Programming Web Graphics with Perl and GNU Softwar

Chapter 10. Web Graphics Cookbook

Several idioms have evolved on the Web that are worth examining in depth; the phenomenon of web counters, for example, or the culture of web cams. This chapter provides some reusable examples of common web graphics applications:

• The BrokenImage module that enables Perl-based CGI scripts that generate graphics to present error messages to a web browser rather than a generic broken image icon.

• A web page access counter that will be updated every time a user views the page. Even though some people rail against counters, they are still a much asked-for feature by new web authors.

• A JavaScript rollover example, because rollovers are also a much requested feature, and they are too widely used to be ignored.

• A Web Cam how-to.

• A section on providing ASCII ALT attributes so that your images may be viewed in some approximation by text-only browsers.

• Thumbnailing scripts that will ease the tedium of making thumbnails of large groups of images.

Each recipe is broken down into three sections: a discussion of the problem, what is required for the example, and a discussion of a possible solution or implementation.

The BrokenImage Module

A web page that accesses a dynamically generated image runs the risk of displaying uninformative “broken” images if the script that generates the image fails to successfully complete its operation. The fact that an <IMG> element that calls a script from its SRC attribute expects a valid stream of image headers and data can make it ...