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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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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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:
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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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:
#AttVal(Alt)"
>
Add the following new definitions:
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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:
Clicking here will take you to a page with an error message if
you don't have client-side image map support:
You can only click here if your browser supports client-side
image maps: 
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The following example shows the use of a map in the same file as the
image:
The following example defines a simple map which describes an image
with a circle in the middle overlapping two large rectangles:
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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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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RFC, FYI, BCP