Moving points that slow down on darker areas of an image.

movers over image

PImage src;
Mover[] movers = new Mover[10000];

void setup(){
 src = loadImage("YOUR_IMAGE.JPG");
 src.resize(src.width/4, src.height/4);
  size(src.width, src.height);
  
  for(int i = 0; i < movers.length; i++){
   movers[i] = new Mover(src, new PVector(random(width), random(height))); 
  }
  
  background(255);
}

void draw(){
 for(int i = 0; i < movers.length; i++){
  movers[i].run();
 } 
}

class Mover {

  PImage src;

  PVector acceleration;
  PVector velocity;
  PVector location;
  
  color c;

  Mover(PImage src_, PVector loc_) {
    src = src_;
    
    acceleration = new PVector();
    velocity = new PVector(int(1)*(random(-1,1)), int(1)*(random(-1, 1)));
    location = loc_;
  } 

  void update() {
    float b;
    c = color(0, 5);
    b = map(brightness(readColor()), 0, 255, 0.1, 2);
    
    velocity.normalize();
    velocity.mult(b);
    
    velocity.add(acceleration);
    location.add(velocity);
    acceleration.mult(0);
  }

  void display() {
    noStroke();
    fill(c);
    pushMatrix();
    translate(location.x, location.y);
    ellipse(0, 0, 1, 1);
    popMatrix();
  }

  void run() {
    update();
    checkEdges();
    display();
  }

  color readColor() {
    src.loadPixels();
    int pixelLoc = int(int(location.y)*src.width + int(location.x));
    if (pixelLoc < src.pixels.length && pixelLoc >= 0) {
      int r = (int) red(src.pixels[pixelLoc]);
      int g = (int) green(src.pixels[pixelLoc]);
      int b = (int) blue(src.pixels[pixelLoc]);      
      
      return color(r, g, b);
    }else{
     return int(0); 
    }
  }
  
  void checkEdges(){
   if(location.x > src.width || location.x < 0){
    velocity.x *= -1;
   }
  if(location.y > src.height || location.y < 0){
   velocity.y *= -1;
  } 
  }
}