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

void setup(){
  size(500,500);
  rectMode(CENTER_DIAMETER);
  
  squareX = width/2;
  squareY = height/2;

}

void loop(){
 background(255,255,255);
 
 dx = mouseX - squareX;
 dy = mouseY - squareY;
 distance = sqrt(dx*dx+dy*dy);
 distance2 =  distance*2;
 
 if(distance2> 255){
 distance2 = 255;
 }
 
 fill(distance2, 0,0);
 rect(squareX  ,squareY, distance,distance);
 
}