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Chapter 6 |  |
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Chapter 6 | |
Normally, an image placed within an anchor simply becomes part of the anchor content. The browser may alter the image in some special way (usually with a special border) to clue the reader that it is a link, but the user clicks the image in the same way they click a textual link.
The HTML standard provides a feature that lets you embed many different links inside the same image. Clicking different areas of the image causes the browser to link to different target documents. Such mouse-sensitive images, known as image maps, open up a variety of creative linking styles.
The traditional HTML mechanism, enabled by the ismap attribute for the <img> tag, requires access to a server and related image map processing applications. Netscape 2.0 and Internet Explorer support the ismap server-side image map, as well as a client-side image map option that is enabled via a special usemap extension for the <img> tag, along with special <map> and <area> tags. Since translation of the mouse position in the image to a link to another document happens on the user's machine, usemap image maps don't require a special server connection, and even can be implemented in non-Web environments, such as on a local hard drive or on a CD-ROM-based document collection. [<map>, 6.5.3] [<area>, 6.5.4]
You add an image to an anchor simply by placing an <img> tag within the body of the <a> tag. Make that embedded image into a mouse-sensitive one by adding the ismap attribute to the <img> tag. This special <img> attribute tells the browser that the image is a special map containing more than one link. (The ismap attribute is ignored by the browser if the <img> tag is not within an <a> tag.) [<img>, 5.2.6]
When the user clicks some place within the image, the browser passes the coordinates of the mouse pointer along with the URL specified in the <a> tag to the document server. The server uses the mouse pointer coordinates to determine which document to deliver back to the browser.
When ismap is used, the href attribute of the containing <a> tag must contain the URL of a server application or, for some HTTP servers, a related map file that contains the coordinate and linking information. If the URL is simply that of a conventional document, errors may result and the desired document will most likely not be retrieved.
The coordinates of the mouse position are screen pixels counted from the upper-left corner of the image beginning with (0,0). The coordinates are added to the end of the URL, preceded by a question mark (?).
For example, if a user clicks 15 pixels down and 43 pixels over from the upper-left corner of the image displayed from the following link:
<a href="/cgi-bin/imagemap/toolbar.map"> <img ismap src="pics/toolbar.gif"> </a>
the browser sends the following search parameters to the HTTP server:
/cgi-bin/imagemap/toolbar.map?43,15
In the example, toolbar.map is a special image map file inside the cgi-bin/imagemap directory and containing coordinates and links. A special image map process uses that file to match the passed coordinates (43,15 in the example) and return with the selected hyperlink document.
With mouse-sensitive <ismap>-enabled image maps, the browser is only required to pass along the URL and mouse coordinates to the server. Converting these coordinates into a specific document is handled by the document server. The conversion process differs between servers and is not defined by the HTML standard.
You need to consult with your Web server administrators and perhaps even read your server's documentation to determine how to create and program an image map. Most servers come with some software utility, typically located in a cgi-bin/imagemap directory, to handle image maps. And most of these use a text file containing the image map regions and related hyperlinks that is referenced by your image map URL to process the image map query.
Here's an example image map file that describes the sensitive regions in our example image:
# Imagemap file=toolbar.map default dflt.html circle 100,30,50 link1.html rectangle 180,120,290,500 link2.html polygon 80,80,90,72,160,90 link3.html
Each sensitive region of the image map is described by a geometric shape and defining coordinates in pixels, such as the circle with its center point and radius, the rectangle's upper-left and lower-right edge coordinates, and the loci of a polygon. All coordinates are relative to the upper-left corner of the image (0,0). Each shape has a related URL.
An image map processing application typically tests each shape in the order it appears in the image file and returns the document specified by the corresponding URL to the browser if the user's mouse x,y coordinates fall within the boundaries of that shape. That means it's okay to overlap shapes; just be aware which takes precedence. Also, the entire image need not be covered with sensitive regions: if the passed coordinates don't fall within a specified shape, the default document gets sent back to the browser.
This is just one example for how an image map may be processed and the accessory files required for that process. Please huddle with your webmaster and server manuals to discover how to implement a server-side image map for your HTML documents and system.
