[Boost-users] Boost.accumulators : weighted_median

Subject: [Boost-users] Boost.accumulators : weighted_median
From: Maximilian Matthe (Maxi.Matthe_at_[hidden])
Date: 2010-09-01 05:19:25

Hi!

I want to calculate the median color (along some other data) of the 
pixels of an image from its histogram:

std::vector<int> m_Hist;
// m_Hist[i] contains number of pixels with color i.

using namespace boost::accumulators;
accumulator_set<double,
        features<tag::weighted_mean,
                tag::weighted_median(with_p_square_quantile),
                tag::weighted_variance(lazy),
                tag::weighted_skewness
	>,
        double> acc;

for(int i = 0; i < 256; i++)
        acc(i, weight = m_Hist[i]);

float mm = weighted_mean(acc);
float stdD = sqrtf(weighted_variance(acc));
float med = weighted_median(acc);
float sk = weighted_skewness(acc);

mean und stdD work correctly, but the median is too large. It gives me 
about 200, but the real value should be around 75 (I checked that 
manually). The skewness is around 11000 (which also seems quite large to 
me).

So, what am I doing wrong with accumulators?