I hope you will forgive my ignorance as this is probably simple, but as a beginner, I can't see why it doesn't work.  I am trying to access pixel data and simply change any that are 'near black' to black.  The resulting image has no changes made to it; pixels of rgb values like (2, 4, 3) still exist.  It may seem trivial but it is a step in learning to get me to what I really want to do. 

 

PImage img;
color blk = color(0, 0, 0);

void setup() {
  size(491, 464);
  img = loadImage("ctdarkred17.jpg");
}

void draw() {
  loadPixels();
  noLoop();
  for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
      int loc = x + y*width;
     
      float r = red(img.pixels[loc]);
      float g = green(img.pixels[loc]);
      float b = blue(img.pixels[loc]);
       
      //if colour is blackish, set to black
      if((r < 20) && (g < 20) && (b < 20))  {
   
          pixels[loc] = color(blk);
       
      }  else  {
           color same = color(r, g, b);
           pixels[loc] = color(same);
      }
    }
     
    }
      updatePixels();
      print("finished");
      save("ctdarkred17.jpg");
  }