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Graphic s Files and Formats
Vector
and Raster
Graphic s
files fall into two general categories: vector and raster. Vector
graphics describe shapes mathematically, whereas raster (or bitmap) files
describe shapes as a pattern of dots or pixels. Vector graphics are
not entirely supported on the web, but bitmap graphics enjoy widespread
compatibility and use on the Internet. For this reason this discussion
will focus on bitmap graphics.
Raster or Bitmap
Images
Raster or Bitmap graphics are produced by many small dots or pixels.
Pixels are dots of color arranged on a regular grid in a pattern representing
the image. The more dots a graphic image has, the better it looks, the fewer
dots it has, the poorer the appearance.
Bitmap images on
the web basically come in two formats: GIF
(Graphics Interchange Format) and JPG (Joint Photographic Experts
Group). [PNG (Portable Network Graphic) is another format supported
by newer browsers but not yet commonly used.] GIF and JPG were specifically
designed for network or Internet use combining quality image display
with compressed file size for quicker downloading.
GIF or JPG :
Which
do I use?
GIF
GIF is a good format for quality and file size when using simple images
that only have a few colors. GIFs have a maximum of 256 colors making
it suitable for flat-colored illustration. This format compresses
an image by counting rows of like colored pixels as a single unit.
A graphic containing a number of colors may not be efficiently compressed.
GIF files can be given special effects such as background transparency
and animation.
JPG
JPG format contains a maximum of 16 million colors and is suitable
for images that have an abundance of gradual colors. This format is
useful for scanned photographs and other complex images. JPG compression
divides the image into a certain number of boxes using a grid format
and then saves the predominant color within that box. The compression
rate, on most graphic programs, can usually be controlled by the user.
The smaller the file size the lower the image quality.
The rule of
thumb when selecting a format is where effects such as transparency
or animation are desired or where graphics with few colors are used,
GIF is preferred. For more complex graphics and photographs JPG is the
better choice.
| Graphic using flat areas of color. |
 |
|
| GIF
(3k) |
JPG
(4k)
compression used to save this JPG has compromised the flat areas
of color. |
 |
| Photographic
image |
 |
 |
GIF
(5k)
To reduce file size this image was saved as a 16 color GIF causing
the "banding" effect of color. |
JPG
(5k) |
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