scale = 256;
range = 1;
rough = .5;

// CREATE THE CORNER POINT

ds_grid_set(map, 0, 0, range - random(range*2));

// THE FRACTAL ITERATION

while(scale > 1) {

  rep = cells/(scale*scale);
  mid = scale >> 1;
  
  // DIAMOND STEP
  
  xx = 0;
  yy = 0;
  
  repeat(rep) {
  
    val0 = ds_grid_get(map, xx, yy);
    val1 = ds_grid_get(map, (xx+scale)mod size, yy);
    val2 = ds_grid_get(map, xx, (yy+scale)mod size);
    val3 = ds_grid_get(map, (xx+scale)mod size, (yy+scale)mod size);
    
    val  = mean(val0, val1, val2, val3) - range + random(range*2);
    ds_grid_set(map, xx+mid, yy+mid, val);
    
    xx   = (xx+scale)mod size;
    if (xx == 0) yy += scale;
    
  }
  
  // SQUARE STEP
  
  xx = 0;
  yy = 0;
  
  repeat(rep) {
  
    val0 = ds_grid_get(map, xx, yy);
    val1 = ds_grid_get(map, (xx+scale)mod size, yy);
    val2 = ds_grid_get(map, xx+mid, yy+mid);
    val3 = ds_grid_get(map, xx+mid, ((yy-mid)+size)mod size);
    
    val  = mean(val0, val1, val2, val3) - range + random(range*2);
    ds_grid_set(map, xx+mid, yy, val);
    
    val1 = ds_grid_get(map, xx, (yy+scale)mod size);
    val3 = ds_grid_get(map, ((xx-mid)+size)mod size, yy+mid);
    
    val  = mean(val0, val1, val2, val3) - range + random(range*2);
    ds_grid_set(map, xx, yy+mid, val);
    
    xx   = (xx+scale)mod size;
    if (xx == 0) yy += scale;
    
  }

  // REDUCE THE RANGE AND SCALE

  range *= power(2, -rough);
  scale  = mid;

}