ConstrainExample

/* Constrain Example (c)2015 Giles Cartmel */

int borderX, borderY;

void setup() {
  size(800,600);
  borderX = width/5;
  borderY = height/5;
}

void draw() {
  background(0);

  int x1, y1, x2, y2;
  x1 = borderX;
  y1 = borderY;
  x2 = width-borderX;
  y2 = height-borderY;

  // draw a rectangle based on 10% border of the canvas
  rectMode(CORNER);
  fill(20);
  stroke(80);
  rect(x1,y1,x2-x1,y2-y1);

  // draw an ellipse at the mouse position
  // constraining it's centre to within the rectangle
  ellipseMode(RADIUS);
  fill(128,255,128,128);
  stroke(255);
  ellipse(constrain(mouseX, x1, x2)
         ,constrain(mouseY, y1, y2)
         ,borderX/5
         ,borderY/5
         );
}