Raster and Photoshop
First off, a raster graphic (also sometimes called a bitmap graphic,
but not to be confused with the .bmp file format) is an image that's
comprised of tiny blocks of colour called pixels. Zoom in close enough
on a raster image, and it begins to pixelate, like a mosaic. Raster
graphics are what we call "resolution dependent" images. Essentially
what that means is that quality is always a concern. Without getting
into a lengthy conversation about resolution and re-sampling, let's just
keep it simple for now and say that resizing your raster graphics is
where problems, sometimes major problems, arise.
Examples of raster-based images include photographs, scans, digital
paintings, website components like buttons and header graphics, and any
other image that's made up of a lot (like, millions) of colours.
Raster-based file formats include JPEG, GIF, PSD, PNG, and a few others.
So if there's a bunch of up front know-how, why even bother with
raster graphics? Cuz they're cool! And this is where Photoshop comes in.
Photoshop is our raster image editor. This is where we'll be doing all
our colour correction on our photographs, touching things up, or
creating special effects (like putting two heads on your
mother-in-law...I recall there's a menu command for that one).
Conceptual artists, digital painters, and comic book artists also turn
to Photoshop for a lot of their work, too. As a web designer, I
constantly use Photoshop to help mock up layouts, create web interfaces,
and develop a lot of graphical components for sites I'm working on. As
you can see, Photoshop is very flexible.
Vector and Illustrator
we have the world of vector graphics. Unlike raster graphics, vector
images are "resolution independent," meaning you and I don't have to
worry about quality, pixels, resolution, or re-sampling. Rather than
being based on pixels, vector graphics are based on mathematical lines
and curves. Of course, this is where Illustrator comes into play. And
what I love so much about Illustrator is I can take something I've
created, like a cartoon, and I can scale it larger or smaller, and not
give a second thought to quality. A vector graphic is always top
quality. Usually, but not always, vector graphics are images that are
comprised of solid blocks of colour, like a cartoon, a company logo, or a
block of text. So who's making use of Illustrator? Graphic designers,
visual artists, apparel companies, and even technical illustrators.
Vector-based file formats include AI, SVG, and Flash's FLA and SWF.
http://msauer.mvps.org/vector%20bitmap.htm
Vector vs. Bitmap
Programs like MS Paint and Photo Shop are both bitmap
applications, treating the images you work with as a fixed-size resource made
up of a fixed number of dots, or pixels. Once a line or curve or piece of text
has been 'committed' to the canvas you cannot go back and change it without
undoing and starting over again.
Also, since the information in a picture is
represented by dots, you cannot enlarge the image without exaggerating the
effect of these dots and making the picture look jagged. One advantage of using
this scheme is that scanned photographs are always represented as bitmaps (the
detail in the average photograph is way too complex to be represented as
vectors), so if you want to work with these items you'll need a bitmap based
program. Another advantage is the mathematical functions that can be performed
on a bitmap, such as averaging the pixels to create a blur, or edge detection
routines that emboss.
Vector based applications such as Corel Draw and Adobe
Illustrator treat images as collections of vectors and shapes. A line would have
a starting point, direction and length, a rectangle would have a starting
point, width and height, circles would have a center and radius, and so on.
After drawing a rectangle you can go back and change its width and height,
bring it to the foreground or send it to the background, even after other
shapes had been drawn on top of it later. When saved to file, vector images
also take up less disk space, since, for example in the case of a rectangle,
the program is only storing four numbers no matter what its size: the x and y
starting point, plus the width and height. In comparison, a bitmap application
would have to store colour information for the 10,000 pixels that make up a
100x100 pixel rectangle.
Another advantage of Vector based applications is that
you can re-size the final image to be as large or as small as you like and
never obscure the detail with jagged edges.