The overlapping area will blend automatically if one of the rectangles is drawn with a transparent color:

1. void setup(){
2.   size(200,200);
3.   noStroke();
4. }
5. void draw(){
6.  background(0);
7.  fill(0,0,255);
8.  rect( 20,20,100,120);
9.  fill(255,0,0,128);
10.  rect(mouseX-30,mouseY-40, 60, 80);
11. }

Thanks for the reply, but if I want to use the additive blending mode then I can do that only with a function like blendColor() right? And from what I know of the function I can use the blended color after specifying a region right? Or is there another way to do it?

With regard to the question of how to find the overlapping area of two rects...

Here's how to do it with math.

Code Example

1. PVector p1 = new PVector(300, 300);
2. float w1 = 150;
3. float h1 = 200;
4. float w2 = 200;
5. float h2 = 150;
6.  
7. void setup() {
8.   size(800, 800);
9. }
10.  
11. void draw() {
12.   background(255);
13.   PVector p2 = new PVector(mouseX, mouseY);
14.   noFill();
15.   rect(p1.x, p1.y, w1, h1);
16.   rect(p2.x, p2.y, w2, h2);
17.   if (rectCollision(p1,w1,h1, p2,w2,h2)) {
18.     PVector q1 = new PVector(max(p1.x, p2.x), max(p1.y, p2.y));
19.     PVector q2 = new PVector(min(p1.x+w1, p2.x+w2), min(p1.y+h1, p2.y+h2));
20.     float wq = q2.x - q1.x;
21.     float hq = q2.y - q1.y;
22.     fill(255, 0, 0);
23.     rect(q1.x, q1.y, wq, hq);
24.   }
25. }
26.  
27. boolean rectCollision(PVector p1, float w1, float h1, PVector p2, float w2, float h2) {
28.   return !(p1.x > p2.x + w2 || p1.x + w1 < p2.x || p1.y > p2.y + h2 || p1.y + h1 < p2.y);
29. }