Canvas-based image white removal

Canvas background: Image: Threshold: Distance:

var limit = 255 - threshold, r, g, b;

for (var i = 0, n = pix.length; i < n; i += 4) {
  
  r = pix[i];
  g = pix[i+1];
  b = pix[i+2];
      
  if (r > limit && 
      g > limit && 
      b > limit && 
      abs(r-g) < distance && 
      abs(r-b) < distance &&
      abs(g-b) < distance) {

    pix[i+3] = 1;
  }
}