Creating Client-Side Imagemaps

As mentioned previously, client-side imagemaps offer several improvements over serverside imagemaps. The most significant improvement is that the link doesn't need to be processed on the server. All modern browsers process the imagemap locally on the users' computers.

Getting an Image

To create an imagemap, you'll need an image (of course). This image will be the most useful if it has several discrete visual areas that can be selected individually. For example, use an image that contains several symbolic elements or that can be easily broken down into polygons. Photographs generally don't make good imagemaps because their various elements tend to blend together or are of unusual shapes. Figures 7.18 and 7.19 show examples of good and poor images for imagemaps.

Figure 7.18. A good image for an imagemap.

Figure 7.19. A not-so-good image for an imagemap.

Determining Your Coordinates

Client-side imagemaps consist of two parts; the first is the image used for the imagemap. The second is the set of HTML tags used to define the regions of the imagemap that serve as links. To define these tags, you must determine the exact coordinates on your image that define the regions you'll use as links.

You can determine these coordinates either by sketching regions and manually noting the coordinates or by using an imagemap creation program. The latter method is easier because the program automatically generates a map file based on the regions you draw with the mouse.

The Mapedit program for Windows, Linux, and the Mac OS can help you create clientside imagemaps. (See Appendix A, "Sources for Further Information," for a full list of related FTP sites.) In addition, many of the latest WYSIWYG editors for HTML pages and web graphics enable you to generate imagemaps. Table 7.1 lists the current tools for generating imagemaps.

Table 7.1. Imagemap Creation Software

Name	Platform	URL
Imaptool	Linux/X Window System	http://www.sspitzer.org/_imaptool/
Mapedit	Windows/UNIX/Mac	http://www.boutell.com/_mapedit/
Poor Person's Image Mapper	Web-based	http://www.pangloss.com/seidel/ClrHlpr/imagemap.html

If you must create your imagemaps by hand, here's how. First, make a sketch of the regions that will be active on your image. Figure 7.20 shows the three types of shapes that you can specify in an imagemap: circles, rectangles, and polygons.

Figure 7.20. There are three types of shapes available for creating imagemaps.

You next need to determine the coordinates for the endpoints of those regions. Most image-editing programs have an option that displays the coordinates of the current mouse position. Use this feature to note the appropriate coordinates. (All the mapping programs mentioned previously will create a map file for you, but for now, following the steps manually will help you better understand the processes involved.)

Defining a Polygon

Figure 7.21 shows the x,y coordinates of a polygon region. These values are based on their positions from the upper-left corner of the image, which is coordinate 0,0. The first number in the coordinate pair indicates the x value and defines the number of pixels from the extreme left of the image. The second number in the pair indicates the y measurement and defines the number of pixels from the top of the image.

Figure 7.21. Getting the coordinates for a polygon.

Note

The 0,0 origin is in the upper-left corner of the image, and positive y is down.

Defining a Circle

Figure 7.22 shows how to get the coordinates for circles. Here you note the coordinates for the center point of the circle and the radius, in pixels. The center point of the circle is defined as the x,y coordinate from the upper-left corner of the image.

Figure 7.22. Getting the coordinates for a circle.

Defining a Rectangle

Figure 7.23 shows how to obtain coordinates for rectangle regions. Note the x,y coordinates for the upper-left and lower-right corners of the rectangle.

Figure 7.23. Getting the coordinates for a rectangle.

The <map> and <area> Tags

If you're creating your imagemap manually and you've written down all of the coordinates for your regions and the URLs they'll point to, you can include this information in the client-side imagemap tags on a web page. To include a client-side imagemap inside an HTML document, use the <map> tag, which looks like the following:

<map name="mapname"> coordinates and links </map>

The value assigned to the name attribute is the name of this map definition. This is the name that will be used later to associate the clickable image with its corresponding coordinates and hyperlink references. So, if you have multiple imagemaps on the same page, you can have multiple <map> tags with different names.

