Stefan Morgenroth - Image Maps


Image maps are an important feature of the point-and-click interface that makes the World Wide Web so popular. The most common use of image maps is to allow users to access different documents by clicking on different areas in an image.

There are several limitations of the current image map implementation as it applies to this use. First, it only works over the HTTP protocol, making it unusable for reading local files or files accessed via alternate protocols. Second, a server transaction is required merely to determine where the link is directed. This can degrade performance noticeably when accessing distant sites. Third, unlike for normal links, there is no way for a browser to provide visual feedback to the user by showing where a portion of an image map leads before the user actually clicks on it. Lastly, the implementation of image maps is server-dependent, compromising portability of documents.

While HTML+1 contains provisions for "hypertext buttons" on images via use of the FIG element, this method is an unworkable short-term solution for several reasons. First, complete support of the FIG element requires significant additional processing by the browser. Second, it cannot degrade gracefully on browsers that do not support it. Third, it requires the map description to be specified when the image appears, which is inappropriate for some applications. The extension to support client-side image maps addresses these issues.


Adding a USEMAP attribute to an IMG element indicates that it is a client-side image map. The USEMAP attribute can be used with the ISMAP attribute to indicate that the image can be processed as either a client-side or server-side image map. The argument to USEMAP specifies which map to use with the image, in a format similar to the HREF attribute on anchors. If the argument to USEMAP starts with a "#", it is assumed to be in the same document as the IMG tag. A few examples would be:

You can only click here if your browser supports client-side image maps:
<IMG SRC="../images/tech/pic1.gif" USEMAP="maps.html#map1">

This image map will work regardless:
<A HREF="/cgi-bin/image map/pic2"> <IMG SRC="../images/tech/pic2.gif" USEMAP="maps.html#map2" ISMAP></A>

Clicking here will take you to a page with an error message if you don't have client-side image map support:
<A HREF="no_csim.html"> <IMG SRC="../images/tech/pic3.gif" USEMAP="maps.html#map3"> </A>

The different regions of the image are described using a MAP element. The map describes each region in the image and indicates where it links. The basic format for the MAP element is as follows:

<MAP NAME="name">
<AREA [SHAPE="shape"] COORDS="x,y,..." [HREF="reference"] [NOHREF]>

The name specifies the name of the map so that it can be referenced by an IMG element. The shape gives the shape of this area. Currently supported shapes include rectangles (RECT), polygons (POLY), circles (CIRCLE), and default (DEFAULT), but the syntax is defined in a way that allows other shapes to be added. If the SHAPE tag is omitted, SHAPE="RECT" is assumed. The COORDS tag gives the coordinates of the shape, using image pixels as the units. For a rectangle, the coordinates are given as "left,top,right,bottom". The rectangular region defined includes the lower right corner specified (i.e. to specify the entire area of a 100-by-100-pixel image, the coordinates would be "0,0,99,99"). Circles are defined as a center point and then a radius (a total of three numbers).

The NOHREF tag indicates that clicks in this region should perform no action. An HREF tag specifies where a click in that area should lead. Note that a relative anchor specification will be expanded using the URL of the map description as a base, rather than using the URL of the document from which the map description is referenced. If a BASE tag is present in the document containing the map description, that URL will be used as the base.

An arbitrary number of AREA tags may be specified. If two areas intersect, the one that appears first in the map definition takes precedence in the overlapping region. For example, a button bar in a document might use a 160-by-60-pixel image and appear like this:


<MAP NAME="buttonbar">
<AREA SHAPE="RECT" COORDS="10,10,49,49" HREF="about_us.html">
<AREA SHAPE="RECT" COORDS="60,10,99,49" HREF="products.html">
<AREA SHAPE="RECT" COORDS="110,10,149,49" HREF="index.html">

<IMG SRC="../images/tech/bar.gif" USEMAP="#buttonbar">
This example includes a region encompassing the entire image with a NOHREF tag, but this is actually redundant. Any region of the image that is not defined by an AREA tag is assumed to be NOHREF.


This syntax provides maximum flexibility to the document author for dealing with browsers that do not support this extension, since such browsers will ignore the MAP and AREA elements. If the document resides on an HTTP server, the server can still provide ISMAP-style support. Otherwise, the author can choose to have the image not appear as an anchor at all, or can choose to have a click anywhere within it lead to another page, perhaps providing an equivalent textual list of options.

The demand for a non-HTTP-based mechanism for image maps will also increase as archives of material in HTML format begins to appear on CD-ROM. The expected increase in pay-per-access servers will also lead to users saving copies of documents locally, which they would then expect to function identically to those on the original server. The extensions described here could serve as a basis to satisfy these needs.



1. Ragget, Dave, HTML+ Discussion Document

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