//Initial variables setup
public int
  canvasSize = 720,  // Set the square canvas size.
  rectLength = 432,  // length of rects. This will be the length and width of the square.
  rectAmount = 10;   // Set rect density of square.

void settings() {
  size(canvasSize, canvasSize); // Set canvas dimensions in setup so that canvasSize can be used as a variable.
}

void setup(){
  noStroke(); // Do  not use stokes by default.
  noSmooth(); // Romove antialiased effect, not needed on square geometry.
  fill(0);    // Set rect fill to black.
}

void draw()
{

//**** EXTRAS **** --------------------------------------------------------------------------
                   // Rect amount is relative to mouse Y.
                   // rectAmount = (mouseY > 4)? mouseY/2 : rectAmount;
                   // Rect length is relative to mouse X.
                   // rectLength = mouseX;
                   // Canvas size is relative to Rect length.
                   // surface.setSize(rectLength+200, rectLength+200); canvasSize = width;

    // Add background color each frame to clear trash.
    background(255);
    // Calculate height of rect by Y position of mouse and amount of rects.
    int rectHeight = (mouseY > 4)? mouseY / (canvasSize / (rectLength/rectAmount + 1)) +1 : 1;
    // Run a loop to create and position the rect elements.
    for (int i = 0; i < rectAmount; i++)
      {
        // Create a rect
        rect(
            // Calculate and set X position.
            (canvasSize - rectLength) / 2,
            // Calculage and set Y position.
            rectLength * i / (rectAmount-1) - rectHeight * i / (rectAmount-1)+(canvasSize - rectLength) / 2,
            // Set Rect length
            rectLength,
            // Set Rect Height
            rectHeight
        );
    };
};