Dynamic Graphics

Generating graphics, charts and graphs for your web site is easy following Mr. Lerner's instructions.

Mark Andreessen, a co-founder of Netscape, is often credited with having turned the Web from an academic playground into a mass medium. But just what did Andreessen do? After all, Tim Berners-Lee invented the browser, HTML and URLs. You could even argue that the original browser was superior in some ways, in that it allowed people to write pages of HTML as well as read them.

Historians might take issue with this, but I would argue that Andreessen's greatest idea was allowing for graphics alongside text in web documents. As a text-only medium, the Web was interesting mainly to physicists and other academics, but with the introduction of graphics, it began to appeal to an entirely new segment of the population.

Today, graphics are not just used for decoration, but often stand on their own. Nearly every professional web site now hires one or more graphic artists to design the site—even when the site will deal mainly with text. Some sites would not be possible or even worthwhile were it not for the use of graphics. In some cases, these graphics are dynamically generated, produced by a program, instead of sitting in a static file on disk.

This month, we will look at ways in which we can create such dynamic graphics with CGI programs. We will look at the GD library, which allows us to create arbitrary images, and will quickly move on to creating different kinds of dynamically generated charts and graphs. After looking at some simple examples of such charts, we will examine a more sophisticated example, one which draws its inputs from a relational database.

Perl, Dynamic Graphics and GD

Writing a CGI program that outputs HTML is not particularly difficult, as we have demonstrated in many previous installments of “At the Forge”. Here, for example, is a simple program that, when invoked, returns some HTML to the user's browser:

#!/usr/bin/perl -wT
    use strict;
    use diagnostics;
    use CGI;
    use CGI::Carp qw(fatalsToBrowser);
    # Create an instance of CGI
    my $query = new CGI;
    # Send an appropriate MIME header
    print $query->header(-type =>
    # Send some content
    print $query->start_html(-title =>
    "This is a test.");
    print "<H1>Testing!</H1>\n";
    print "<P>This is a test.</P>\n";
    print $query->end_html;

If we want to return graphics to the user's browser, we must modify the “Content-type” header in the HTTP response, generated with the call to “header”. If we want to generate a GIF, we will have to change our call to header such that it outputs “image/gif” instead. By the same token, we can tell the user's browser that a JPEG (image/jpeg) or PNG (image/png) graphic will be sent. Once we have described the content to the user's browser, we must generate a graphic of this type. How can we do that?

Perl's scalar variables can contain any data we might like. If we were more familiar with the GIF standard, we could stick a GIF into a scalar, then send that value to the user's browser. Of course, most of us are unfamiliar with the intimate details of the GIF standard, which makes this a less than ideal solution. A better idea would be to take advantage of Perl's object-oriented capabilities, using someone else's solution to the same problem.

Sure enough, Lincoln Stein (author of CGI.pm, the standard module for CGI programs) has written and distributed GD.pm. This module, available on CPAN (see “Resources”), gives us access to the popular “gd” libraries for C written by Thomas Boutell.

GD gives your program the ability to draw in a manner similar to popular drawing programs. You can choose from an array of paint brushes, colors and built-in shapes, as well as any fill shapes you have drawn. GD has its own internal drawing format, but as we will see, it supports the conversion of drawn images into GIF format.

A Simple Graphics Program

A simple program that uses GD, gd-intro.pl, is shown in Listing 1. If you install it in your CGI directory and invoke it from your browser, you should see a blue-filled green square.

Listing 1.

As you can see, our program manipulates two objects—an instance of CGI and an instance of GD. Each object handles its own affairs, keeping its nose out of the other object's business. $query, our instance of CGI, neither knows nor cares what sort of data we are receiving from the user or returning to his or her browser. By the same token, $image, our instance of GD, does not know that its output is going to be sent to a browser. Such compartmentalizing of tasks is one reason why objects make programming easier and software more maintainable.

When we create $image, we declare it to be of type GD::Image and to be 100 pixels wide by 100 tall. GD will not warn you if your image is cut off by the boundaries of this declared “canvas”; when I first started to play with GD, I was puzzled by the fact that no output appeared. I finally realized that my image was 100x100, but I was drawing a circle with a 400-pixel diameter. GD dutifully performed the task I requested, which meant that no picture appeared in the end.

After declaring $image, we allocate some colors, using GD's colorAllocate method. Each color is defined as red-green-blue (RGB), with each of those parameters varying between 0 and 255. I find it useful to declare color names within a hash, as with %COLORS in gd-intro.pl, but you may prefer to assign them to individual variables or to work with colorAllocate directly.

Next, we tell $image that it should create GIFs in “interlaced” mode. Interlacing means that instead of drawing every horizontal line of an image, the computer should first draw all the even lines, and then all the odd lines. You can see this in action on an ordinary television set. When TV standards were defined, televisions were unable to draw all the horizontal lines at once. Because of this, your television draws all the odd horizontal lines, followed by the even ones, followed by the odd ones again.

Making a GIF interlaced is not related to the speed of your computer or its ability to display images quickly; rather, it has to do with the speed of a user's connection. If the user has a slow connection, a GIF will load slowly. Making the graphic interlaced allows the user to see the graphic as it loads. Otherwise, the graphic will not be displayed until it is completely loaded, which might take a while.

We also set the “transparent” color, which is the color selected to melt into the background. By setting white as the transparent color, we indicate that anything drawn in white should actually be drawn in the color of the background. Since GD drawings have a white background by default, setting white as the transparent color means our graphic will appear to be floating in the user's browser, rather than set against a white background.

After all this, we can finally draw. We create a rectangle between 20,20 and 80,80, which should fill most of the 100x100 area defined when we created $image. We choose to draw the rectangle in green, using %COLORS, which we defined earlier. Finally, we fill the rectangle with blue by pointing GD to a point inside of the rectangle and asking it to fill the area.

GD has a number of other functions, including the ability to draw polygons, create custom brushes and fill with specified patterns. You can add text labels, which are incorporated into the final graph. You can even save graphics to disk in GD's own format, then load them again and continue to manipulate them before turning them into GIFs.