Indexed color
From Wikipedia, the free encyclopedia
Indexed color is a type of color space for digital images. Whereas an RGB image specifies a red, green, and blue value separately for each pixel in the image, an indexed color image maintains a table that defines a number of predefined colors, and each pixel refers to a color in that table.
Indexed color can greatly reduce file sizes for images—especially large ones—using only a few different colors (because each pixel needs only to use a few bits to refer to a space in the color table, rather than many bits to refer to subtle hues), but often results in very large file sizes for photographs or images with many subtle color shades. If an indexed color image has too small of a color table, gradients and other shadings can appear blocky (although dithering can be used in some cases to reduce this effect).
This table is usually called a Colour Look-Up Table or palette.
GIF images—which are very common on the Internet—use indexed color, as do some but not all PNG and TIFF images.
Although indexed color is usually used only with the RGB color model, the PDF file format does support indexed color in other colorspaces, notably CMYK, and Adobe Distiller by default will convert images to indexed color whenever the total number of colors in an image is less than 256. This occasionally causes confusion for people who have been trained to think that the word "indexed" always implies RGB. PDF seems to be alone at the moment in this use; although an extension to TIFF was proposed in 1995 [1], there does not seem to be any software which can produce images in this format, nor do any other image file formats use indexed color with CMYK (or any other color spaces other than RGB).
