page 135

12.6 COLORS

•It has been long know that the eye can be tricked into seeing a wide range of colors by blending three different colors in different intensities.

•In the additive color scheme we add red blue and green (RGB).We start with black and add shades of these colors.

•In the subtractive color scheme we start a white pixel. The intensity is reduced by filtering the colors magenta, cyan and yellow.

•Most computers use the RGB scheme, but the subtractive color scheme is popular in printing and photographic reproduction techniques.

•Some of the techniques used when limited numbers of colors or shades are available are,

-colors maps

-dithering

12.6.1 Color Maps

•A color map is a list of colors that the computer can use to draw with.

•The eye is very sensitive and can sense millions of different colors. And current trends are to go to 24 bit color systems that have 8 bits for each primary color. This gives the ability to display different colors so close that the human eye cannot detect the difference.

•For various reasons we will use machines that have limited numbers of colors available (256 is common).

•When this occurs colors that should look like smooth transitions tend to look more like bands of color.

•One approach to providing colors is to construct a well distributed pallet (a fixed set of colors) that the user can select from. They must always find the best match to their desired colors.

•The eye tends to be more sensitive to certain colors, and so one approach is to map the colors into a pallet using color bit assignments. For example for a pallet of 256 (8 bits) we may choose to assign 3 bits to blue, 3 bits to blue and only 2 bits to red. This means that there will be 8 intensity levels for both green and blue, but only four for red.