Adobe Illustrator CS3 - Manual (Page 264)

ILLUSTRATOR CS3
User Guide
258
Importing bitmap images
About bitmap images
Bitmap images—technically called raster images—use a rectangular grid of picture elements (pixels) to represent
images. Each pixel is assigned a specific location and color value. When working with bitmap images, you edit pixels
rather than objects or shapes. Bitmap images are the most common electronic medium for continuous-tone images,
such as photographs or digital paintings, because they can more efficiently 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 to high magnifications on-screen or if they are printed at a lower resolution
than they were created for.
Example of a bitmap image at different levels of magnification
Bitmap images sometimes require large amounts of storage space, and often need to be compressed to keep file sizes
down when used in certain Creative Suite components. For instance, you compress an image file in its original appli-
cation before you import it into a layout.
Note: In Adobe Illustrator, you can create bitmap effects in your artwork using filters, effects, and graphic styles.
See also
“About vector graphics” on page 52
Image resolution guidelines for final output
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.
For imported bitmap images, image resolution is determined by the source file. For bitmap effects, you can specify
a custom resolution. To determine the image resolution to use, consider the medium of final distribution for the
image. The following guidelines can help you determine your requirements for image resolution:
Commercial printing Commercial printing requires 150 to 300 ppi (or more) images, depending on the press (dpi)
and screen frequency (lpi) you’re using; always consult your prepress service provider before making production
decisions. Because commercial printing requires large, high-resolution images, which take more time to display
while you’re working with them, you may want to use low-resolution versions for layout and then replace them with
high-resolution versions at print time.
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