically perceived images, even if the monitors are the same make and model. Another implication is that images rendered to different types of devices will look different. An example is the difference between an image rendered to a monitor and one rendered to a color hardcopy device. Although there are numerous schemes designed to minimize color-matching problems, none is wholly satisfactory.
For these and other reasons, the accurate rendition of color is full of difficulties, and much work continues to be done. Although a number of mechanical devices have recently appeared on the market, they are for the most part designed to work with one type of output device. The ultimate arbiter of color quality will always be the person who views the image on the output device.
Several different mathematical systems exist which are used to describe colors. This section describes briefly the color systems most commonly used in the graphics file formats described in this book.
NOTE:Keep in mind that perception of color is affected by physiology, experience, and viewing conditions. For these reasons, no system of color representation has yet been defined, or indeed is likely to be, which is satisfactory under all circumstances.
For purposes of discussion here, colors are always represented by numerical values. The most appropriate color system to use depends upon the type of data contained in the file. For example, 1-bit, gray-scale, and color data might each best be stored using a different color model.
Color systems used in graphics files are typically of the trichromatic colorimetric variety, otherwise known as primary 3-color systems. With such systems, a color is defined by specifying an ordered set of three values. Composite colors are created by mixing varying amounts of three primary colors, which results in the creation of a new color. Primary colors are those which cannot be created by mixing other colors. The totality of colors that can be created by mixing primary colors make up the color space or color gamut.
Color systems can be separated into two categories: additive color systems and subtractive color systems. Colors in additive systems are created by adding colors to black to create new colors. The more color that is added, the more the resulting color tends towards white. The presence of all the primary colors in sufficient amounts creates pure white, while the absence of all the primary colors creates pure black. Additive color environments are self-luminous. Color on monitors, for instance, is additive.
Color subtraction works in the opposite way. Conceptually, primary colors are subtracted from white to create new colors. The more color that is subtracted, the more the resulting color tends towards black. Thus, the presence of all the primary colors theoretically creates pure black, while the absence of all primary colors theoretically creates pure white. Another way of looking at this process is that black is the total absorption of all light by color pigments. Subtractive environments are reflective in nature, and color is conveyed to us by reflecting light from an external source. Any color image reproduced on paper is an example of the use of a subtractive color system.
No color system is perfect. As an example, in a subtractive color system the presence of all colors creates black, but in real-life printing the inks are not perfect. Mixing all ink colors usually produces a muddy brown rather than black. The blacks we see on paper are only approximations of the mathematical ideal, and likewise for other colors.
The next few sections describe some common color systems. Table 2-1 shows corresponding values for the primary and achromatic colors using the RGB, CMY, and HSV color systems.
RGB is perhaps the most widely used color system in image formats today. It is an additive system in which varying amounts of the colors red, green, and blue are added to black to produce new colors. Graphics files using the RGB color system represent each pixel as a color triplet--three numerical values in the form (R,G,B), each representing the amount of red, green, and blue in the pixel, respectively. For 24-bit color, the triplet (0,0,0) normally represents black, and the triplet (255,255,255) represents white. When the three RGB values are set to the same value--for example, (63,63,63) or (127,127,127), or (191,191,191)--the resulting color is a shade of gray.
| RGB | CMY | HSV |
|---|---|---|---|
Red |
255,0,0 |
0,255,255 |
0,240,120 |
Yellow |
255,255,0 |
0,0,255 |
40,240,120 |
Green |
0,255,0 |
255,0,255 |
80,240,120 |
Cyan |
0,255,255 |
255,0,0 |
120,240,120 |
Blue |
0,0,255 |
255,255,0 |
160,240,120 |
Magenta |
255,0,255 |
0,255,0 |
200,240,120 |
Black |
0,0,0 |
255,255,255 |
160,0,0 |
Shades of Gray |
63,63,63 |
191,191,191 |
160,0,59 |
of |
127,127,127 |
127,127,127 |
160,0,120 |
Gray |
191,191,191 |
63,63,63 |
160,0,180 |
White |
255,255,255 |
0,0,0 |
160,0,240 |
CMY is a subtractive color system used by printers and photographers for the rendering of colors with ink or emulsion, normally on a white surface. It is used by most hard-copy devices that deposit color pigments on white paper, such as laser and ink-jet printers. When illuminated, each of the three colors absorbs its complementary light color. Cyan absorbs red; magenta absorbs green; and yellow absorbs blue. By increasing the amount of yellow ink, for instance, the amount of blue in the image is decreased.
