Hi,

I'm trying to solve this question:

I would find the exact angle on mouse click in order to generate an ellipse that moves in a circle,
I wrote this code based on the idea suggested in "Processing" by Reas and Fry (pg 123).

It works "quarter a way", because of if I click on the top-left quarter of the screen
the ellipse starts moving in circle counter-clockwise,
clicking in the bottom-right it moves in circle clockwise,
clicking on the top-right the ellipse moves in ellipse clockwise,
clicking on the bottom-left it moves in ellipse counter-clockwise.

I can't figure out what's the problem, I guess i miss some trigonometry knowledges... :(

I would appreciate if someone could point me in the right way.

Here's the code:

1. /* from Processing by Reas  and Fry (pg 123)
    float a = HALF_PI;
    float c = cos(a);
    float ac = acos(c);
    // Prints "3.1415927 : -1.0 : 3.1415927"
    println(a + " : " + c + " : " +  ac);*/
   
   float a= 100, b=100, raggio=30;
   
   void setup() {
     size(200,200);
     smooth();
   }
   
   void draw() {
     background(120);
     stroke(0);
     line(width/2,0,width/2,height);
     line(0,height/2,width,height/2);
     noStroke();
     fill(0);
     ellipse(cos(a)*raggio+width/2,sin(b)*raggio+height/2,10,10);
     a=a+PI/100.0;
     b=b+PI/100.0;
   }
   
   
   
   void mousePressed() {
     raggio=dist(mouseX,mouseY,width/2,height/2);
     float xT=((mouseX-width/2)/dist(width/2,height/2,mouseX,mouseY));
     float yT=((mouseY-height/2)/dist(width/2,height/2,mouseX,mouseY));
     a=acos(xT);
     b=asin(yT);
     println("the value of a is :"+a+" the value of b is "+b);
   }