int squareX;
int squareY;
int dx;
int dy;
int x;
int y;
float distance;
float distance2;

void setup(){
  size(300,300);
  rectMode(CENTER_DIAMETER);
 
  squareX = width/2;
  squareY = height/2;

}

void loop(){
  background(255,255,255);
  
  for(y = 0;y<300;y +=10){
     for(x= 0; x <300; x+=10){
  
//-------calculate distance--------
  dx = mouseX - x;//distance on X axis
  dy = mouseY - y;//distance on Y axis
  distance = sqrt(dx*dx + dy*dy);//2D distance
  
 //----DRAW================
  fill(distance,dx,dy);
  rect(x,y,4-distance/10.0,4-distance/10.0);
  }
  }
}