Image modes and compression formats

A color mode determines the color model used to display and print images. Photoshop bases its color modes on established models for describing and reproducing color. Common models include HSB (hue, saturation, brightness); RGB (red, green, blue); CMYK (cyan, magenta, yellow, black); and CIE L*a*b*. Photoshop also includes modes for specialized color output such as Indexed Color and Duotone. ImageReady uses RGB mode to work with images. In addition to determining the number of colors that can be displayed in an image, color modes affect the number of channels and the file size of animage.

RGB

• This mode assigns an intensity value to each pixel ranging from 0 (black) to 255 (white). It is additive, meaning that color is obtained by adding light emitting pixels.

• When the values of all three components are equal, the result is a shade of neutral gray. When the value of all components is 255, the result is pure white; whenÊthe value is 0, pure black.

• RGB uses three color channels, red green and blue.

• 16 bit RGB can produce 16.7 million different colors

Bitmap

• This mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1-bit images because they have a bit depth of 1.

Grayscale

• This mode uses up to 256 shades of gray.

• Every pixel of a grayscale image has a brightness value ranging from 0 (black) to 255 (white).

• Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black).

• You can convert both Bitmap-mode and color images to grayscale.

• To convert a color image to a high-quality grayscale image, Photoshop discards all color information in the original image. The gray levels (shades) of the converted pixels represent the luminosity of the original pixels.

Duotone

• This mode creates duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using two to four custom inks.