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rfc:rfc1980

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

 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

Seidman Informational [Page 3] 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
  >

Seidman Informational [Page 4] 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">

Seidman Informational [Page 5] 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.

Seidman Informational [Page 6] 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

Seidman Informational [Page 7]

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