Microsoft Office Tutorials and References
In Depth Information
Color Depth
Images from other sources may be any of dozens of graphics formats, including PCX, BMP,
GIF, or PNG.
Different graphic formats can vary tremendously in the size and quality of the image they
produce. The main differentiators between formats are the color depth they support and
the type of compression they use (which determines the fi le size).
Remember earlier how I explained that each pixel in a 24-bit image requires 3 bytes? (That’s
derived by dividing 24 by 8 because there are 8 bits in a byte.) Then you multiply that by
the height and then by the width to determine the image size. Well, that formula was not
completely accurate because it does not include compression. Compression is an algorithm
(basically a math formula) that decreases the amount of space that the fi le takes up on the
disk by storing the data about the pixels more compactly. A fi le format will have one of
these three states in regard to compression:
No compression. The image is not compressed.
Lossless compression. The image is compressed, but the algorithm for doing so
does not throw out any pixels so there is no loss of image quality when you resize
the image.
Lossy compression. The image is compressed by recording less data about the pix-
els, so when you resize the image, there may be a loss of image quality.
Table 11.1 provides a brief guide to some of the most common graphic formats. Generally
speaking, for most on-screen presentations JPEG should be your preferred choice for graph-
ics because it is compact and Web-accessible (although PNG is also a good choice and uses
lossless compression).
TABLE 11.1 Popular Graphics Formats
Mai ntains
Transparency
Extension
Pronunciation
Compression
Notes
JPEG or JPG
“Jay-peg”
Yes
No
Stands for Joint Photographic
Experts Group. Very small
image size. Uses lossy compres-
sion. Common on the Web. Up
to 24-bit.
GIF
“gif” or
“jif”
Yes
No
Stands for Graphic Interchange
Format. Limited to 8-bit (256
color). Uses proprietary com-
pression algorithm. Allows ani-
mated graphics, which are
useful on the Web. Color depth
limitation makes this format
unsuitable for photos.
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