Between the <map> and the </map> tags, enter the coordinates for each area in the imagemap and the destinations of those regions. The coordinates are defined inside yet another new tag: the <area> tag. To define a rectangle, for example, you would write the following:

<area shape="rect" coords="41,16,101,32" href="test.html">

The type of shape to be used for the region is declared by the shape attribute, which can have the values rect, poly, circle, and default. The coordinates for each shape are noted using the coords attribute. For example, the coords attribute for a poly shape appears as follows:

<area shape="poly" coords="x1,y1,x2,y2,x3,y3,...,xN,yN" href="URL">

Each x,y combination represents a point on the polygon. For rect shapes, x1,y1 is the upper-left corner of the rectangle, and x2,y2 is the lower-right corner:

<area shape="rect" coords="x1,y1,x2,y2" href="URL">

For circle shapes, x,y represents the center of a circular region of size radius:

<area shape="circle" coords="x,y,radius" href="URL">

The default shape is different from the othersit doesn't require any coordinates to be specified. Instead, the link associated with the default shape is followed if the user clicks anywhere on the image that doesn't fall within another defined region.

Another attribute you need to define for each <area> tag is the href attribute. You can assign href any URL you usually would associate with an <a> link, including relative pathnames. In addition, you can assign HRef a value of "nohref" to define regions of the image that don't contain links to a new page.

Note

When you're using client-side imagemaps with frames, you can include the target attribute inside an <area> tag to open a new page in a specific window, as in this example:

<area shape="rect" coords="x1,y1,x2,y2" href="URL" target= "window_name">

You need to include one more attribute in HTML 4.01. Earlier in this lesson, you learned how to specify alternate text for images. In HTML 4.01, the alt attribute is an additional requirement for the <area> tag that displays a short description of a clickable area on a client-side imagemap when you pass your cursor over it. Using the <area> example that I cited, the alt attribute appears as shown in the following example:

<area shape="rect" coords="41,16,101,32" href="test.html" alt="test link">

The usemap Attribute

After you've created your <map> tag and defined the regions of your image using <area> tags, the next step is to associate the map with the image. To do so, the usemap attribute of the <img> tag is used. The map name that you specified using the name attribute of the <map> tag, preceded by a #, should be used as the value of the usemap attribute, as shown in this example:

<img src="/books/2/631/1/html/2/image.gif" usemap="#mapname">

Note

The value assigned to usemap is a standard URL. This is why mapname has a pound symbol (#) in front of it. As with links to anchors inside a web page, the pound symbol tells the browser to look for mapname in the current web page. If you have a very complex imagemap, however, you can store it in a separate HTML file and reference it using a standard URL.

Task: Exercise 7.3. A Clickable Jukebox

Let's take a look at how to create a client-side imagemap for a real image. In this example, you'll define clickable regions on an image of a jukebox. The image you'll be using appears in Figure 7.24.

Figure 7.24. The jukebox image.

First, define the regions that will be clickable on this image. There are six rectangular buttons with musical categories on them, a center area that looks like a house, and a circle with a question mark inside it. Figure 7.25 shows regions on the image.

Figure 7.25. The jukebox with areas defined.

Now that you know where the various regions are, you need to find the exact coordinates of the areas as they appear in your image. You can use a mapping program like Mapedit, or you can do it manually. If you try it manually, it's helpful to keep in mind that most image-editing programs display the x and y coordinate of the image when you move the mouse over it.

Getting Image Coordinates from the Browser

You don't have an image-editing program? If you use Netscape as your browser, here's a trick: Create an HTML file with the image inside a link pointing to a fake file, and include the ismap attribute inside the <img> tag. You don't need a real link; anything will do. The HTML code might look something like the following:

<a href="nothing"><img src="/books/2/631/1/html/2/myimage.gif" ismap></a>

When you load this into your browser, the image is displayed as if it were an imagemap. When you move your mouse over it, the x and y coordinates appear in the status line of the browser. Using this trick, you can find the coordinates for the map file of any point on that image.

With regions and a list of coordinates, all you need are the web pages to jump to when the appropriate area is selected. These can be documents, scripts, or anything else you can call from a browser as a jump destination. For this example, I've created several documents and stored them inside the music directory on my web server. These are the pages you'll define as the end points when the clickable images are selected. Figure 7.26 identifies each of the eight clickable areas in the imagemap. Table 7.2 shows the coordinates of each and the URL that's called up when it's clicked.

