Internet DRAFT - draft-seidman-html-clientsideimagemap
draft-seidman-html-clientsideimagemap
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INTERNET-DRAFT James L. Seidman
<draft-ietf-html-clientsideimagemap-01.txt> Spyglass, Inc.
Expires SIX MONTHS FROM---> August 8, 1995
A Proposed Extension to HTML : Client-Side Image Maps
Status of this Memo
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Abstract
The markup language known as "HTML/2.0" provides for image maps.
Image maps are document elements which allow clicking on different
areas of an image to reference different network resources, as
specified by Uniform Resource Locators (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 Purpose
1.2 Overall Operation
2. Client-Side Image Map Extension
2.1 Syntax
2.2 Required Changes to HTML/2.0 DTD
2.3 Backwards Compatibility
2.4 Examples
3. References
4. Author's Address
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 on 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 on
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 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 on 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 on 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 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:
<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
circle. 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 %URI; #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
>
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 on 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">
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. References
[1] T. Berners-Lee, D. Connolly. "HyperText Markup Language
Specification - 2.0" Internet-Draft (work in progress),
June 16, 1995.
[2] J. Seidman, "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] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen,
"Hypertext Transfer Protocol -- HTTP/1.0" Internet-Draft
(work in progress), March 8, 1995.
4. Author's Address
James L. Seidman
jim@spyglass.com
Senior Software Engineer
Spyglass, Inc.
1230 East Diehl Road
Naperville, IL 60563