The obvious downside to server-side ismap-enabled image maps is that they require a server. That means you need access to the required HTTP server or its /cgi-bin/ directory. That's not always the case. And, server-side image maps limit portability since not all image map processing applications are the same.
Server-side image maps also mean delays for the user while browsing since the browser must get the server's attention to process the image coordinates. That's even if there's no action to take, such as a section of the image that isn't hyperlinked and doesn't lead anywhere.
Netscape and Internet Explorer support what are known as client-side image maps that suffer from none of these difficulties. Enabled by the usemap attribute for the <img> tag, and defined by special <map> and <area> extension tags, client-side image maps let HTML authors include in their documents a map of coordinates and links that describe the sensitive regions of an image. The extended browser on the client computer translates the coordinates of the mouse position within the image into an action, such as loading and displaying another document.
Include the usemap attribute itself as part of the <img> tag. Its value is the URL of a <map> segment in an HTML document that contains the map coordinates and related link URLs. The document in the URL identifies the HTML document containing the map; the fragment identifier in the URL identifies the map to be used. Most often, the map is in the same document as the image itself, and the URL can be reduced to the fragment identifier: a hash (#) symbol followed by the map name.
For example, the following source fragment tells the Netscape or Internet Explorer browser that the map.gif image is a client-side image map and that its mouse-sensitive coordinates and related link URLs are found in the map section of the document named map:
<img src="pics/map.gif" usemap="#map">
For client-side image maps to work, you must include somewhere in the HTML document a set of coordinates and URLs that define the mouse-sensitive regions of a client-side image map and the hyperlink to take for each region that is clicked by the user. You include those coordinates and links as values of attributes in special <area> tags; the collection of <area> specifications are enclosed within the <map> tag and its end tag </map>.
The <map> segment may appear anywhere in the body of any HTML document. Currently, only Netscape 2.0 and Internet Explorer recognize client-side image maps, so all other browsers, including earlier versions of Netscape, ignore the contents of the <map> tag. The <map> contents never get displayed in the browser window.
The required name attribute and its quote-enclosed value in the <map> tag is the name used by the usemap attribute in an <img> tag to locate the image map specification. The name must be unique and not used by another <map> in the document, but more than one image map may reference the same <map> specifications. [usemap, 6.5.2]
The guts of a client-side image map are the <area> tags within the map segment. These <area> tags define each mouse-sensitive region and the action the browser should take if that region is selected by the user in an associated client-side image map.
When the user moves the mouse pointer over any region defined by an <area> tag in the image's <map>, the pointer icon changes into a hand and the browser displays the URL of the related hyperlink at the bottom of the browser window. Regions of the client-side image map not defined in at least one <area> tag are not mouse-sensitive.
The required coords attribute of the <area> tag defines coordinates of a mouse-sensitive region in a client-side image map. The number of coordinates and their meaning depend upon the region's shape as determined by the shape attribute, below. Currently, Netscape 2.0 supports only rectangular-shaped regions, while Internet Explorer supports rectangles, circles, and polygons. The appropriate values for each shape are:
coords="x,y,r" where x and y define the position of the center of the circle ("0,0" is the upper-left corner of the image) and r is its radius in pixels.
coords="x1,y1,x2,y2,x3,y3,..." where each pair of x,y coordinates define a vertex of the polygon, with "0,0" being the upper left corner of the image. At least three pairs of coordinates are required to define a triangle, higher order polygons require a larger number of vertices. The polygon is automatically closed, so it is not necessary to repeat the first coordinate at the end of the list to close the region.
coords="x1,y1,x2,y2" where the first coordinate pair is one corner of the rectangle and the other pair is the diagonally opposite corner, with "0,0" being the upper-left corner of the image. Note that a rectangle is just a shortened way of specifying a polygon with four vertices.
For example, the following fragment defines a single mouse-sensitive region in the lower-right quarter of a 100x100 image and another circular region smack in the middle:
<map name="map1"> <area shape=rectangle coords="75,75,99,99"> <area shape=circle coords="50,50,25"> </map>
If the coordinates in one <area> tag overlap with another region, the first <area> tag takes precedence. The browsers ignore coordinates that extend beyond the boundaries of the image.
Like the href attribute for hyperlinks, the href attribute for the <area> tag defines the URL of the desired link if its region in the associated image map is clicked. The value of the href attribute is any valid URL, relative or absolute.
