NOTE-imagemap-961127
NOTE-imagemap-961127
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This memo discusses appropriate handling of documents containing imagemapped images in cases where the user-agent cannot display or otherwise process inline images. Typical examples where this is so are (i) text-only browsers such as lynx, text-to-speech or braille browsers; (ii) graphical browsers with disabled image loading; or (iii) information-gathering robots that do not retrieve or process inlined image files.
Because imagemapped images are processed by the user agent and a server-side process, this memo discusses requirements of both these utilities. The focus is on three issues: appropriate HTML markup, and in particular the use of ALT attributes with IMG elements; appropriate URL encoding by user agents of imagemap requests in the absence of user agent-specified image coordinate data; and the appropriate processing by a server resource of imagemap requests in the absence of user agent-specified image coordinate data.
This memo addresses ISMAP and FORM-based imagemaps as specified in the official specification of HTML 2.0 (RFC 1866) [1].
Conformance to these recommendations will help provide a standard, graphic-free interface to Web documents, and will improve the accessiblity of web resources to users or automated web robots that are unable to view images or process image-based data.
The ALT attribute to an IMG element provides a text alternative to an image, used by user-agents that do not process inline images. Consequently, it is essential that document authors include appropriate ALT text descriptions if they wish their documents to be usable when images are not displayed. This is particularly important for images used as hypertext anchors or as imagemaps. In both these cases, the ALT text should clearly describe the purpose and intent of the link or of the imagemap and related image map database.
If an image is purely decorative, it is recommended that an author use an ALT attribute of the form ALT="" (a null string). This form should never be used for IMG elements inside an anchor, as some user agents will entirely hide such an anchor from the user.
There are two cases where imagemaps can be presented to a user-agent. The first is as an ISMAP image contained within a hypertext anchor. Here, an image element taking the ISMAP attribute is enclosed within an anchor element, the anchor element referencing a server resource (often a gateway program) that processes the coordinate data selected by the user. The second case is the TYPE="image" INPUT element within an HTML FORM. These cases are discussed in the next two sections.
ISMAP imagemapped hypertext anchors take the form:
<A HREF="http://some.domain.name/path/resource">
<IMG SRC="path/to/image.gif"
ALT="[alternative text description]"
ISMAP >
</A>
where the ISMAP attribute declares the image to be active, and where the anchor element enclosing the IMG element specifies the server resource to which the coordinate data should be sent for processing. A user agent that can process imagemaps must take the user-selected integer pixel coordinates (x,y), measured from the upper left hand corner of the image, and send these to the server using the GET method, appending the selected coordinates to the URL as the encoded [2,3] query string "x,y". For example:
http://some.domain.name/path/resource?23,111
The behaviour in the absence of user-selected coordinate data is at present unspecified, and is the main subject of this memo.
In the absence of a mechanism for specifying coordinates on the imagemapped image, a user agent should access the specified URL without any appended coordinate data: that is, using the URL specified in the anchor element, without appended query data. For example (following the example markup in Section 3):
http://some.domain.name/path/resource
The referenced server resource that processes this request should interpret a request containing no coordinate data as a request from a user agent that cannot display or process image files. As a result, the server resource should return a text-only HTML document providing alternative access to the imagemapped data: for example, a list of hypertext links, with related descriptions.
Some current user agents, such as lynx, return the default coordinates (0,0) when accessing imagemapped anchors in that absence of user agent-selected coordinate data. This behaviour is deprecated, but authors of server-side programs that process imagemap data should treat the coordinate pair (0,0) as equivalent to the absence of coordinate data.
Query string data is forbidden within the URL specifing the server resource for an ISMAP imagemapped hypertext anchor. Thus, markup of the form
<A HREF="http://some.domain.name/path/resource?query-data">
<IMG SRC="path/image.gif"
ALT="[alternative text]"
ISMAP >
</A>
is disallowed. If there is query data within the resource URL, the error handling mechanism of a user agent should truncate this query data, and append the appropriate coordinate data (if any) obtained from user input. This error should be reported to the user, as this change may reflects loss of information.
Fragment identifiers may be appended to the URL specifying the server resource for an ISMAP hypertext anchor. Thus, markup of the form:
<A HREF="http://some.domain.name/path/resource#fragment">
<IMG SRC="path/image.gif"
ALT="[alternative text]"
ISMAP >
</A>
is allowed. Since a fragment identifier is not part of a URL, the user agent must cache the fragment identifier, strip the string (including the leading # character) from the URL, and then append, as a query string, the appropriate coordinate data (if any) obtained from user input. When data are returned to the user agent from the server, the fragment identifier must be used to locate the appropriate named location in the returned document, where appropriate.
Many current user-agents do not support fragment identifiers appended to the URL specifying the server resource for an ISMAP hypertext anchor.
Within a FORM, an author can declare an input element of the form
<INPUT TYPE="IMAGE"
NAME="tstname" VALUE="testvalue"
SRC="path/image.gif">
This specifies an image resource to be displayed, in which the user can select pixel coordinates (x,y) measured from the upper left hand corner of the image. This input element type is analogous to a button of the form TYPE="submit" -- when the user selects coordinates in the TYPE="image" element, the form as a whole is immediately submitted, with the image coordinates being passed to the server as an URL query string field of the form:
tstname.x=x_coord&tstname.y=y_coord
Here "tstname.x" and "tstname.y" are the variable names composed by combining the encoded [1] INPUT element NAME attribute value with the strings ".x" and ".y", and where "x_coord" and "y_coord" are the positive integer x and y pixel coordinates selected by the user, relative to the upper left-hand corner of the image. The VALUE attribute is unused, and is currently ignored by most user agents.
If the FORM is submitted without the user selecting image coordinates, for example by selecting a TYPE="input" element, then no data associated with the TYPE="image" input element are included with the message sent by the user agent. the the URL query string due to this element contains no data This is analogous to an unselected TYPE="checkbox" input element.
If a user agent incapable of processing images (for whatever reason) submits a FORM via the TYPE="image" input element, then the encoded string from the TYPE="image" element should take the (URL encoded) form:
tstname.x=&tstname.y=
That is, there should be no coordinate data passed to the server (the coordinate data are null strings). The server resource processing the FORM data can use the presence of "tstname.x" and "tstsname.y" NAMEs, and the absence of coordinate data VALUEs, to infer that the user agent is not capable of processing images, but that the user did select the TYPE="image" input element. If possible, the server resource can then return a FORM more appropriate to the user agent's capabilities.
Some user agents currently send encoded data of the form
tstname.x=0&tstname.y=0
when they are unable to process the image. This behavior is deprecated, but writers of server-side programs that process FORM data resulting from TYPE="image" INPUT elements should where possible treat the coordinate pair (0,0) as equivalent to the absence of coordinate data,
It is recommended that authors use the VALUE attribute to contain a text alternative to the active image. User agents incapable of processing images can use this text in place of the image, providing some guidance to the user. Ideally, however, the document author should provide an alternative representation of the active image to such users, for example a series of NAMEd TYPE="submit" buttons.
The author would like to thank the following individuals who provided extensive feedback and commentary on initial drafts:
Foteos Macrides <MACRIDES@sci.wfbr.edu> Roy Fielding <fielding@avron.ics.uci.edu> Dave Seibert <seibert@prism.physics.mcgill.ca> Murray Altheim <murray@spyglass.com>
[1] RFC 1866 - The Hypertext Markup Language, Version 2.0
[2] RFC 1738 - Uniform Resource Locators (URL encoding of query strings)
[3] RFC 1808 - Relative URLs