Information Technology Reference
In-Depth Information
1 byte is used to store the intensity of each of these colors. Thus,
each pixel has 3 bytes of information: one for red, one for green,
and one for blue. A pure red pixel would have intensity 11111111
for red, and 00000000 for both green and blue. Because the color
white contains a full measure of red, green, and blue, the code for a
white pixel would have intensity 11111111 for all three hues. In
contrast, black is the absence of color, so a black pixel's code would
specify 00000000 for red, green, and blue.
This use of pixels and red/green/blue intensities to store image
information yields what is called a digital picture . In digital technol-
ogy , a wide range of color intensities is possible for any pixel (256
possible reds, 256 greens, and 256 blues), but this range is not infi
nite. In contrast, a painter mixing paint has complete control of the
combinations and hues of each color she or he creates, allowing a
much fuller range of possibilities. Also, the painter need not worry
about dots, but applies the paint continuously over the canvas or
paper. The physical approach of using the paint itself without re
striction is called analog technology , and is different both conceptu
ally and practically from the digital technology used with bits and
bytes in a computer.
A color picture uses 3 bytes of color information for each pixel,
so a full picture would require three times the storage of a full black
and white picture (with shading). For the 3 5" image stored at
300 bpi, this yields a requirement for 4,050,000 bytes of storage—
about 4 megabytes.
The pixelbypixel storage of all color information provides
complete detail about every aspect of a picture, and this collection
of information is called a bit map . Typically, a full file for a bit map
of one picture would include a header (with some type, format, and
size information), general information (such as width, height, and
resolution), and the image information of 3 bytes per pixel. On
many computer systems, this material is labeled as a simple version
of a “.BMP” file. Unfortunately, as our computation indicates, stor
age of all color information in such a file yields rather large files—
even for relatively small pictures—taking up considerable space if
they are stored or requiring significant time to download (particu
larly if one's Internet connection is fairly slow).
Because bit maps require large amounts of data, various ap
proaches have been developed to reduce the amount of data actually
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