PImage miImagen;
color colorPixel;

int anchoPixel;
int altoPixel;

float anchoRect;
float altoRect;

void setup(){
  size(500, 500, P3D);
  miImagen = loadImage("VanGogh-self-portrait-dedicated_to_gaugin.jpg");
  rectMode(CENTER);
}

void draw(){
  background(255);
  
  anchoPixel = int(map(mouseX, 0, width, 3, width/10));
  altoPixel = int(map(mouseY, 0, height, 3, height/10));
  
  miImagen.loadPixels();
  for(int i=0;i<miImagen.width;i+=anchoPixel){
    for(int j=0;j<miImagen.height;j+=altoPixel){
      colorPixel = miImagen.get(i,j);
      fill(0);
      noStroke();
      
      anchoRect = map(brightness(colorPixel), 0, 255, 0, anchoPixel-1);
      altoRect = map(brightness(colorPixel), 0, 255, 0, altoPixel-1);
      rect(i, j, anchoRect, altoRect);
    }
  }
}