Perl Graphics Programming

Introduction to File Formats

Over time there have been many different file formats for representing graphics. Most are footnotes to history, with only a few ever becoming wildly popular. In this book we talk about the image formats PNG (Portable Network Graphics), JPEG (Joint Pictures Expert Group, the committee that designed the format), and GIF (Graphic Interchange Format); the animation formats SVG (Scalable Vector Graphics), SWF (used by Macromedia Flash), and to a lesser extent GIF again; and the document formats PostScript and PDF (Portable Document Format).

All of these formats fall into one of two camps: raster or vector. A raster image is composed of dots, called pixels. Almost every image on a web page is a raster image, and the PNG, JPEG, and GIF formats are raster formats. The alternative to representing a picture as a series of dots is to represent it as you might draw it, with lines, curves, and filled colors. This is a vector image. The most noticeable difference between raster and vector images is how they scale. If you zoom in on a raster image, each pixel simply becomes a larger and larger spot, which leads to chunky images. A vector image, in contrast, gives infinite magnification.

Each raster file format represents an image’s pixels differently. These formats vary in how much color information they store, whether they can handle transparency in an image, and whether you have to pay to use them. For this reason, not all graphics formats are suited for all jobs. JPEG is generally much better for photos because it has good compression, for example, and GIF is the only raster image format that can handle animated sequences.

The limitations of a file format highlight its design goals. The flexible PNG format, for example, has a maximum size of approximately 2 Gigapixels × 2 Gigapixels. At a standard screen resolution of 72 pixels per inch, could store an image of approximately 470 miles × 470 miles. This would allow you to save a 1:1 life-sized image of most of New England in a single PNG file! By comparison, a GIF file has a maximum size of 75 feet × 75 feet, which would let you store a 1:1 image of a good-sized house. So, the next time you need a life-sized image of Rhode Island, you’ll know which file format to use.

Table 1-1 gives you some of the important details of the file formats covered in this book.

Table 1-1. File format details

Format	Type	Max. color depth	Alpha channel	Max. image size (pixels)	Multiple images	Toolkits
PNG	Raster	8-bit indexed 16-bit grayscale 48-bit RGB +16 bits w/alpha	16 bit	2Gig x 2Gig	No	GD, GD::Graph, ImageMagick, Gimp; can be embedded in SWF, SVG
JPEG	Raster	12-bit grayscale 36-bit RGB 32-bit CMYK	No	64K x 64K	No	ImageMagick, GD, Gimp; can be embedded in SWF, SVG
GIF	Raster	8-bit indexed	1-bit transparency	64K x 64K	Yes	ImageMagick, Gimp
SVG	Vector	24-bit	Limited by user agent	Limited by user agent	Yes	ImageMagick, Gimp
SWF	Vector	24-bit	8-bit	1Gig x 1Gig (twips)	Yes	Ming library
PostScript	Document	Limited by display device	No	Limited by display device	Yes	PostScript module, ImageMagick, Gimp
PDF	Document	Limited by user agent	Limited by user agent	Limited by user agent	Yes	PDF::API2, ImageMagick

Let’s now nail down a few definitions that are used for all graphics formats, starting with color.