Color Palettes

 

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Feb 26, 2001

Color Representations

There are basically three modes of representing color combinations on a display:

Palette: an indexed array usually with 256 entries, each three bytes long, representing the amounts of R,G, and B to be used to represent the color at the pixel in question.

Hi-Color: Sometimes referred to as "thousands of colors", uses 16 bits to represent colors, thus allowing 64K colors to be displayed.

True-Color: Sometimes referred to as "millions of colors", uses 24 bits (3 bytes) to represent colors, thus allowing 256*256*256 colors to be displayed.

On the WWW, images with no more than 256 colors are sent as palettized images (usually .gif files), to reduce bandwidth and time for downloading.  If the image has more than 256 colors, it is usually sent as a compressed .jpg file.


Example: Show the following image in different representations:

Pseudo-Color

Since the human eye is so limited in distinguishing shades of gray, scientific data images are frequently pseudo-colored, with each data value being assigned some color.  Since the eye is very sensitive to color changes, this allows subtle changes in the data to be more easily discerned.  For psychological reasons, specialized palettes are used that have a natural interpretation of less-to-more, as for instance from blue to red, indicating increasing temperature.  In general, of course, there is no natural way to specify one color as "more" than another.

 

Last modified on February 28, 2001