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Computing Gini Index of an image (measure of Impurity)

Using my previous posts Class file and this reference:
http://people.revoledu.com/kardi/tutorial/DecisionTree/how-to-measure-impurity.htm

float computeGiniIndex(Mat r, Mat g, Mat b)
    {
        float giniIndex = 0.0;
        float frequency = getFrequencyOfBin(r);
        for( int i = 1; i < histSize; i++ )
        {
            float Hc = abs(getHistogramBinValue(r,i));
            giniIndex += Hc*Hc;
        }
        frequency = getFrequencyOfBin(g);
        for( int i = 1; i < histSize; i++ )
        {
            float Hc = abs(getHistogramBinValue(g,i));
            giniIndex += Hc*Hc;
        }
        frequency = getFrequencyOfBin(b);
        for( int i = 1; i < histSize; i++ )
        {
            float Hc = abs(getHistogramBinValue(b,i));
            giniIndex += Hc*Hc;
        }
        giniIndex = 1 - (giniIndex);
        //cout << giniIndex <<endl;
        return giniIndex;
    }
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