周辺光量落ち

/**
* Decrease in the peripheral light quantity
*
* @author aa_debdeb
* @date 2015/11/06
*/

void setup(){
  size(640, 424);
  PImage image = loadImage("image.jpg");
  image(image, 0, 0);
  noFill();
  stroke(0);
  strokeWeight(1);
  for(int i = 0; i < 400; i++){
    float alpha = map(i, 0, 400, 0, 255);
    stroke(0, alpha);
    ellipse(width/2, height/2, 500 + i, 600 + i);
  }
}