HTML and CSS Reference
Certain bitmap formats such as GIF and PNG provide the option to display a degraded fast preview of an image before
it is fully downloaded. The method is known as interlacing . Advanced image-processing software applications support
interlacing and offer the choice to save such files either as interlaced or as noninterlaced . A similar effect can be
achieved in photographs by applying a frequency decomposition hierarchy in progressive JPEG files. In the era of slow,
dial-up connections, interlacing was a useful feature but lost most of its significance after the widespread distribution
of broadband Internet access.
Partially transparent images are very popular, and can be used for design effects and images that can be displayed on
different background colors or textures (Figure 9-8 ).
Figure 9-8. An image on three different backgrounds (first row). The second row is magnified to 600 percent. The
transparent shadow perfectly fits to all backgrounds
Not all raster image formats support transparency, though. GIF is generally a good (sometimes the best)
option for partially transparent geometric shapes and cartoon-style images, while PNG is ideal for more complex
(photorealistic) partially transparent images. With the exception of really simple shapes with relatively large areas of
the same color, PNG is the better option.
Raster File Formats
The most common raster image file formats on the Web are JPEG, GIF, and PNG.
Joint Photographic Experts Group (JPEG) is a common lossy compression format for digital photographs. Part 1
has been standardized in ISO/IEC 10918-1:1994  and ITU-T T.81 (09/92) , Part 2 in ISO/IEC 10918-2:1995 
and ITU-T T.83 (11/94) , Part 3 in ISO/IEC 10918-3:1997  and ITU-T T.84 (07/96) , Part 4 in ISO/IEC 10918-
4:1999  and ITU-T T.86 (06/98) , and Part 5 in ISO/IEC FCD 10918-5 .
The original version of the Graphics Interchange Format, GIF87a, was introduced in 1987 , and the enhanced
version with transparency and interlacing support, GIF89a, was introduced in 1989 .
The Portable Network Graphics (PNG) format was standardized by IETF RFC 2083 in 1997  and
ISO/IEC 15948 in 2004 . However, the PNG format has not gained full support until recently. Although Internet
Explorer supports PNG images from version 4.01b, the implementation was neither complete nor correct until IE9