http://hotwired.lycos.com/webmonkey/design
Bitmap
(or raster) graphics are stored as a series of tiny dots called pixels. Each
pixel is assigned a color, and when they are viewed together, they form a
picture.
Vector
graphics are not based on pixel patterns, but instead use mathematical formulas
consisting of lines and curves that make shapes. With the exception of Flash
(vector graphics program) most browsers do not support vector graphics on
the web.
A
computer screen generally displays 72 pixels per inch and most printers¹ work
with 600-2400 pixels per inch. The terms ppi, pixels/inch, dpi, dots/inch
and spi, samples/inch all are different terms for describing how the grid
is laid out which defines an image¹s resolution. The only difference between the terms is the devices being
used. Scanners and monitors generally use PPI or spi. Output devices use dpi
or lpi. Lpi is lines per inch.
Drawing
programs like Freehand and Illustrator are vector based programs. They use
math to describe the shapes that make up an image. The advantage of using
math to describe a shape lies in its flexibility. It doesn't have a set resolution,
i.e. it is ³scalable². An image displays and prints at the highest resolution
that the output device supports. This is very different from raster images,
which have a fixed resolution. Fonts (text) are usually vector based.
The
way each format handles resolution is very important. In Photoshop to achieve
the highest quality you need to work at the correct resolution from the beginning.
Each time you resize or alter the resolution the quality suffers.
As
you can see a raster based image at 72 dpi has fairly jagged edges. When increasing
the size to 200% the resolution drops to 36 dpi and it is really jagged. On
the other hand a vector based image looks much less jagged since the curves
are defined mathematically and resizing the image has no effect on quality.
Raster
based image at 100%
Raster
based image at 200%
Vector
based image at 100%
Vector
based image at 200%
1.
Digital Photographs
1.
Logos, which are generally a few solid colors and need to be shown at a variety
of sizes
2.
Text
1.
Resolution independent
2.
Smooth curves
3.
Small file sizes for computer drawn artwork (vector art and type)
1.
Photographs usually look more accurate as
raster images. The gradients are smoother and it maintains the subtle details
within the photograph.
2.
Photographs are almost always smaller as
raster images than vector images.