PImage bkg;

color px;

float centerX;
float centerY;
float run;
float rise;
float distSq;
float invDist;
float hypotSq;
float red;
float green;
float blue;
float smooth = 0.013;
float invSmooth = 1 - smooth;
float scalar = 0.125;

void setup() {
  size(512, 256);
  bkg = loadImage("corfu.png");
  bkg.resize(width, height);
  hypotSq = scalar * (width * width + height * height);
}

void draw() {

  // Move flashlight to mouse.
  centerX = invSmooth * centerX + smooth * mouseX;
  centerY = invSmooth * centerY + smooth * mouseY;

  background(bkg);
  loadPixels();
  for (int y = 0, i = 0, x; y < height; ++y) {
    for (x = 0; x < width; ++x, ++i) {

      // Calculate rise and run.
      rise = centerY - y;
      run = centerX - x;

      // Find distance squared and normalize.
      distSq = (run * run + rise * rise) / hypotSq;
      invDist = 1 - distSq;

      // Decompose color.
      px = pixels[i];
      red = (px >> 16 & 0xff) * invDist;
      green = (px >> 8 & 0xff) * invDist;
      blue = (px & 0xff) * invDist;

      // Boundary check new values.
      red = red < 0 ? 0 : red > 255 ? 255 : red;
      green = green < 0 ? 0 : green > 255 ? 255 : green;
      blue = blue < 0 ? 0 : blue > 255 ? 255 : blue;

      // Composite into hex color.
      pixels[i] = 0xff000000
        | round(red) << 16
        | round(green) << 8
        | round(blue);
    }
  }
  updatePixels();
}