Perl Graphics Programming

Raster File Formats

A raster image is one that is described by a grid of colored pixels. Even though particular image formats may use compression algorithms to store the image data differently, or alpha channels to create the illusion of non-rectangular shapes, all raster graphics translate into a rectangular grid of pixels.

Two techniques have been developed to make raster images easier to transfer over the Internet. Each image format has its own compression system, where advanced mathematics or data structures are used to exploit regularity in the image data so that the full image can be reconstructed from a smaller number of bytes. Many raster image files are arranged so that they may be displayed while partially downloaded (a technique called interlacing).

Table 1-2 gives you an idea of the relative effectiveness of each compression mechanism and the interlacing styles used by each format.

Table 1-2. Raster graphic compression and interlacing comparison

Format	Compression ratio	Compression algorithm	Method	Progressive display	Interlacing style
PNG	4:1 to 10:1	Deflate	lossless	Yes	Adam7
JPEG	5:1 to 100:1	JPEG	lossy	Yes	PJPEG
GIF	3:1 to 5:1	LZW	lossless	Yes	Scan line

The rest of this section describes compression and interlacing styles. The three raster image formats are then described in greater detail.

Compression

Image transmission is always a tradeoff between two limiting factors: the time it takes to transfer the image over the network and the time it takes to decode the image. JPEG, for example, is a ...