PNG is a graphic file format and provides a patent-free
replacement for GIF and can also replace many common uses of TIFF.
Indexed-color, grayscale, and truecolor images are supported, plus an optional
alpha channel. Sample depths range from 1 to 16 bits.
One
reviewer of the .png format writes, “PNG is designed to work well in online
viewing applications, such as the World Wide Web, so it is fully streamable
with a progressive display option. PNG is robust, providing both full file
integrity checking and simple detection of common transmission errors. Also,
PNG can store gamma and chromaticity data for improved color matching on
heterogeneous platforms.”
If you are a graphics designer or develop web pages, PNG
files are used in HTML documents and STATISTICA 6.1 graphs embed into the HTML
documents as PNG files. Such graphic
design packages as Adobe Photoshop and web-design programs as Macromedia’s
Flash support the excellent design features that a PNG file offers.
Andrew Zolli, senior technologist at Siegel & Gale, an
international strategic communications, design, and interactive media
development company and one of the developers of the software PNG Live
has outline these important advantages:
- More colors. While GIF images are
limited to 8-bit color, PNG images can be of any bit depth up to 48-bit
color, allowing for images that contain literally trillions of colors -
more colors than the human eye can see. This greatly expanded palette is
essential for professional Web site developers, who must faithfully
reproduce color logos and marketing materials for their clients.
- Better compression. Because it uses a
better compression method than the LZW (Lempel Zev Welch) algortihm used
in GIF, 8-bit PNG images are 10-30% smaller than identical GIF images.
Obviously, smaller files mean faster pages: the W3C has estimated that
replacing GIF with PNG, using Cascading Style Sheets (CSS), and
implementing HTTP 1.1 will make the Web run two to eight times faster,
with no other infrastructure improvements. Such speed improvement will
allow developers to be more visually expressive without damaging the
user's experience.
- Alpha channels. The PNG format supports
a completely different kind of transparency than GIF. In a GIF image, each
pixel is either transparent or opaque: This is called binary
transparency. In a PNG image, each pixel can have one of 256 levels of
relative transparency, from completely opaque, to semitransparent, to
completely transparent. This improved transparency has several benefits.
First, a wide range of creative possibilities emerge, such as allowing Web
page designers to layer semitransparent images on top of one another.
Second, with PNG, images become much more portable. Today, each
transparent GIF image which appears in a Web page must be antialiased to
the specific background against which it appears. If the background of the
page is changed in any way, a "halo" of pixels appears around
the GIF image content.With the PNG format's built-in alpha channels,
however, this halo disappears, and images can be made to blend seamlessly
with any background. This is a subtle but critical improvement, since the
reproduction of GIF images for different backgrounds is an enormous drain
on many developers schedules and inflates the cost of site development.
- Gamma and chromaticity correction. PNG supports true gamma
correction and color correction, so that images created on one operating
system look "correct" when viewed on any other operating system.
This is particularly important for many Web designers who create images on
the Mac OS, which typically has a much "brighter" display than
the Windows operating system. In principal, this means that Web page
designers will be able to reduce the amount of cross-platform quality
assurance testing that they do, lowering the cost and development time for
Web sites.
- Searchable Meta-Data. A PNG image can contain
meta-data, non-displaying information that identifies the image's
contents, revision history, or authorship. Users will be able to use
search engines to look for individual images rather than having to search
for pages that contain those images. Also, developers will be able to
track image assets in a much more straightforward and powerful way.
- Improved Interlacing. PNG employs a new
interlacing method that "blurs" an image onto the screen
horizontally and vertically. This method is far superior to the GIF
"window shade" approach, and delivers a usable image eight times
faster than GIF. Also, according to several usability analysis studies,
small, bitmapped text in an interlaced PNG image can generally be read two
times faster than in a GIF. This improved usability comes in addition to
the speed improvements measured by the World Wide Web Consortium's study.
- Royalty-free license: Work on PNG began in
response to the announcement by Compuserve and UNISYS that they would
charge users and developers for using the GIF format. Unlike GIF, PNG was
developed from the start as a royalty-free format that could be used and
developed without cost to developers and users. What's more, there are
already source code libraries for parsing and creating PNG images
available freely online, including some early Java classes for displaying
PNG images. This royalty-free source code is a tremendous aid to
developers who wish to embed PNG support in their existing or planned
applications.
- Easy extensibility: Unlike GIF, PNG uses
an easily extensible format, similar in concept to HTML: data in a PNG
image is encapsulated in "chunks" (like HTML tags) which are
read by the PNG parser. If a PNG parser encounters an unrecognized chunk
type, it will ignore that chunk. This allows developers to easily add new,
application-specific data to PNG images.
- "Smart" signing: A PNG file contains an
internal signature that can detect the most common types of file
corruption and report on the nature of the problem. Unlike most file
signatures, the PNG file signature contains network sensitive characters -
such as the escape, new line, and line reset characters - which are often
altered if the PNG file is transmitted improperly, for example when the
PNG MIME type is not set correctly in the server's MIME.TYPES file. This
allows developers to much more easily diagnose what went wrong when an image
was transmitted improperly. [i]
If you are
working with graphic design needs in mind, in the important areas of file size,
importing, masks, and transparent gradients, you should be “PNG-ing” to capture
the excellent advantages.[ii]
[i]
http://developer.netscape.com/viewsource/png.html
[ii]
http://www.eyewire.com/magazine/columns/scott/png/