float x = 100;
float y = 100;
float angle1 = 0.0;
float segLength = 50;

  float horizon;
void setup() {
  size(640, 360);
  strokeWeight(20.0);
  stroke(255, 100);
  size( 600, 400 );
  horizon=  height/3;
}

void draw() {
  background(50,182,255);
  fill( 155, 255, 0);
  ellipseMode( CORNER );
  ellipse(10, 0, 50, 50);
  fill( 100, 255, 100 );  
 //--  rectMode( CENTER );
//--  rect( 300, 200, 300, 200);
  rectMode( CORNER );
  rect( 0, horizon, width, height*2/3); 
  
  float dx = mouseX - x;
  float dy = mouseY - y;
  angle1 = atan2(dy, dx);  
  x = mouseX - (cos(angle1) * segLength);
  y = mouseY - (sin(angle1) * segLength);
 
  segment(x, y, angle1); 
  ellipse(x, y, 20, 20);
  
  
}

void segment(float x, float y, float a) {
  pushMatrix();
  translate(x, y);
  rotate(a);
  line(0, 0, segLength, 0);
  popMatrix();
}