Bitmap images

Photoshop and other paint and image-editing programs generate bitmap images, also called raster images. Bitmap images use a grid (the bitmap or raster) of small squares known as pixels to represent images. Each pixel is assigned a specific location and color value. For example, a bicycle tire in a bitmap image is made up of a mosaic of pixels in that location. When working with bitmap images, you edit pixels rather than objects or shapes.

A bitmap image is resolution-dependent—that is, it contains a fixed number of pixels to represent its image data. As a result, a bitmap image can lose detail and appear jagged if viewed at a high magnification on-screen or printed at too low a resolution. Bitmap images are the best choice for representing subtle gradations of shades and color—for example, in photographs or painted images.

Bit depth

also called pixel depth or color depth—measures how much color information is in the image to display or print pixels. Greater bit depth (more bits of information per pixel) means more available colors and more accurate color representation in the digital image. For example, a pixel with a bit depth of 1 has two possible values: black and white. A pixel with a bit depth of 8 has 28, or 256, possible values. And a pixel with a bit depth of 24 has 224, or roughly 16 million, possible values. Common values for bit depth range from 1 to 64 bits per pixel. Photoshop supports up to 16 bits per pixel for each channel in an image. A 24-bit RGB image, for example, would have 8 bits per pixel for each of the red, green, and blue channels.

Converting to indexed color

Specifying the bit depth

Using the Uniform or Adaptive palette, you can specify the bit depth of an indexed-color image—the number of bits of color information per pixel. Bit depth, also known as color depth, determines the number of colors used to display (or print) an image. For example, if you choose 4 bits per pixel, the image is composed of 16 colors; 6 bits per pixel, 64 colors; 8 bits per pixel, 256 colors. (The number of colors used is displayed in the Colors text box.) In addition, you can specify the exact number of colors to be displayed (up to 256) by choosing Other for Color Depth and entering a value for Colors.

The options in the Indexed Color dialog box control only how the indexed color table is created. Adobe Photoshop still treats the image as an 8-bit, 256-color image.