Page 121 - Digital Analysis of Remotely Sensed Imagery

92 Cha pte r T h ree

scope of this section. Instead, the discussion will center around four
commonly used image formats: generic binary, Graphic Interchange
Format (GIF), Joint Photographers Experts Group (JPEG), and Tagged
Image File Format (TIFF). They are recognizable by most image pro-
cessing systems, if not all. These formats play an essential role in trans-
ferring raw or processed image data among different image analysis
systems, and between image analysis systems and GIS.

3.3.1 Generic Binary
In the generic binary format, all image pixel values are represented
as binary data of 0s and 1s without header information. Each pixel is
represented as a byte. Thus, an image of 512 rows by 512 columns
requires a space of 262,144 bytes to store. Ancillary image information,
such as the number of rows and columns, and the number of spectral
bands, is stored in the header, separate from the image data. The
image analyst needs to specify these parameters when importing a
generic binary image into an image processing system. If the header
information is lumped together with the binary image file, the
analyst needs to specify the exact number of bytes used to store it as
well as the number of spectral bands when importing the data. In
this way the computer knows how many bytes to skip before it starts
to read the image data. Importing a generic binary image cannot be
successful unless all of the above information is supplied correctly
and completely.
Generic binary is an image format that enables image data to be
stored faithfully without any loss of information. However, it is also
cumbersome to read generic binary images. They have to be converted
to other graphic image formats as not many systems can read them
directly. So this format is not widely used for storing remote sensing
imagery. For exchanging image data between different systems, other
generic image formats are more user-friendly than generic binary,
and thus preferred.

3.3.2 GIF
Initially developed by CompuServe in 1987, GIF is a standard defining
a mechanism for storing and transmitting satellite imagery data
(Fulton, 2008). A GIF file is made up of several parts, including a
signature, a screen descriptor, global color map, image descriptor,
local color map, and finally raster data (CompuServ, 1987). Unlike the
other components, the last three parts are repeated many times. The
screen descriptor contains all ancillary information about the image,
such as the number of bits used, image width and height, and
background color, and so on. Though optional, the global color map
is recommended for accurately rendering color images. If the global
color map is present, 3 bytes are used to specify the relative intensities
of red, green, and blue, respectively. The image descriptor defines the