Image ImageAutolevel(const Image& img){
	uint64 histogram[256];
	for(int i=0;i<256;++i)
		histogram[i]=0;
	
	const RGBA *start = img;
	const RGBA *s = start;
	const RGBA *e = s + img.GetLength();
	while(s < e){
		histogram[(s->r+s->g+s->b)/3]++;
		s++;
	}
	int v_lower = -1;
	int v_upper = -1;
	
	for(int i=0;i<256&&(v_lower==-1||v_upper==-1);++i){
		if(v_lower==-1&&histogram[i]!=0)
			v_lower = i;
		if(v_upper==-1&&histogram[255-i]!=0)
			v_upper = 255-i;
	}
	
	// return if not sense to autolevel
	//if(v_lower==v_upper==-1)
	
	if(v_lower==-1)
		v_lower = 0;
	
	if(v_upper==-1)
		v_upper = 255;
	
	s = start;
	ImageBuffer w(img.GetSize());
	Unmultiply(w);
	RGBA *t = w;
	
	while(s < e) {
		t->r = s->r*255/v_upper;
		t->g = s->g*255/v_upper;
		t->b = s->b*255/v_upper;
		t->a = s->a;
		t++;
		s++;
	}
	Premultiply(w);
	w.SetHotSpots(img);
	return w;
}