RFC1980 - A Proposed Extension to HTML : Client-Side Image Maps

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.

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

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:

<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

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 (topmiddlebottom) #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 (RECTCIRCLEPOLYGON) RECT #IMPLIED

COORDS CDATA #REQUIRED

HREF %URI; #IMPLIED

NOHREF (NOHREF) #IMPLIED

ALT CDATA #IMPLIED

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: <IMG SRC="http://www.qqread.com/net-protocol/pic3.jpg" USEMAP="maps.html#map3">

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:

<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", RFC1866, 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)", RFC1738, December 1994.

5. Author's Address

James L. Seidman

Senior Software Engineer

Spyglass, Inc.

1230 East Diehl Road

Naperville, IL 60563

EMail: jim@spyglass.com