/** Weighted per-channel variance of the box. It's used to decide which channel to split by */
static f_pixel box_variance(const hist_item achv[], const box* box)
{
f_pixel mean = box->color;
f_pixel variance(0.0, 0.0, 0.0, 0.0);
f_pixel goodEnough(2.0/256.0, 1.0/256.0, 1.0/256.0, 1.0/256.0);
goodEnough.square();
for (uint i=0; i<box->colors; ++i) {
f_pixel px = achv[box->ind + i].acolor;
double weight = achv[box->ind + i].adjusted_weight;
f_pixel diff = mean - px;
diff.square();
if (diff.alpha < goodEnough.alpha) diff.alpha *= 0.5;
if (diff.r < goodEnough.r) diff.r *= 0.5;
if (diff.g < goodEnough.g) diff.g *= 0.5;
if (diff.b < goodEnough.b) diff.b *= 0.5;
variance += diff * weight;
}
variance *= f_pixel(4.0/16.0, 7.0/16.0, 9.0/16.0, 5.0/16.0);
return variance;
}
static double sqrtIter(double x, double guess) {
if(goodEnough(x, guess)) return guess;
return sqrtIter(x, improveGuess(x, guess));
}
