function sigma( x ) {
  return x / (Math.abs(x) + 0.4);
}

function avgYUV( data ) {
  var rgb = { r : 0, g : 0, b : 0 };
  var yuv = { y : 0, u : 0, v : 0 };
  var count = 0;
    for ( i = 0; i<data.data.length; i=i+4 ) {
      // Ignore white color. Suppose we hate it.
      if ( data.data[i]     == 255 &&
           data.data[i + 1] == 255 &&
         data.data[i + 2] == 255 ) {
        continue;
      }
      rgb.r = data.data[i] / 255;
      rgb.g = data.data[i + 1] / 255;
      rgb.b = data.data[i + 2] / 255;

    yuv.y +=  0.299 * rgb.r + 0.587 * rgb.g + 0.114 * rgb.b;
    yuv.u += -0.147 * rgb.r - 0.289 * rgb.g + 0.436 * rgb.b;
    yuv.v +=  0.615 * rgb.r - 0.515 * rgb.g - 0.100 * rgb.b;

    count += 1;
  }

  yuv.y = yuv.y/count;
  yuv.u = yuv.u/count;
  yuv.v = yuv.v/count;

  yuv.y = sigma(yuv.y);
  yuv.u = sigma(yuv.u);
  yuv.v = sigma(yuv.v);

  rgb.r = yuv.y + 1.3983 * yuv.v;
  rgb.g = yuv.y - 0.3946 * yuv.u - 0.58060 * yuv.v;
  rgb.b = yuv.y + 2.0321 * yuv.u;

  rgb.r = ~~(rgb.r * 255);
  rgb.g = ~~(rgb.g * 255);
  rgb.b = ~~(rgb.b * 255);

  return rgb;
}