Raster Images vs. Vector Graphics
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Computer graphics can be created as either raster
or vector images. Raster graphics are bitmaps.
A bitmap is a grid of individual pixels that collectively
compose an image. Raster graphics render images
as a collection of countless tiny squares. Each
square, or pixel, is coded in a specific hue or
shade. Individually, these pixels are worthless.
Together, they're worth a thousand words.
Raster graphics are best used for non-line art
images; specifically digitized photographs, scanned
artwork or detailed graphics. Non-line art images
are best represented in raster form because these
typically include subtle chromatic gradations,
undefined lines and shapes, and complex composition.
However, because raster images are pixel-based,
they suffer a malady called image degradation.
Just like photographic images that get blurry
and imprecise when blown up, a raster image gets
jagged and rough. Why? Ultimately, when you look
close enough, you can begin to see the individual
pixels that comprise the image. Hence, your raster-based
image of Wayne Newton, magnified to 1000%, becomes
bitmapped before you can isolate that ravenous
glint in his eye. Although raster images can be
scaled down more easily, smaller versions often
appear less crisp or "softer" than the
original.
To maximize the quality of a raster image, you
must keep in mind that the raster format is resolution-specific
- meaning that raster images are defined and displayed
at one specific resolution. Resolution in raster
graphics is measured in dpi, or dots per inch.
The higher the dpi, the better the resolution.
Remember also that the resolution you actually
observe on any output device is not a function
of the file's own internal specifications, but
the output capacity of the device itself. Thus,
high resolution images should only be used if
your equipment has the capability to display them
at high resolution.
Better resolution, however, comes at a price.
Just as raster files are significantly larger
than comparable vector files, high resolution
raster files are significantly larger than low
resolution raster files. Overall, as compared
to vector graphics, raster graphics are less economical,
slower to display and print, less versatile and
more unwieldy to work with. Remember though that
some images, like photographs, are still best
displayed in raster format. Common raster formats
include TIFF, JPEG, GIF, PCX and BMP files. Despite
its shortcomings, raster format is still the Web
standard - within a few years, however, vector
graphics will likely surpass raster graphics in
both prevalence and popularity.
Unlike pixel-based raster images, vector graphics
are based on mathematical formulas that define
geometric primitives such as polygons, lines,
curves, circles and rectangles. Because vector
graphics are composed of true geometric primitives,
they are best used to represent more structured
images, like line art graphics with flat, uniform
colors. Most created images (as opposed to natural
images) meet these specifications, including logos,
letterhead, and fonts.
Inherently, vector-based graphics are more malleable
than raster images - thus, they are much more
versatile, flexible and easy to use. The most
obvious advantage of vector images over raster
graphics is that vector images are quickly and
perfectly scalable. There is no upper or lower
limit for sizing vector images. Just as the rules
of mathematics apply identically to computations
involving two-digit numbers or two-hundred-digit
numbers, the formulas that govern the rendering
of vector images apply identically to graphics
of any size.
Further, unlike raster graphics, vector images
are not resolution-dependent. Vector images have
no fixed intrinsic resolution, rather they display
at the resolution capability of whatever output
device (monitor, printer) is rendering them. Also,
because vector graphics need not memorize the
contents of millions of tiny pixels, these files
tend to be considerably smaller than their raster
counterparts. Overall, vector graphics are more
efficient and versatile. Common vector formats
include AI, EPS, CGM, WMF and PICT (Mac).
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