Java code...

[Stephen Lime]

Ok. Here's my working version. It's Java 1.1 code and compiles fine
under 1.2 except for a few "depricated methods" messages. If anyone
is interested in porting to 1.2 and swing let me know.

To install: Copy the mapimage.class file someplace on you system that
is web accessible. If you need to rebuild from source you'll need to make
sure that the Netscape java classes (either a .zip or a .jar) are in your
CLASSPATH environment variable.

To use:

  1. Make sure your normal MapServer form has a name attribute (eg. name="mapserv")
  2. Add the following javascript code to the HTML header section:

    <script language="javascript">
      function SetImageXY(i) { document.mapserv.imgxy.value = i; }
      function SetImageBox(i) { document.mapserv.imgbox.value = i; }
    </script>

     These functions are called by the applet, so the names can't be changed. However, you
     could change the function code itself to, for example:
    
     - submit the form immediately
     - update the status line of the browser

  3.  Add a couple of hidden variables to the form:

    <input type="hidden" name="imgbox" value="-1 -1 -1 -1">
    <input type="hidden" name="imgxy" value="149.5 199.5">
  
    imgbox allows the MapServer to process a pair of mouse clicks (ie. a box). It is ignored if the box is 
    simply a point, hence the initialization value. imgxy is a way to pass a single mouse click to the
    MapServer. This mimics the standard forms based method. The initial value should be something
    meaningful like the center of the image. Remember that the coordinates on the image start at 0
    and end at nrows-1 or ncols-1. To compute center x use (ncols-1)/2 and y use (nrows-1)/2 and
    keep the decimal value. This allows the user to zoom using a button rather than the image.

  4. Add a call to the applet (this replaces the normal form-based call to the image <input type="image"...>).
      Here's an example from a template file (note the [img] substitution):

    <applet codebase="/java/mapimage" code="mapimage.class" width="300" height ="400" name="mapimage" MAYSCRIPT>
        <param name="image" value="http://www.dnr.state.mn.us[img]">
        <param name="jitter" value="1">
        <param name="color" value="255 0 0">
      </applet>   

     The parameters are:

       image - fully qualified URL to the image. This can be a CGI call.
       jitter - allowable slop in mouse clicking. Basically sets a minimum box size.
       color - RGB triplet that sets the color of the box or cursor.

That covers the basic use. There are 3 public methods that can be called from javascript that change
the behavior of the applet.

boxOn and boxOff toggle the applets ability to display a dragged box. Useful when adding features to
an interface. For example, it doesn't make sense to drag a box when zooming out. No arguments.

newMap(string image_url) allows you to swap images without changing the rest of the page. The function
uses the java MediaTracker interface to delay image loading until the new image is completely loaded. This
is useful with advanced interface setup. It's possible to manage extents, layers etc completely on the client
using javascript requesting a new image only when necessary via the applet. It's actually pretty cool once
it's set up that way. The string supplied to newMap must be a fully qualified URL for an image (can be a CGI
call).

Examples of this applet can be found at:

  - http://deli.dnr.state.mn.us/
  - http://www.dnr.state.mn.us/mis/gis/gislis99/example1.html (click "next step" until you hit example 5)
  - http://www.ra.dnr.state.mn.us/

Possible enhancements:

  - displaying coordinates on the image as the mouse moves
  - bubble attributes (eg. move mouse over feature label and display attributes or whatever)

Hope this helps, have fun!

Steve

Stephen Lime
Internet Applications Analyst
MIS Bureau - MN DNR

(651) 297-2937
steve.lime at dnr.state.mn.us

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