Perl Graphics Programming

Appendix A. Serving Graphics on the Web

When creating graphics, it’s good to keep in mind that not all web browsers handle all “standard” HTML features in the same way. At various times, different browsers have had their own idiosyncratic interpretations of the ALT tag, client-side image maps, the usemap attribute, GIF89a animation, image spacing attributes, transparency, inline PNG/XBM/Progressive-JPEG images, the lowsrc tag, borders on image links, alignment tags, and scaling tags. In short, many features that should have a standard implementation have had different levels of compliance to the standard on different browsers.

You can retrieve a good deal of information about the user agent that is looking at your web page. You may want to know which browser the client is using (accessible through the HTTP_USER_AGENT environment variable), or the MIME types that the client accepts. Some web servers offer options for modifying the image request negotiation process to accommodate different browser capabilities, so browsers that can’t see PNG files are automatically sent a GIF version instead, for example. Consult your web server’s documentation for these features.