PNG Compression 293PNGPNG CompressionWeb Design in a Nutshell, eMatter EditionUpdated browser support information can be found at the PNG Home Page main-
tained by Greg Roelofs (http://www.cdrom.com/pub/png/pngapbr.html).
8-Bit Palette, Grayscale, and Truecolor
PNG was designed to replace GIF for online purposes and the inconsistently
implemented TIFF format for image storage and printing. As a result, there are
three types of PNG files: indexed color (palette images), grayscale, and truecolor.
8-Bit Palette Images
Like GIFs, PNGs can be saved as 8-bit indexed color. This means they can contain
up to 256 colors, the maximum number that 8 bits of information can define.
Indexed color means the set of colors in the image, its palette, are stored in a
color table. Each pixel in the image contains a reference (or “index”) to its corre-
sponding color and position in the color table.
Although 8-bit is the maximum, PNGs may be saved at lower bit-depths (1-, 2-,
and 4-bit, specifically) as well, thus reducing the maximum number of colors in
the image (and the file size).
Grayscale
PNGs can also support 16-bit grayscale images—that’s as many as 65,536 shades of
gray (2^16 ), enabling black and white photographs and illustrations to be stored
with enormous subtlety of detail. This is useful for medical imaging and other
types of imaging where detail must be maintained, but it is not much of an advan-
tage for images intended for web delivery. Grayscale images are also supported at
1-, 2-, 4-, and 8-bit depths as well.
Truecolor
PNG can support 24-bit and 48-bit truecolor images. “Truecolor” refers to the full
color range (millions of colors) that can be defined by combinations of red, green,
and blue (RGB) light on a computer monitor. Truecolor images do not use color
tables and are limited only by the number of bits available to describe values for
each color channel. In PNG format, each channel can be defined by 8-bit or 16-bit
information.
PNG Compression
The most notable aspect of PNG compression is that it is “lossless,” meaning no
information is lost in the compression process. A decompressed PNG image will
be identical to the original.
PNGs use a “deflate” compression scheme. Like GIFs, PNG’s compression works
on rows of pixels, taking advantage of repetition in bytes of information. By use of
filters (discussed in the next section), it can take advantage of some vertical
patterns as well; however, filters are not recommended for use with palette
images. PNG’s compression engine typically compresses images 5–25% better than
GIF.