You've
almost certainly seen imagemaps on your trips across the Web. They
are pictures with different parts that you can click, and each part
takes you to a different URL. Imagemaps let web sites offer pictorial
melanges where you can select where you want to go, as an alternative
to presenting a boring list of text items.
In this
book, the imagemap is generated and interpreted within the program.
But you should probably see how most people use conventional imagemaps.
They start with a crisp graphic image (preferably GIF, as it is
more portable than JPEG). Once they select an
image, they must define various hotspots (or areas where users can
click), and identify them in an imagemap file in the following format:
shape URL coordinate1, coordinate2, ... coordinaten
where shape can be "circle," "poly,"
or "rect"; URL is the file you want to display
in response to the user's click; and the coordinates are measured
in pixels. Programs exist to help you determine the coordinates
of the regions you want to mark within an image. Here is an example
of an imagemap file (the following applies to the NCSA
server only):
default https://my.company.com
rect https://some.machine.com 0, 0, 50, 50
poly https://www.machine.com/graphic.gif 100, 120, 230, 250, 320, 75
circle https://their.machine.com/circle.gif 100, 100, 150, 150, 100
The next step is
to edit the imagemap.conf configuration file
and add an entry like the following:[1]
dragon: /graphics/dragon.map
The
first part of this statement is the name of the imagemap, while
the second part is the relative path to the imagemap data file.
Now, the imagemap is all but set up. The only step that needs to
be performed is to add the appropriate HTML in
a document to access the imagemap:
<A HREF="/cgi-bin/imagemap/dragon"><IMG SRC="/graphics/dragon.gif" ISMAP></A>
When
the user clicks on a point in the image, the client sends the coordinates
as query information, and the imagemap name as an extra path to
the imagemap CGI program (which comes with most servers). Here is
what a typical HTTP client request might look
like:
GET /cgi-bin/imagemap/dragon?53,87
First, the CGI program reads the imagemap configuration file,
in order to determine the imagemap data file for the clicked image.
It then opens the data file and determines the appropriate URL to
access. This is a very inefficient process, as two separate files
have to be opened. As a result, many webmasters do not allow users
to set up imagemaps.
While this should be enough information
to get you started with imagemaps, we will do something much more
efficient and fun in our last example--we'll generate the imagemap
without using auxiliary files.
