int screensize = 300;
int x1, y1, x2, y2;
float p_left, p_right, p_up, p_down;
float direction_x, direction_y;
int stride;
int counter;

void setup() {
  size(screensize, screensize);
  background(255, 255, 255);
  colorMode(HSB, screensize);
  x1 = y1 = x2 = y2 = 150;

  stride = 3;
  counter = 0;
}

void loop() {
  counter++;

  // To regulate of probability x, y
  if (mousePressed) {
    // probability of x
    if (x1mouseX) {
      p_right = 2;
      p_left = -2;
    }

    // pobability of y
    if (y1mouseY) {
      p_down = 2.8;
      p_up = -2;
    }

    // To regulate of probability x, y to default
  } else {
    p_left = -1;
    p_right = 2;
    p_up = -1;
    p_down = 2.8; // To regulate imcomplate random. Why?
  }

  // to get the new x-coordinate
  direction_x = random(p_left, p_right);

  if (direction_x < 0) {
    x2 = x1 - stride;
  } else if (direction_x > 1) {
    x2 = x1 + stride;
  }

  if (x2 < 0 || x2 > screensize) {
    x2 = x1;
  }

  // to get the new y-coordinate
  direction_y = random(p_up, p_down);

  if (direction_y < 0) {
    y2 = y1 - stride;
  } else if (direction_x > 1) {
    y2 = y1 + stride;
  }

  if (y2 < 0 || y2 > screensize) {
    y2 = y1;
  }

  // to draw
  fill(x2, y2, counter%300);
  ellipse(x2, y2, 8, 8);

  x1 = x2;
  y1 = y2;
}