As in all subtractive systems, we say that in the CMY system colors are subtracted from white light by pigments to create new colors. The new colors are the wavelengths of light reflected, rather than absorbed, by the CMY pigments. For example, when cyan and magenta are absorbed, the resulting color is yellow. The yellow pigment is said to "subtract" the cyan and magenta components from the reflected light. When all of the CMY components are subtracted, or absorbed, the resulting color is black. Almost. Whether it's possible to get a perfect black is debatable. Certainly, a good black color is not obtainable without expensive inks.
In light of this, the CMY system has spawned a practical variant, CMYK, with K standing for the color black. To compensate for inexpensive and off-specification inks, the color black is tacked onto the color system and treated something like an independent primary color variable. For this reason, use of the CMYK scheme is often called 4-color printing, or process color. In many systems, a dot of composite color is actually a grouping of four dots, each one of the CMYK colors. This can be readily seen with a magnifying lens by examining a color photograph reproduced in a glossy magazine.
CMYK can be represented as either a color triple, like RGB, or as four values. If expressed as a color triple, the individual color values are just the opposite of RGB. For a 24-bit pixel value, for example, the triplet (255,255,255) is black, and the triplet (0,0,0) is white. In most cases, however, CMYK is expressed as a series of four values.
In many real-world color composition systems, the four CMYK color components are specified as percentages in the range of 0 to 100.
HSV is one of many color systems that vary the degree of properties of colors to create new colors, rather than using a mixture of the colors themselves. Hue specifies "color" in the common use of the term, such as red, orange, blue, and so on. Saturation (also called chroma) refers to the amount of white in a hue; a fully (100 percent) saturated hue contains no white and appears pure. By extension, a partly saturated hue appears lighter in color due to the admixture of white. Red hue with 50 percent saturation appears pink, for instance. Value (also called brightness) is the degree of self-luminescence of a color--that is, how much light it emits. A hue with high intensity is very bright, while a hue with low intensity is dark.
HSV (also called HSB for Hue, Saturation, and Brightness) most closely resembles the color system used by painters and other artists, who create colors by adding white, black, and gray to pure pigments to create tints, shades, and tones. A tint is a pure, fully saturated color combined with white, and a shade is a fully saturated color combined with black. A tone is a fully saturated color with both black and white (gray) added to it. If we relate HSV to this color mixing model, saturation is the amount of white, value is the amount of black, and hue is the color that the black and white are added to.
The HLS (Hue, Lightness, and Saturation) color model is closely related to HSV and behaves in the same way.
There are several other color systems that are similar to HSV in that they create color by altering hue with two other values. These include:
None of these is widely used in graphics files.
The YUV model is a bit different from the other colorimetric models. It is basically a linear transformation of RGB image data and is most widely used to encode color for use in television transmission. (Note, however, that this transformation is almost always accompanied by a separate quantization operation, which introduces nonlinearities into the conversion.) Y specifies gray scale or luminance. The U and V components correspond to the chrominance (color information). Other color models based on YUV include YCbCr and YPbPr.
Black, white, and gray are considered neutral (achromatic) colors that have no hue or saturation. Black and white establish the extremes of the range, with black having minimum intensity, gray having intermediate intensity, and white having maximum intensity. One can say that the gamut of gray is just a specific slice of a color space, each of whose points contains an equal amount of the three primary colors, has no saturation, and varies only in intensity.
White, for convenience, is often treated in file format specifications as a primary color. Gray is usually treated the same as other composite colors. An 8-bit pixel value can represent 256 different composite colors or 256 different shades of gray. In 24-bit RGB color, (12,12,12), (128,128,128), and (199,199,199) are all shades of gray.
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