indexed color

(redirected from color map)
Also found in: Wikipedia.

indexed color

A method of compressing an image that enables 8 bits per pixel to look almost as good as 24 bits per pixel. The technique determines the 256 most frequently used colors in the image and creates a color lookup table, also called a "color map" or "color palette," that is stored with the image. Rather than each pixel in the image having all three RGB colors (one 8-bit red, one 8-bit green and one 8-bit blue), each pixel contains one 8-bit number that indexes into the 256-color lookup table, which contains the RGB values (see illustration below).

It Used to Be a Major Problem
When early computer screens were commonly limited to 256 colors, indexed color methods were essential. Even then, two indexed photos on screen at the same time with vastly different color schemes would overload the hardware's color capacity and display improperly. Today, computer hardware easily renders full 24-bit color, but 8-bit indexed images are still widely used to save bandwidth and storage space. See bit depth, color palette and color lookup table.


Color Palette Index
The pixels in the image contain index numbers that point to the RGB value in the color lookup table. The RGB values are the ones used by the display system.







24-Bit Vs. 8-Bit
Indexed color images can look nearly identical to their 24-bit originals, because the 256 most frequently used colors are identified. The 256-color palette (top) was created in Photoshop from the original 24-bit image (left) to generate the image as 8 bits (right).


24-Bit Vs. 8-Bit
Indexed color images can look nearly identical to their 24-bit originals, because the 256 most frequently used colors are identified. The 256-color palette (top) was created in Photoshop from the original 24-bit image (left) to generate the image as 8 bits (right).
References in periodicals archive ?
Once digitized, the data are displayed in a 3-D color map that visualizes geometric variation.
It creates a color map that shows the deviation of the part surfaces between the two sources.
When hail occurs, a regional color map is created along with city-level maps for cities within the region.
In a seven-page section on the Russian River wine country, color labels of several wineries are displayed with entries on them, and the location of 16 of the more acclaimed wineries are noted on a detailed, color map.
The PC-based touchscreen features on-screen, touch activated controls and a customized color map floor plan of resident rooms.
VISTA's unique color map provides an overview of the risks within a one-mile radius.
The design offers b&w and color historical and contemporary photos and illustrations, plus color map charts throughout and detailed color maps on the inside covers.
The Touchscreen is a PC-based solution that offers on-screen touch-activated controls and a customized color map of resident rooms.
It performs draft analysis to identify undercuts and zero-draft areas though a user-defined color map of draft angles.
The PC-based Graphical Touchscreen Master offers on-screen touch-activated controls and a color map floor plan of patient rooms.
Then there's the eyewear trend that isn't anyway near the color map.
It also handles all popular image file formats: TGA (8-bit grey scale, 8-bit color map, 16/24/32-bit color), PCX (8-bit grey scale, 8-bit color map), GIF (8-bit grey scale, 8-bit color map), TIFF (8-bit grey scale, 8-bit color map, 24-bit color), BMP (Windows and Presentation Manager 4/8-bit color map, 24-bit color), JFF (JPEG JFIF File Interchange Format, which extends as JPG), JTF (JPEG TIF) and CMP (proprietary LEAD Technologies format) files.