HTML and CSS Reference
In-Depth Information
5.2.1.4. PNG
The Portable Network Graphics (PNG) technology originated to replace
GIF, but not because GIF wasn't up to the job. Indeed, GIF was and
probably still is the most widely implemented graphics format on the In-
ternet. Instead, many Internet users got enraged when in 1993, after
GIF had attained its popularity and widespread use, Unisys decided to
enforce its patent and collect royalties on GIF's essential compression
technology. That action ran against the widespread philosophy of free
exchange and use enjoyed by the mostly academic community of In-
ternet users, and prompted an informal Internet working group led by
Thomas Boutell to develop the PNG alternative.
PNG's advantages over GIF and JPEG, besides providing a litigation-
free alternative format, include a broader selection of color formats
(24-bit true-color RGB, a grayscale and GIF-like 8-bit palette) and better
lossless compression. PNG's unique and attractive features include alpha
channels which let you specify many more than GIF's one layer of trans-
parency (more than 65,000, actually) and can simulate 3D imagery,
gamma correction which controls cross-platform image brightness for
more vivid graphics, and two-dimensional interlacing which provides for
a finer progressively developing image.
PNG does not support animation. Though you may hesitate to use PNG
on that basis alone, we encourage you to try it out anyway, especially
for high-color and high-quality images.
5.2.2. When to Use Images
Most pictures are worth a thousand words. But don't forget that no one
pays attention to a blabbermouth. First and foremost, think of your doc-
ument images as visual tools, not as gratuitous trappings. They should
support your text content and help readers navigate your documents.
Use images to clarify, illustrate, or exemplify the contents. Content-sup-
porting photographs, charts, graphs, maps, and drawings are all natural
and appropriate candidates. Product photographs are essential compon-
ents in online catalogs and shopping guides, for example. And link-en-
