Page 205 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 12: Image Format

Table 12.3 provides a comparison of the Web bitmapped formats largely taken from the electronic notes
documentation for Macromedia’s Fireworks image creation program.

table 12.3: Comparison of Bitmapped formats used for Web pages

Gif JpG pnG
Colour depth 8-bit maximum (256 Up to 24-bit (16.77 million Up to 32-bit [24-bit colour
colours) colours) and 8-bit alpha channel]
Compression Lossless (Means that there Lossy (means there is Lossless
is no loss of image quality irretrievable loss of image
in the compression); data in the compression);
compresses solid areas of compresses subtle colour
colour well transitions well
transparency Yes No Yes
support
advantages • Lossless compression • Ability to control • Lossless compression
• Transparency quality loss in • Alpha transparency
• Most efficient for compression • High colour support
illustrations or • Excellent compression
drawings of photographic images
Disadvantages • Maximum of 256 • No transparency • Despite its
colours • Loss of quality advantages, not
• Does not compress when compressed widely used
gradient colours well • Can’t be displayed
by older browsers
typical uses • Carton images • Scanned photographs • Images with high
• Logos • Images with complex numbers of colours
• Animated banners textures • Complex, live
• Images with complex transparency
gradient colours

12.2.2 structured Graphics
Rather than saving each pixel of an image, the structured formats save formulas for drawing
the images (e.g., use a line this colour and width, start drawing at this point, move to this point,
fill with this colour, etc.). As a result, the file sizes are much smaller than those for bitmapped
images. However, the structured formats are generally only used for illustrations (images which
look like drawings), rather than photo-realistic images. The most common “generic” structured
formats for Windows are the Windows Meta File (.wmf format), the standard vector-based file
format for Windows and the WordPerfect Graphic format (.wpg extension) used by DrawPerfect
and the clip art which comes with WordPerfect. Adobe’s illustrator, Macromedia’s FreeHand and
Corel’s CorelDraw are all structured graphic programs.

Web browsers cannot display the structured formats; clip art would have to be changed into one
of the bitmapped formats to be displayed on a page. As a result, structured file formats would
typically be used for graphic elements in presentations. However, because of the bandwidth
problems on the Web and because structured graphics produce small file sizes, there is a trend
to introduce some structured formats into Web usage. Macromedia’s Flash, recently made an
open standard for everyone to use, works with structured formats that (at this point) require
plug-ins to display.

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