A Proposed Extension to HTML : Client-Side Image Maps
RFC 1980

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Network Working Group                                         J. Seidman
Request for Comments: 1980                                Spyglass, Inc.
Category: Informational                                      August 1996

         A Proposed Extension to HTML : Client-Side Image Maps

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The markup language known as "HTML/2.0" provides for image maps.
   Image maps are document elements which allow clicking different areas
   of an image to reference different network resources, as specified by
   Uniform Identifier (URIs).  The image map capability in HTML/2.0 is
   limited in several ways, such as the restriction that it only works
   with documents served via the "HTTP" protocol, and the lack of a
   viable fallback for users of text-only browsers.  This document
   specifies an extension to the HTML language, referred to as "Client-
   Side Image Maps," which resolves these limitations.

Table of Contents

   1.  Introduction ...............................................  1
       1.1  Purpose ...............................................  1
       1.2  Overall Operation .....................................  2
   2.  Client-Side Image Map Extension ............................  2
       2.1  Syntax ................................................  2
       2.2  Required Changes to HTML/2.0 DTD ......................  4
       2.3  Backwards Compatibility ...............................  5
       2.4  Examples ..............................................  5
   3.  Security Considerations ....................................  6
   4.  References .................................................  6
   5.  Author's Address ...........................................  7

1. Introduction

1.1  Purpose

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

Seidman                      Informational                      [Page 1]
RFC 1980                 Client-Side Image Maps              August 1996

   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 showing where a portion of an image map
   leads before the user actually clicks it. Lastly, the method for
   specifying the active regions of image maps is server-dependent,
   compromising portability of documents.  This extension to support
   client-side image maps addresses these issues.

   It is proposed that this extension be included in a future revision
   of the HTML specification.

1.2  Overall Operation

   Client-side image maps work by placing a complete representation of
   the active areas of an image, including their shape, size, and
   destination (URI), into an SGML-compliant textual form.  This markup
   may also optionally include a textual description for each area for
   display on non-textual browsers.  This representation, or "map," is
   given a name to identify it.

   When an image is included in an HTML document, it may include an
   attribute specifying a map to use.  The map may be contained in the
   same file which references the image, but this it not required.  If
   the map is in a different file, a URI to that file must be provided.

   The browser will parse the map and remember the contents.  When the
   user clicks the map, the browser will match up the location with the
   specified destination for that location and access that URI.  In the
   case of a non-graphical browser, the browser could display the
   textual descriptions for each area instead of the image.  Clicking a
   given textual description would then go to the associated
   destination.

2. Client-Side Image Map Extension

2.1 Syntax

   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, by specifying the URI for
   the file containing the map, followed by a '#', followed by the name

Seidman                      Informational                      [Page 2]
RFC 1980                 Client-Side Image Maps              August 1996