Figure 7.26. Eight hotspots, numbered as identified in Table 7.2.

Table 7.2. Clickable Areas in the Jukebox Image

Number	Type	URL	Coordinates
1	rect	music/classics.html	101,113,165,134
2	rect	music/country.html	101,139,165,159
3	rect	music/rockpop.html	101,163,165,183
4	poly	music/home.html	175,152,203,118 220,118,247,152 237,153,237,181 186,181,186,153
5	rect	music/swing.html	259,113,323,134
6	rect	music/jazz.html	259,139,323,159
7	rect	music/gospel.html	259,163,323,183
8	circle	music/help.html	379,152,21

For the jukebox image, the <map> tag and its associated <area> tags and attributes look like the following:

<map name="jukebox"> <area shape="rect" coords="101,113, 165,134"   href="/music/classics.html"   alt="Classical Music and Composers" /> <area shape="rect" coords="101,139, 165,159"   href="/music/country.html"   alt="Country and Folk Music" /> <area shape="rect" coords="101,163, 165,183"   href="/music/rockpop.html"   alt="Rock and Pop from 50's On" /> <area shape="poly" coords="175,152, 203,118, 220,118, 247,152,   237,153, 237,181, 186,181, 186,153"   href="code/music/home.html"   alt="Home Page for Music Section" /> <area shape="rect" coords="259,113, 323,134"   href="/music/swing.html"   alt="Swing and Big Band Music" /> <area shape="rect" coords="259,139, 323,159"   href="/music/jazz.html"   alt="Jazz and Free Style" /> <area shape="rect" coords="259,163, 323,183"   href="/music/gospel.html"   alt="Gospel and Inspirational Music" /> <area shape="circle" coords="379,152, 21"   href="/music/help.html"   alt="Help" /> </map>

The <img> tag that refers to the map coordinates uses usemap, as follows:

<img src="/books/2/631/1/html/2/jukebox.gif" usemap="#jukebox">

Finally, put the whole thing together and test it. Here's a sample HTML file for The Really Cool Music Page with a client-side imagemap, which contains both the <map> tag and the image that uses it:

Input

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/transitional.dtd"> <html> <head> <title>The Really Cool Music Page</title> </head> <body bgcolor="#ffffff"> <div align="center"> <h1>The Really Cool Music Page</h1> <p>Select the type of music you want to hear.<br />  You'll go to a list of songs that you can select from.</p> <p> <img src="/books/2/631/1/html/2/jukebox.gif" alt="Juke Box" usemap="#jukebox" /> <map name="jukebox"> <area shape="rect" coords="101,113, 165,134"   href="/music/classics.html"   alt="Classical Music and Composers" /> <area shape="rect" coords="101,139, 165,159"   href="/music/country.html"   alt="Country and Folk Music" /> <area shape="rect" coords="101,163, 165,183"   href="/music/rockpop.html"   alt="Rock and Pop from 50's On" /> <area shape="poly" coords="175,152, 203,118, 220,118, 247,152,   237,153, 237,181, 186,181, 186,153"   href="code/music/home.html"   alt="Home Page for Music Section" /> <area shape="rect" coords="259,113, 323,134"   href="/music/swing.html"   alt="Swing and Big Band Music" /> <area shape="rect" coords="259,139, 323,159"   href="/music/jazz.html"   alt="Jazz and Free Style" /> <area shape="rect" coords="259,163, 323,183"   href="/music/gospel.html"   alt="Gospel and Inspirational Music" /> <area shape="circle" coords="379,152, 21"   href="/music/help.html"   alt="Help" /> </map></p> <p> <a href="code/music/home.html">Home</a> | <a href="code/music/classics.html">Classics</a> | <a href="code/music/country.html">Country</a> | <a href="code/music/rockpop.html">Rock/Pop</a> | <a href="code/music/swing.html">Swing</a> | <a href="code/music/jazz.html">Jazz</a> | <a href="code/music/gospel.html">Gospel</a> | <a href="code/music/help.html">Help</a> </p> </div> </body> </html>

Figure 7.27 shows the imagemap in a browser.

Output

Figure 7.27. The finished Really Cool Music Page with client-side imagemap.