Figure 6.2 shows one way of thinking about additive color, in a two-dimensional color space. The largest circles represent beams of light in red, green, and blue. Where the beams overlap, they produce other colors. For example, red and green combine to produce yellow. Red and blue add up to magenta, and green and blue produce cyan. The center portion, in which all three colors overlap, is white. If the idea that overlapping produces no color, rather than some combined color seems confusing, remember that the illumination is being added together, and combining all three of the component colors of light produces a neutral white with equal amounts of each.
However, if you look at the figure, you'll see that it shows overlapping circles that are each more or less the same intensity of a single color (allowing some artistic license for the 3D "shading" effect added to keep the individual circles distinct). For that reason, this two-dimensional model doesn't account for the lightness or darkness of a colorthe amount of white or black. That added dimension is dealt with by, literally, adding a
Figure 6.3. The RGB color space can be better represented by a three-dimensional cube, simplified to corners and edges in this illustration.
The figure shows red, green, and blue colors positioned at opposite corners of the cube, with their complementary colors arranged between them. White and black are located opposite one another, as well. Any shade that can be produced by adding red, green, and blue together can be represented by a position within the cube.
No widely used display device available today produces pure red, green, or blue light. Only lasers, which output at one single frequency of light, generate absolutely pure colors, and they aren't used for display devices. We see images through the glow of phosphors, LEDs, or LCD pixels, and the ability of these to generate absolutely pure colors is limited. Color representations on a display differ from brand to brand and even from one display to another within the same brand.
Moreover, the characteristics of a given display can change as the monitor ages and the color-producing elements wear out. Some phosphors, particularly blue ones, change in intensity as they age, at a different rate than other phosphors. So, identical signals rarely produce identical images on displays, regardless of how closely the devices are matched in type, age, and other factors.
In practice, most displays show far fewer colors than the total of which they are theoretically capable. Actually, the number of different colors a display can show at one time is limited to the number of individual pixels. At 1024 x 768 resolution, there are only 786,432 different pixels. Even if each one were a different color, you'd view, at most, only around three-quarters of a million colors at once. The reason your digital camera, scanner, display, and Photoshop itself need to be 24-bit compatible is so the right 786,432 pixels (or whichever number is actually required) can be selected from the available colors that can be reproduced by a particular color model's gamut. In practice, both scanners and digital cameras capture more than 24 bits worth of color, to allow for the inevitable information lost in translating a full-spectrum image to digital form. To understand why, you need to understand a concept called bit depth.
As you know from working with Photoshop, the number of theoretical colors that can be represented is measured using that bit depth yardstick. For example, "4-bit" color can represent the total number of colors possible using four bits of binary information (0000 to 1111), or 16 colors. Similarly, 8-bit color can represent 256 different colors or grayscale tones, while "high color" 15- or 16-bit displays can represent 32,767 or 65,535 colors. You'll sometimes encounter high color images when you're displaying at very high resolutions using video cards which don't have enough memory for 24-bit color at that resolution.
For example, perhaps you own a 21-inch or larger monitor capable of displaying images at 2048 x 1536 pixels of resolution, and you are working with an older video card that can display only 65,535 colors at that resolution. This example is a bit farfetched, because most video cards today have enough memory to display at least 16.8 million colors at whatever their maximum resolution is. You will, however, sometimes encounter a card that must step down to a lower color depth to display its absolute top resolution.
In the past, when you used Photoshop with so-called 32-bit color images, you were actually working with ordinary 24-bit images, plus an extra 8 bits used to store grayscale alpha channel information. The image itself was usually a normal 24-bit image. In this case, the extra 8 bits store your selections, layer masks, and so forth. On the other hand, in Photoshop CS2 terminology, "16bit color" usually doesn't refer to those "high-color," 65,535-hue images, either. Within Photoshop
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