This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

Distance 2D.

Move the mouse across the image to obscure and reveal the matrix. Measures the distance from the mouse to each square and sets the size proportionally.

 
float max_distance;

void setup() {
  size(640, 360); 
  noStroke();
  max_distance = dist(0, 0, width, height);
}

void draw() {
  background(0);

  for(int i = 0; i <= width; i += 20) {
    for(int j = 0; j <= height; j += 20) {
      float size = dist(mouseX, mouseY, i, j);
      size = size/max_distance * 66;
      ellipse(i, j, size, size);
    }
  }
}