Communicating Applets

The independent operators described in the previous section can be embedded and interact in an HTML document for display in a Web page. This can be achieved by the following piece of HTML code:

     <APPLET CODE="AddNoise.class" WIDTH=800 HEIGHT=400>
     <PARAM NAME=images  VALUE="images/holesquare2.gif">
     </APPLET>

which adds the applets to an HTML page.

If the parameter named images is given a value (as in this case) then it used, otherwise a default image is loaded.

Then, the output image from each applet is linked to the next one down the page so that it can be further processed. The way this is achieved is by giving each applet the ability to communicate with the next one.

The procedure which JAVA employs to grant applets the privilege of communicating with each-other is fairly simple and works as follows:

• The applet must be given a name using the NAME parameter in the relevant HTML document e.g.

       <APPLET CODE="AddNoise.class" WIDTH=800 HEIGHT=400 NAME="addnoise">
  

• Then, one must use the getApplet() method from the applet context with the name of that applet as a parameter. This will return a reference to that applet. One can use this applet as if it were an object and manipulate it accordingly. For example, if one wished to obtain the applet named ``addnoise'' belonging to the AddNoise class one would need to declare an AddNoise variable and cast the applet obtained by getApplet() to that class:

       AddNoise addnoise_applet;
       addnoise_applet = (AddNoise)getAppletContext().getApplet("addnoise");
  

• All one needs to do now is define the functions that will implement the communication protocol.
  Each applet has been equipped with two extra functions. One of these functions is send_image(), which activates when the ``Forward Results'' button has been pressed, thus sending the whole integer array of the final image produced by this applet to the next applet on the page. For example, consider Figure 9. The applets pictured are called ``gamma'' and ``addnoise'' respectively. After ``Forward Results'' has been pressed and the gamma operation repeated, the resulting image is passed on to the ``addnoise'' applet (Figure 9).
  This is achieved by a call to send_image() which, in turn, calls the function set_src_image() of the ``addnoise'' applet in the usual object-oriented way:

       addnoise_applet.set_src_image(img_1d);
  

  This function's purpose is to set the source image of the applet which defines it to be the image sent by the applet that calls it.