329
ADOBE ILLUSTRATOR CS2
User Guide
Bitmap images are the most common electronic medium for continuous-tone images, such as photographs or digital
paintings, because they can represent subtle gradations of shades and color. Bitmap images are resolution-
dependent—that is, they contain a fixed number of pixels. As a result, they can lose detail and appear jagged if they
are scaled on-screen or if they are printed at a lower resolution than they were created for.
200%
800%
Bitmap image at different levels of magnification
You can import bitmap images into an Illustrator document using the Open, Place, and Paste commands. You can
also create bitmap effects in your artwork using filters, effects, and graphic styles. When combining vector graphics
and bitmap images, it’s important to remember that how your artwork looks in Illustrator isn’t always how it will look
in its final medium (whether commercially printed, printed on a desktop printer, or viewed on the web). The
following factors influence the quality of your final artwork:
Transparency
Many effects add partially transparent pixels to your artwork. When your artwork contains trans
parency, Illustrator performs a process called
flattening
before printing or exporting. In most cases, the default
flattening process produces excellent results. However, if your artwork contains complex, overlapping areas and you
require high-resolution output, you will probably want to preview the effects of flattening. (See “Printing and
exporting transparent artwork” on page 484.)
Image Resolution
The number of pixels per inch (ppi) in a bitmap image. Using too low a resolution for a printed
image results in
pixelation
—output with large, coarse-looking pixels. Using too high a resolution (pixels smaller than
what the output device can produce) increases the file size and slows the printing of the artwork.
Printer resolution and screen frequency
The number of ink dots produced per inch (dpi) and the number of lines
per inch (lpi) in a halftone screen. The relationship between image resolution, printer resolution, and screen
frequency determines the quality of detail in the printed image. (See “Printer resolution and screen frequency” on
page 472.)
See also
“About vector graphics” on page 144
About image resolution
Bitmap images contain a fixed number of pixels, usually measured in pixels per inch (ppi). An image with a high
resolution contains more, and therefore smaller, pixels than an image of the same printed dimensions with a low
resolution. For example, a 1-inch-by-1-inch image with a resolution of 72 ppi contains a total of 5184 pixels (72
pixels wide x 72 pixels high = 5184). The same 1-inch-by-1-inch image with a resolution of 300 ppi would contain a
total of 90,000 pixels.