    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.

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    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

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    RFC 1980                 Client-Side Image Maps              August 1996

       of the map.  If the argument to USEMAP starts with a '#', the map is
       assumed to be in the same document as the IMG tag.  The presence of a
       USEMAP attribute overrides the effect of an enclosing anchor (A)
       element.

       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 to. The basic format for the MAP element is as follows:

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

       The NAME attribute specifies the name of the map so that it can be
       referenced by an IMG element.  Each AREA element contained inside the
       map element specifies a single clickable area of the image.  The
       SHAPE attribute gives the shape of this area.  Possible shapes are
       "RECT", "CIRCLE", and "POLYGON", which specify rectangular, circular,
       and polygonal regions respectively.  If the SHAPE tag is omitted,
       SHAPE="RECT" is assumed.

       The COORDS tag describes the position of an area, using image pixels
       as the units with the origin at the upper-left corner of the image.
       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
       100x100 image, the coordinates would be "0,0,99,99".

       For a circular region, the coordinates are given as
       "center_x,center_y,radius", specifying the center and radius of the
       ircle.  All points up to and including those at a distance of
       "radius" points from the center are included.  For example, the
       coordinates "4,4,2" would specify a circle which included the
       coordinates (2,4) (6,4) (4,2) and (4,6).

       For a polygonal region, the coordinates specify successive vertices
       of the region in the format "x1,y1,x2,y2,...,xn,yn".  If the first
       and last coordinates are not the same then a segment is inferred to
       close the polygon.  The region includes the boundary lines of the
       polygon.  For example, "20,20,30,40,10,40" would specify a triangle
       with vertices at (20,20) (30,40) and (10,40).  No explicit limit is
       placed on the number of vertices, but a practical limit is imposed by
       the fact that HTML limits an attribute value to 1024 characters.

       The NOHREF attribute indicates that clicks in this region should
       perform no action.  An HREF attribute specifies where a click in that
       area should lead.  A relative anchor specification will be expanded

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    RFC 1980                 Client-Side Image Maps              August 1996

       using the URI of the map description as a base, rather than using the
       URI of the document from which the map description is referenced. If
       a BASE tag is present in the document containing the map description,
       that URI will be used as the base.

       An arbitrary number of AREA tags may be specified.  If two areas
       intersect, the one which appears first in the map definition takes
       precedence in the overlapping region.  Multiple areas may share the
       same destination to create composite shapes.  Any portion of an image
       which is not described by an AREA tag defaults to having no action.

       The ALT attribute specifies optional text which describes a given
       area.  A text-only browser can display the textual contents for each
       area as a substitute for the image.

    2.2  Required Changes to HTML/2.0 DTD

       The required changes to the HTML/2.0 DTD to support this syntax would
       be as follows:

      Change the IMG element definition to be:
        <!ELEMENT IMG    - O EMPTY>
        <!ATTLIST IMG
                SRC CDATA #REQUIRED
                ALT CDATA #IMPLIED
                ALIGN (top|middle|bottom) #IMPLIED
                ISMAP (ISMAP) #IMPLIED
                USEMAP %URI; #IMPLIED
                %SDAPREF; "<Fig><?SDATrans Img: #AttList>#AttVal(Alt)</Fig>"
        >

      Add the following new definitions:
        <!ELEMENT MAP    - - +(AREA)>
        <!ATTLIST MAP
                NAME %linkName; #REQUIRED
        >

        <!ELEMENT AREA   - O EMPTY>
        <!ATTLIST AREA
                SHAPE (RECT|CIRCLE|POLYGON) RECT #IMPLIED
                COORDS CDATA #REQUIRED
                HREF %URI; #IMPLIED
                NOHREF (NOHREF) #IMPLIED
                ALT CDATA #IMPLIED
        >

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    RFC 1980                 Client-Side Image Maps              August 1996

    2.3  Backwards Compatibility

       This extension is specifically designed to provide a variety of
       fallback options for browsers which do not support it.  These options
       are based on the assumption that browsers will ignore any attributes
       or elements which are not present in the HTML/2.0 DTD.

       An document can be written so that a client-side image map can have
       three different fallback behaviors.  First, the document can use the
       server-side image map capability, by specifying the ISMAP attribute
       as well as USEMAP.  In situations where this is possible, the image
       map will work whether or not the browser supports the client-side
       extension.

       Second, clicking the image can direct the user to a single URI,
       regardless of where on the image he clicks.  This is accomplished by
       placing the image inside an anchor (A) element.  The fallback
       destination could provide the user with an error or a textual list of
       destinations.

       Lastly, the image can appear to not be a link at all (i.e. missing
       whatever visual cues a browser provides to indicate a hyperlink).
       This will be the result if the image element neither contains an
       ISMAP attribute nor is inside an anchor.

    2.4  Examples

       The following three examples show markup demonstrating the three
       fallback mechanisms described in section 2.3:

          This image map will work with any graphical browser:
          <A HREF="/cgi-bin/imagemap/pic1">
          <IMG SRC="pic1.jpg" USEMAP="maps.html#map1" 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="pic2.jpg" USEMAP="maps.html#map2"></A>

          You can only click here if your browser supports client-side
          image maps: <IMG SRC="pic3.jpg" USEMAP="maps.html#map3">

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    RFC 1980                 Client-Side Image Maps              August 1996

       The following example shows the use of a map in the same file as the
       image:

          <IMG SRC="picture.jpg" USEMAP="#mymap">

       The following example defines a simple map which describes an image
       with a circle in the middle overlapping two large rectangles:

          <MAP NAME="welcomemap">
          <AREA SHAPE=CIRCLE COORDS="50,50,40" HREF="about_us.html"
                ALT="About our company">
          <AREA SHAPE=RECT COORDS="0,0,100,50" HREF="products.html"
                ALT="Our products">
          <AREA SHAPE=RECT COORDS="0,51,100,100 HREF="technology.html"
                ALT="Technology for the next century">
          </MAP>

    3. Security Considerations

       Clicking a portion of a client-side image map may cause a URI to be
       dereferenced.  In this case, the security considerations related to
       URLs [5] apply.

    4. References

       [1] Berners-Lee, T., and D. Connolly, "HyperText Markup Language
           Specification - 2.0", RFC 1866, November 1995.

       [2] Seidman, J., "An HTML Extension to Support Client-Side Image
           Maps", The Second Internation WWW Conference '94 Advance
           Proceedings, pp 927-930.

       [3] "Standard Generalized Markup Language"  ISO Standard 8879:1986
           Information Processing Text and Office Systems.

       [4] Berners-Lee, T., Fielding, R., and H. Frystyk Nielsen,
           "Hypertext Transfer Protocol -- HTTP/1.0", Work in
           Progress.

       [5] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
           Resource Locators (URL)", RFC 1738, December 1994.

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    RFC 1980                 Client-Side Image Maps              August 1996

    5. Author's Address

    James L. Seidman
    Senior Software Engineer
    Spyglass, Inc.
    1230 East Diehl Road
    Naperville, IL 60563

    EMail: jim@spyglass.com

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