For example, Netscape will load and display the link4.html document if the user clicks in the lower-left quarter of a 100x100-pixel image, as defined by the following example image map <area> tag:
<map name="map"> <area coords="75,75,99,99" href="link4.html"> </map>
The nohref attribute for the <area> tag lets you define a mouse-sensitive region in a client-side image map for which no action is taken even though the user may select it. You must include either an href or a nohref attribute for each <area> tag.
This attribute defines the shape of the mouse-sensitive region. Netscape supports only one shape, a rectangle, specified by the value rect. Internet Explorer also supports circles and polygons as well as shortened forms of the shape names. Thus, Internet Explorer supports the values circ, circle, poly, polygon, rect, and rectangle.
The value of this attribute affects how the value of the coords attribute, above, is interpreted by the browser.
The following example fragment draws together the various components of a client-side image map. It includes the <img> tag with the image reference and usemap attribute with a name that points to a <map> that defines four mouse-sensitive regions and related links:
<body>
...
<img src="pics/map.gif" usemap="#map1">
...
<map name="map1">
<area shape=rectangle coords="0,0,49,49" href="link1.html">
<area shape=polygon coords="50,0,99,0,99,49,50,49"
href="link2.html">
<area shape=circle coords="50,50,25"
href="link3.html">
<area coords="50,50,99,99" href="link4.html">
</map>
Unlike its server-side ismap counterpart, the client-side image map tag (<img usemap>) doesn't need to be included in an <a> tag. But it may be, so that you can gracefully handle browsers that are unable to process client-side image maps.
For example, Mosaic or early versions of Netscape simply load a document named main.html if the user clicks the map.gif image referenced in the following source fragment. The extended browsers, on the other hand, will divide the image into mouse-sensitive regions, as defined in the associated <map>, and link to a particular name anchor within the same main.html document if the image map region is selected by the user:
<a href="main.html"> <img src="pics/map.gif" ismap usemap="map1"> </a> ... <map name="map1"> <area coords="0,0,49,49" href="main.html#link1"> <area coords="50,0,99,49" href="main.html#link2"> <area coords="0,50,49,99" href="main.html#link3"> <area coords="50,50,99,99" href="main.html#link4"> </map>
To make an image map fully backward-compatible with all image map-capable browsers, you may also include both client-side and server-side processing for the same image map. Netscape 2.0 and Internet Explorer will honor the faster client-side processing; all other browsers will ignore the usemap attribute in the <img> tag and rely upon the referenced server process to handle user selections in the traditional way. For example:
<a href="/cgi-bin/images/map.proc"> <img src="pics/map2.gif" usemap="map2" ismap> </a> ... <map name="map2"> <area coords="0,0,49,49" href="link1.html"> <area coords="50,0,99,49" href="link2.html"> <area coords="0,50,49,99" href="link3.html"> <area coords="50,50,99,99" href="link4.html"> </map>
Some of the most visually compelling pages we've seen on the Web have mouse-sensitive images: maps with regions that when clicked, for example, lead to more information about a country or town, or result in more detail about the location and who to contact at a regional branch of a business. We've even seen a mouse-sensitive image of a fashion model whose various clothing parts lead to their respective catalog entries, complete with detailed description and price tag for ordering.
The visual nature of mouse-sensitive images coupled with the need for an effective interface means that you should strongly consider having an artist, user-interface designer, and even a human-factors expert evaluate your mouse-sensitive imagery. At the very least, engage in a bit of user testing to make sure people know where to click to move to the desired document. Make sure the ``mouseable'' areas of the image indicate this to the user using a consistent visual mechanism. Consider using borders, drop shadows, or color changes to indicate those areas that can be selected by the user.
Finally, always remember that the decision to use mouse-sensitive images is an explicit decision to exclude text-based and image-restricted browsers from your pages. This includes the many millions of browsers connecting to the Internet via slow modem connections. For these people, downloading your beautiful images is simply too expensive. To keep from disenfranchising a growing population, make sure any page that has a mouse-sensitive image has a text-only equivalent easily accessible from a link on the image-enabled version. Some thoughtful webmasters even provide separate pages for users preferring full graphics versus mostly text.
