PNG is a 5 year old lossless image file format that many
people are hoping will become the primary format for graphics
on the Internet. It stands for "Portable Network Graphics" and
is always pronounced as "ping".
Currently the majority of web based images are stored in
either the JPEG or GIF formats. JPEG's lossy compression
format has proven to be a very effective format for high
resolution complex images such as photographs. GIF uses a
lossless compression format like PNG and is most effective
with uniform images typically produced by computer graphic
applications.
So what is special about PNG when compared to GIF and
JPEG?
The fact is both JPEG and GIF have significant advantages
over PNG. GIF for example supports animation which is not
part of the PNG format. JPEG's lossy compression is capable
of producing smaller image sizes than either PNG and GIF
and will undoubtedly remain popular for displaying large
photographs.
PNG on the other hand, combines many of the advantages of
both GIF and JPEG with capabilities not available in either.
GIF only supports a maximum of 8 bit color tables while
JPEG is restricted to True Color or Gray Scale images. PNG
supports all 3 formats up to a maximum color depth of 64 bits
per pixel. This provides greater flexibility to compress an
image without sacrificing quality. In fact, a PNG image can be
compressed, expanded, edited and recompressed indefinitely
without loosing any information. At 8 bits, GIF images start
out without the ability to store all of the information in a
complex image while a JPEG image will progressively loose
information each time it is compressed even at the highest
quality setting.
The most powerful feature of PNG is its Alpha Channel
support. For those of you that are not familiar with an Alpha
channel, it is used to store transparency information in an
RGBA format. As many of you know, you can assign a
transparency value to a GIF image by selecting one if its color
tables as a transparency color. This provides 1 bit transparency
where a pixel is either visible or invisible. PNG's alpha
channel provides gradient transparency where a pixel can have
up to 256 ranges of transparency. This means that a PNG
image can be anti-aliased into its background to remove the
jagged edges that are apparent when a transparent GIF is
displayed. It also means that drop shadows can be blended into
any background image.
PNG supports gamma correction. This means that a PNG
image can be dynamically color corrected to display correctly
on different computer monitors. GIFs in particular can take on
a different color appearance when viewed on a MAC or PC.
PNG supports interlacing that is similar to GIF's line-
interlacing but closer in appearance to progressive JPEG. This
means that a PNG image can be quickly displayed with a
minimum of distortion that improves as the image is
progressively displayed.
Finally, PNG is not encumbered by patents, as is GIF.
Although it is not apparent to the majority of designers, GIF
uses an LZW compression algorithm that is patented by
UNISYS. UNYSIS in turn has imposed a license fee and other
restrictions on developers using GIF. This is a major
hindrance for many software developers that are actively
seeking an alternative to GIF. See the article at:
http://www.cloanto.com/users/mcb/19950127giflzw.html for
more information on this interesting topic.
So where does this leave PNG? Unfortunately with still a way
to go. Although PNG is officially sanctioned by the W3C and
preferred by the majority of graphic application developers, it
still has several hurdles to overcome. The primary hurdle is it's
lack of full support in several Browsers. IE5 and NN5 will
implement many of PNG's capabilities. Several older browsers
however, have either limited or incompatible support. Web
designers will consequently have to design only for the most recent
browsers or will have to provide alternatives for visitors with
older browsers.
Another limitation is its lack of support for animated images.
Animated GIFs have become very popular on many web sites. An
extension to the PNG standard called MNG will provide more
advanced animation than GIF but it will be some time before it
is fully available. Until it is, PNG will not be able to fully
replace GIF.
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