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456 ADOBE ILLUSTRATOR CS2
User Guide
Web graphics formats and options
Comparing web graphics formats
Web graphics formats fall into two categories: bitmap and vector. The bitmap formats—GIF, JPEG, PNG, and
WBMP—describe artwork as a series of colored dots called pixels. Each pixel in a bitmap image has a fixed size and
is therefore
resolution-dependent , meaning that dimensions of the image depend on the resolution of the monitor on
which it is viewed. The vector formats—SVG and SWF—describe artwork mathematically, as a set of geometric
objects. Because of this, vector graphics are
resolution-independent and can be scaled up or down without losing any
image quality.
GIF and PNG-8
GIF is the standard format for compressing images with flat color and crisp detail, such as line art, logos, or illustra­
tions with type. Like the GIF format, the PNG-8 format efficiently compresses solid areas of color while preserving
sharp detail; however, not all web browsers can display PNG-8 files. (See “Optimization options for GIF and PNG­
8 formats” on page 457.)
PNG-8 and GIF files support 8-bit color, so they can display up to 256 colors. The process of determining which
colors to use is called
indexing , so images in GIF and PNG-8 formats are sometimes called indexed color images. To
convert an image to indexed color, Illustrator builds a color lookup table, which stores and indexes the colors in the
image. If a color in the original image does not appear in the color lookup table, the application either chooses the
closest color in the table or simulates the color using a combination of available colors. (See “To customize the color
table for GIF and PNG-8 images” on page 459.)
JPEG
JPEG is the standard format for compressing continuous-tone images such as photographs. Optimizing an image as
a JPEG format relies on
lossy compression, which selectively discards data. (See “Optimization options for JPEG
format” on page 460.)
PNG-24
PNG-24 is suitable for compressing continuous-tone images; however, it produces much larger files than JPEG
format. The advantage of using PNG-24 is that it can preserve up to 256 levels of transparency in an image. (See
“Optimization options for PNG-24 format” on page 460.)
SWF
The Macromedia® Flash™ (SWF) file format is a vector-based graphics file format for the creation of scalable,
compact graphics for the web. Since the file format is vector-based, the artwork maintains its image quality at any
resolution and is ideal for the creation of animation frames. (See “About Flash graphics” on page 450 and “Optimi­
zation options for SWF format” on page 461.)
SVG
SVG is a vector format that describes images as shapes, paths, text, and filter effects. The resulting files are compact
and provide high-quality graphics on the web, in print, and even on resource-constrained, handheld devices. (See
“About SVG” on page 447 and “Optimization options for SVG format” on page 462.)