PImage img;
void setup() {
  size(500, 500);
  colorMode(HSB, 360, 100, 100);
  img = loadImage("lena_color.jpg");
}

void draw() {
  int grid = 10;
  background(0, 0, 100);
  for (int j = 0; j < height; j+= grid) {
    for (int i = 0; i < width; i+= grid) {
      color c = img.get(i, j);
      int gradation = int(brightness(c)/10) + 1;
      stroke(c);
      rectPointGradation(i, j, grid, gradation);
    }
  }
  noLoop();
}

void rectPointGradation(int x, int y, int grid, int gradation) {
  pushMatrix();
  translate(x, y);
  for (int j = 0; j < grid; j++) {
    for (int i = 0; i < grid; i++) {
      if ((j*grid + i)%gradation == 0) {
        point(i, j);
      }
    }
  }
  popMatrix();
}