You should draw some non usual form because you can check circle and horizontal or vertical line mor quickly : your sketch trap all color intersection so your example is not the best one (i think). You could draw a random line instead of rectangular form.

1.     //Point_intersect
       //Owaun Scantlebury
       //November 17th 2010
       
       
       int w= 255;
       int h = 255;
       PGraphics a,b;
       PImage c,d;
       void setup(){
         size(255,255);
         a = cg(w,h);
         b= cg(w,h);
         c=createImage(w,h,ARGB);
         d=createImage(w,h,ARGB);
         B= new color[b.width*b.height];
         A= new color[a.width*a.height];
       
         lp(a);
         lp(b);
         bd(a);
         bd(b);
         a.smooth();
       
         for (int x=25;x<width-25;x++)
         {
           setx(a,x,25,color(255,0,0,val),"r");
        
           setx(a,x,height-25,color(255,0,0,val),"r");
         
           setx(a,25,x,color(255,0,0,val),"r");
       
           setx(a,width-25,x,color(255,0,0,val),"r");
       
       
           setx(b,x+15,25-15,color(0,0,255,val),"b");
           //a.set(x,height-25,color(0,0,255,255));
           setx(b,x+15,height-25-15,color(0,0,255,val),"b");
       
           setx(b,25+15,x-15,color(0,0,255,val),"b");
       
           setx(b,width-25+15,x-15,color(0,0,255,val),"b");
         }
         up(a);
         up(b);
         ed(b);
         ed(a);
         image(a,0,0);
         background(128);
         background(128);
         image(b,0,0);
       
         arraycopy(a.pixels,c.pixels);
         arraycopy(b.pixels,d.pixels);
         arraycopy(a.pixels,A);
         arraycopy(b.pixels,B);
         loadPixels();
       }
       public color [] A;
       public color [] B;
       void mouseDragged(){
        if (mouseButton==37)c.set(mouseX,mouseY,color(255,0,0));
       
       if(mouseButton==39) d.set(mouseX,mouseY,color(0,0,255));
       if (mouseButton==3){
         c.set(mouseX,mouseY,color(0,255,0));
         d.set(mouseX,mouseY,color(0,255,0));
       }
        
       }
       void draw(){
       
         background(128);
         lp(a);
         lp(b);
         bd(a);
         bd(b);
       
         a.copy(c,0,0,w,h,0,0,w,h);
         b.copy(d,0,0,w,h,0,0,w,h);
       
         a.noFill();
         a.stroke(255,0,0,val);
         a.ellipse(mouseX,mouseY,60,60);
       
         a.stroke(0,255,0,val);
         a.ellipse(mouseX,mouseY,120,120);
       
         b.noFill();
         b.stroke(255,0,255,val);
         b.ellipse(mouseX,mouseY,30,30);
         b.stroke(0,255,0,val);
         b.ellipse(mouseX,mouseY,120,120);
        
      
         ed(b);
         ed(a);
         up(a);
         up(b);
       
         image(a,0,0);
       
         image(b,0,0);
         checkit();
       }
       
       PGraphics cg(int w, int h){
         PGraphics  retval = createGraphics(w,h,JAVA2D);
       
         return retval;
       }
       void bd(PGraphics img){
         img.beginDraw();
       
       }
       
       void ed(PGraphics img){
         img.endDraw();
       }
       
       void lp(PGraphics img){
         img.loadPixels();
       }
       
       void up(PGraphics img){
         img.updatePixels();
       }
       int val = 256;
       void setx(PGraphics  a,int x,int y,color col,String in){
         color retval = 0;
         color src = a.pixels[x+a.width*y];
         color rr1 = _fr2(col);
         color rr2 = _fr2(src);
         color gg1 = _fg2(col);
         color gg2 = _fg2(src);
         color bb1 = _fb2(col);
         color bb2 = _fb2(src);
       
         if (in=="r")retval = color(rr1,gg2,bb2,val);
         if (in=="g")retval = color(rr2,gg1,bb2,val);
         if (in=="b")retval = color(rr2,gg2,bb1,val);
       
       
         a.pixels[x+a.width *y]=retval;
       }
       
       color _fr2(color colx){
         return  colx >> 16 & 0xff;
       }
       color _fg2(color colx) {
         return  colx >> 8 & 0xff;
       }
       color _fb2(color colx){
         return    colx  & 0xff;
       }
       
       void checkit(){
         for (int x=0;x<a.pixels.length;x++){
           int _y=(int)x/width;
           int _x= x-(_y*width);
           color c1 = a.pixels[x];
           color c2  = b.pixels[x];
           float rr=_fr2(c1);
           float bb= _fb2(c2);
           float gg1= _fg2(c1);
           float gg2= _fg2(c2);
       
           if ((rr+bb)>255 || (rr+gg1) > 255 || (bb+gg1)>255 || (bb+gg2)>255 || (rr+gg2)>255) {
             noFill();
             ellipse(_x,_y,6,6);
           }
         }
       }