an easy way to detect wether the mouse is over a certain area is drawing this "area" into an offscreen buffer and check the color at the mouse position. For exaple. if you draw a red prism over a black background. you can use
if(pg.get(mouseX,mouseY)== color(255,0,0)  (or the pixelsarray directly) to detect if the mouse is over it or not...
doing it this way the shape of your area doesnt matter.

1. float boxSize = 100;
2. float boxX, boxY;
3. void setup(){
4.   size(640,480);
5.   boxX = width/2;
6.   boxY = height/2;
7.   rectMode(RADIUS);
8. }
9. void draw(){
10.   background(255);
11.   //make sure all shapes are "transparent"
12.   noFill(); 
13.    
14.   //draw mouse circle
15.   ellipseMode(CENTER);
16.   ellipse(mouseX, mouseY, 30,30);
17.  
18.   //test if mouse is in box's area
19.   //ideas and code taken from:
20.   //http://www.processing.org/learning/basics/mousefunctions.html
21.   if(mouseX>boxX-boxSize && mouseX<boxX+boxSize &&
22.   mouseY>boxY-boxSize && mouseY<boxY+boxSize){
23.     fill(200, 200);
24.     //println("in area");
25.   }else{
26.     ;
27.   }
28.  
29.   //create box "safe" area in center and dividing lines
30.   //USE QUADS, I.E. PRISMS
31.   rect(boxX, boxY, boxSize, boxSize);
32.   //line(0, 0, width, height); //top left to bottom right
33.   //line(0, height, width, 0); //bottom left to top right
34.  
35.   noFill();
36.   //top
37.  
38.   quad(0.0,0.0, float(width),0.0, width/2+boxSize, height/2-boxSize, width/2-boxSize, height/2-boxSize);
39.   //bottom
40.   quad(float(width),float(height), 0.0,float(height), width/2-boxSize, height/2+boxSize, width/2+boxSize, height/2+boxSize);
41.   //left
42.   quad(0.0,0.0, 0.0,float(height), width/2-boxSize, height/2+boxSize, width/2-boxSize, height/2-boxSize);
43.   //right
44.   quad(float(width),0.0, float(width),float(height), width/2+boxSize, height/2+boxSize, width/2+boxSize, height/2-boxSize);
45. }

This is the current code, it's still a work in progress.
It depicts a square surrounded by the 4 prisms.
I understand what yove told me, but I would like to keep the prisms with noFill() until the mouse is over that prism.

Or use toxiclibs, put the corner points in a polygon and check if point lies within:

The Toxiclibs look good, but I'd prefer to keep extra libraries at a minimum the eventual code will have to be accessible for a fair few people so I'd like to keep accessibilty at a maximum.
I have implemented the code depending on colour of the poolygons and it's working well.
Still trying to figure out the area problem though with the generic included librairies.
Any more help? It's been stirling so far!
Sam

Understand your concerns, though accessibility is always relative, IMHO. Easy to read code & not reinventing the wheel is the main idea of using libraries and learning how to use open source culture as a tool itself is essential learning too... Anyhow, my main point was that using the offscreen approach is pretty brute force and not very efficient compared to a purely geometric solution. It's easy enough to extract the containsPoint() function from the Polygon2D class, so here's a version not using any libraries:

Hi guys, got it all sorted in the end.
Many thanks to all.
First time posting in the forums and it's been a positive experience.
Sam
p.s. Going to put the shapes into a separate object.

