Graphics Programs Reference
In-Depth Information
the one we use in the following chapter. If each pixel needs 8 bits to store an
grayscale value, the memory required by the data is 729x713x8=4,158,216
bits or 4,158,216/8=519,777 bytes. This number is exactly what we obtain
by typing
whos
in the command window. Common prefi xes for bytes are
kilobyte, megabyte, gigabyte and so forth.
bit = a 1 or 0 (b)
8 bits = 1 byte (B)
1024 bytes = 1 Kilobyte (KB)
1024 Kilobytes = 1 Megabyte (MB)
1024 Megabytes = 1 Gigabyte (GB)
1024 Gigabytes = 1 Terabyte (TB)
It is important to note that in data communication 1 kilobit = 1,000 bits, while
in data storage 1 kilobyte = 1,024 bytes. A 24-bit or
true color image
then
requires three times the memory needed to store a 8-bit image, or 1,559,331
bytes = 1,559,331/1,024 kilobytes (KB) § 1,523 KB § 1,559,331/1,024
2
=
1.487 megabytes (MB).
In many cases, however, the dimension of an image is not given by the
total number of pixels, but the length and height of the picture and its reso-
lution. The resolution of an image is the number of
pixels per inch
( ppi) or
dots per inch
(dpi). The standard resolution of a computer monitor is 72 dpi
although modern monitors often have a higher resolution such as 96 dpi. As
an example, a 17 inch monitor with 72 dpi resolution displays 1,024x768
pixels. If the monitor is used to display images at a different (lower, higher)
resolution, the image is resampled to match the monitor·s resolution. For
scanning and printing, a resolution of 300 or 600 dpi is enough in most
applications. However, scanned images are often scaled for large printouts
and therefore have higher resolutions such as 2,400 dpi. The image used in
the next chapter has a width of 25.2 cm (or 9.92 inch) and a height of 25.7
cm (10.12 inch). The resolution of the image is 72 dpi. The total number of
pixels is therefore in horizontal direction 72*9,92 § 713, the vertical number
of pixels is 72 *10,12 § 729, as expected.
Numerous formats are available to save vector and raster data into a fi le.
These formats all have their advantages and disadvantages. Choosing one
format over another in an application requires a good knowledge of the
characteristics of the various fi le formats. This knowledge is particularly
important if images are to be analyzed quantitatively. The most popular for-
mats for storing vector and raster data are:
1.
Compuserve Graphics Interchange Format
(GIF) - This format was de-
veloped in 1987 for raster images using a fi xed colormap of 256 colors.
