/*
 * This version of grayscaling is based on a nonuniform
 * weighting of red,green,blue components known as luminosity.
 * luminosity = 0.21 R + 0.72 G + 0.07 B.
 *
 * This is the formula used by PImage.filter(GRAY) function.
 */

// Can either load an image stored locally, as follows:
// PImage slu = loadImage("campus4.jpg");

// Or can load an image from a URL
PImage slu = loadImage("http://business.slu.edu/uploads/2012/11/08/slu-homepage-campus4.jpg");

int w = slu.width;
int h = slu.height;

for (int x=0; x < w; x++) {
  for (int y=0; y < h; y++) {
    color c = slu.get(x,y);
    float gray = (0.21 * red(c) + 0.72 * green(c) + 0.07 * blue(c));
    color replace = color(gray);
    slu.set(x,y,replace);
  }
}

// time to display it
size(w,h);
image(slu,0,0);