Special Features 295PNGSpecial FeaturesWeb Design in a Nutshell, eMatter EditionNot a JPEG Substitute
Although PNG does support 24-bit color and higher, its lossless compression
scheme will nearly always result in larger files than JPEG’s lossy compression
applied to the same image. The high bit-depth support was developed so PNGs
could take the place of TIFF files for saving highly detailed images where loss of
image information is unacceptable (such as medical images). For Web purposes
where every byte counts, photographic and continuous tone images are still best
saved as JPEGs.
A Good GIF Substitute
PNGs are recommended for the type of image that would typically be saved as a
GIF (graphics with areas of flat color or sharp edges). PNG’s better compression
engine can result in a file size that is smaller than a GIF compression of the same
image. Bear in mind, however, that the efficiency of compression largely depends
on how well the PNG format is implemented in the graphics program being used.
PNG also has a more sophisticated interlacing technique than GIF and starts
displaying the image in 1/8th the time.
Special Features
You may choose to use a PNG (in that perfect world) for some of its advanced
features that no other graphic offers, such as variable transparency levels and full
color management systems for automatic image correction, including gamma and
color balance corrections.
Interlacing (Progressive Display)
Like GIFs, PNGs can be encoded for interlaced display. When this option is
selected, the image will display in a series of passes, the first displaying after only
a portion of the file has been downloaded, and each subsequent pass increasing in
detail and clarity until the whole image is rendered.
Interlaced PNGs display over a series of seven passes (using a method known as
“Adam7,” named for its creator, Adam Costello). The first rendering of the image
appears after only 1/64 of the file has downloaded (that’s eight times faster than
GIF). Unlike GIF, which fills in horizontal rows of information, PNGs fill in both
horizontally and vertically.
Gamma Correction
Briefly stated,gammarefers to the brightness setting of a monitor (for more infor-
mation on gamma, see Chapter 3,Web Design Principles for Print Designers).
Because gamma settings vary by platform (and even by manufacturer), the
graphics you create may not look the way you intend. In general, graphics created
on Macs look dark on PCs and graphics created on PCs will look washed out on
Macs.
PNGs can be tagged with information regarding the gamma setting of the platform
on which they were created. This information can then be interpreted by software