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Chapter 13. Creating Graphics on the Fly

Throughout this book we have seen many examples of CGI scripts generating dynamic output. However, in almost all cases, the output has been HTML. Certainly this is the most common format your scripts will generate. However, CGI scripts can actually generate any type of format, and in this chapter we will look at how we can dynamically generate images.

Generating images dynamically has many uses. One of the most common is to generate graphs. If you have a data source that is continually changing, such as the results of an online survey, a CGI script can generate a graph that presents a visual snapshot of this data.

There are also times when generating images dynamically makes less sense. It is much less efficient to generate an image dynamically than for your web server to serve the image from an image file. Thus, just because some of these tools allow you to generate really cool graphics dynamically doesn't mean you must use them only in a dynamic context. Unless the images you generate are based upon data that changes, save the image to a static file and serve that instead.

This chapter presents a broad overview of the different tools available for generating dynamic images online, and includes references with each for finding more information. The goal of this chapter is to explain techniques for generating images dynamically and familiarize you with the most popular tools available to you. A full description of many of these tools along with others is available in a book of its own, Programming Web Graphics with Perl and GNU Software by Shawn Wallace (O'Reilly & Associates, Inc.).

13.1. File Formats

Let's first review the image formats that are used online today. The most common image formats, of course, are GIF and JPEG, which every graphical web browser supports. Other file formats that we will discuss in this chapter include PNG and PDF.

13.1.1. GIF

The Graphics Interchange Format (GIF ) was created by CompuServe and released as an open standard in 1987. It quickly became a very popular image format and, along with JPEG, became a standard format for images on the Web. GIF files are typically quite small, especially for images with few colors, which makes them well suited for transferring online.

GIF only supports up to 256 colors, but it works well for text and images, such as icons, which do not have many colors but have sharp details. The compression algorithm that GIF uses, LZW, is lossless, which means that no image quality is lost during compression or decompression, allowing GIF files to accurately capture details.

The GIF file format has been extended to support basic animation, which can loop. The moving banner ads that you see online are typically animated GIF files. GIF files can also have a transparent background by specifying a single color in the image that should be displayed as transparent.

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