1. float boxSize = 100;
2. float boxX, boxY;
3. void setup(){
4.   size(640,480);
5.   boxX = width/2;
6.   boxY = height/2;
7.   rectMode(RADIUS);
8. }
9. void draw() {
10.   background(255);
11.   fill(0);
12.   ellipse(mouseX, mouseY, 20,20);
13.   noFill();
14.   //test whether mouse is in box
15.   if(mouseX>boxX-boxSize && mouseX<boxX+boxSize &&
16.   mouseY>boxY-boxSize && mouseY<boxY+boxSize){
17.     fill(200, 200);
18.     //println("in area");
19.   }else{
20.     ;
21.   }
22.   //draw center box
23.   rect(boxX, boxY, boxSize, boxSize);
24.  
25.   //following code mod'd
26.   //from toxmeister on Processing forums
27.  
28.   //draw top
29.   PVector[] top=new PVector[4];
30.   // add corner points of quad
31.   top[0]=new PVector(0,0);
32.   top[1]=new PVector(float(width),0);
33.   top[2]=new PVector(width/2+boxSize, height/2-boxSize);
34.   top[3]=new PVector(width/2-boxSize, height/2-boxSize);
35.   // check if mouse pos is inside
36.   if(containsPoint(top,mouseX,mouseY)) {
37.     fill(200,200);
38.   } else {
39.     noFill();
40.   }
41.   beginShape();
42.   for(PVector v : top) {
43.     vertex(v.x,v.y);
44.   }
45.   endShape(CLOSE);
46.  
47.   //draw bottom
48.   PVector[] bottom=new PVector[4];
49.   // add corner points of quad
50.   bottom[0]=new PVector(float(width),float(height));
51.   bottom[1]=new PVector(0.0,float(height));
52.   bottom[2]=new PVector(width/2-boxSize, height/2+boxSize);
53.   bottom[3]=new PVector(width/2+boxSize, height/2+boxSize);
54.   // check if mouse pos is inside
55.   if(containsPoint(bottom,mouseX,mouseY)) {
56.     fill(200,200);
57.   } else {
58.     noFill();
59.   }
60.   beginShape();
61.   for(PVector v : bottom) {
62.     vertex(v.x,v.y);
63.   }
64.   endShape(CLOSE);
65.  
66.   //draw left
67.   PVector[] left=new PVector[4];
68.   // add corner points of quad
69.   left[0]=new PVector(0.0,0.0);
70.   left[1]=new PVector(0.0,float(height));
71.   left[2]=new PVector(width/2-boxSize, height/2+boxSize);
72.   left[3]=new PVector(width/2-boxSize, height/2-boxSize);
73.   // check if mouse pos is inside
74.   if(containsPoint(left,mouseX,mouseY)) {
75.     fill(200,200);
76.   } else {
77.     noFill();
78.   }
79.   beginShape();
80.   for(PVector v : left) {
81.     vertex(v.x,v.y);
82.   }
83.   endShape(CLOSE);
84.  
85.   //draw right
86.   PVector[] right=new PVector[4];
87.   // add corner points of quad
88.   right[0]=new PVector(float(width),0.0);
89.   right[1]=new PVector(float(width),float(height));
90.   right[2]=new PVector(width/2+boxSize, height/2+boxSize);
91.   right[3]=new PVector(width/2+boxSize, height/2-boxSize);
92.   // check if mouse pos is inside
93.   if(containsPoint(right,mouseX,mouseY)) {
94.     fill(200,200);
95.   } else {
96.     noFill();
97.   }
98.   beginShape();
99.   for(PVector v : right) {
100.     vertex(v.x,v.y);
101.   }
102.   endShape(CLOSE);
103. }
104. // taken from:
105. // http://hg.postspectacular.com/toxiclibs/src/tip/src.core/toxi/geom/Polygon2D.java
106. boolean containsPoint(PVector[] verts, float px, float py) {
107.   int num = verts.length;
108.   int i, j = num - 1;
109.   boolean oddNodes = false;
110.   for (i = 0; i < num; i++) {
111.     PVector vi = verts[i];
112.     PVector vj = verts[j];
113.     if (vi.y < py && vj.y >= py || vj.y < py && vi.y >= py) {
114.       if (vi.x + (py - vi.y) / (vj.y - vi.y) * (vj.x - vi.x) < px) {
115.         oddNodes = !oddNodes;
116.       }
117.     }
118.     j = i;
119.   }
120.   return oddNodes;
121. }