Information Technology Reference
In-Depth Information
stored. The idea is that image information will be stored in some
compressed format in such a way that the computer can compute the
full bit map based on the compressed data.
images in a reasonably compressed way?
Several different schemes are used for the compression of im
ages, and some approaches may utilize a combination of these
schemes.
Color Tables: Although a color picture requires 3 bytes of infor
mation for each pixel, few pictures actually contain millions of dif
ferent colors. Instead, one could define a table of colors actually
used, and then specify colors based on that table. This table of col
ors is called a
color table
. For example, if an image contained only
100 different colors, then a table would have 100 distinct entries,
with each entry having a red, blue, and green value for that color.
The color of each pixel in the actual picture then could be specified
by indicating which table entry contained the appropriate color.
A file for an image using this scheme, therefore, would have to
contain the image information as well as a color table that has 3
bytes per entry. The image data, however, would be composed only
of table indices. In our example where an image contains only 100
colors, the numbers for up to 100 colors could be stored as 8bit en
tries in a table, so pixel information would require just 1 byte rather
than 3 (plus the original table). If the picture is large, the size of the
color table itself may be insignificant compared to bitbybit specifi
cation of the image according to colors in the table. Overall, this ap
proach could cut file size by approximately onethird.
Font Tables: Graphical information often involves text, so dis
play information for a specific type font can be stored in a table to
help conserve storage space. This table of font information is a
font
table
. One entry in the table might indicate the details for display
ing the letter “A” in a particular font, another entry would specify
the letter “B”, and so on. When a font table is used, an image con
taining letters would only need to indicate the name of each letter
and where to place it on a display. The table would define its font.
Although this yields no savings in space if a letter is used only once,
such a system can be effective if letters appear multiple times. Also,
