Information Technology Reference
In-Depth Information
processed using such ideas as data averaging and sampling accord
ing to how much detail and clarity a user specifies. These abbrevi
ated files then are made even smaller using data compression. If the
user wants a highquality image, then the 8 by 8 blocks maintain
much of their content, and relatively little compression can take
place. If a relatively lowquality image is acceptable, then informa
tion (particularly about color) may be discarded in order to produce
rather small files. When using the JPEG format, the creator of an
image must determine how much detail and clarity are needed, and
instruct the image processor accordingly. Normally, such decisions
rely upon finding an appropriate balance between image quality and
file size.
Because JPEG files involve adjustments of a picture based on
computations, picture quality usually deteriorates each time a pic
ture is processed and saved. This quality is not true of the GIF or
PostScript formats. On the other hand, using the JPEG file format
can result in remarkable reductions in file size.
Do these differences among various file formats
really matter?
As noted in the previous section, each file format has particular
advantages and disadvantages, which in turn affect when you would
want to choose one format over another. Let's look at each in turn.
The GIF format is based on storing up to 256 colors in a color
table. This produces crisp, clean pictures unless an image contains
considerable shading with many colors. In such cases, shades of
color may be adjusted to fit within the 256 limit, and coloring may
be lost. Because each pixel is stored separated, .gif files may not
scale well. Further, large images have many pixels, and a .gif file
may not contain enough information when a picture is expanded
in size.
The PostScript format is based on drawing an image and works
well when shapes are used. Because pictures are drawn, images
saved in PostScript formats often scale up in size well. On the other
hand, if shapes are sufficiently irregular, drawing can be difficult; at
the extreme, each pixel might need to be drawn separately. This can
result in large files in certain circumstances.
 
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