New Post: Colour manipulation; tonal ranges

Hi, the image I'm working with is of a person, so I've grabbed an image of a sunflower and run it through the same process. The difference between the two corrected images is fairly clear.

The original image is:


The GIMP corrected image is:


The IM corrected image is:


In GIMP I apply +50 to the Red channel in Shadows, Midtones and Highlights to get this effect. In the color balance screen in GIMP there is a "Preserve Luminosity" checkbox which I also uncheck.

In Magick I'm using .Evaluate as below. It's a bit clunky, but I think it's okay. I want to try and use an overload which accepts midpoint so that I can adjust each channel for each range. I can't work out which midpoints to use, though. I've seen a post about ImageMagick which suggests-
Low = (mean - factor * std. dev.
Mid = mean
High = mean + factor * std. dev.

img.Evaluate(
     ImageMagick.Channels.Red,
     settings.Red < 0 ? ImageMagick.EvaluateOperator.Subtract : ImageMagick.EvaluateOperator.Add,
     settings.Red < 0 ? settings.Red * -1 : settings.Red
);

One other thing I've noticed; thinking about how to calculate the midpoint of the range to apply the change to, I started looking at the statistics that IM generates. For the corrected image shown in this post GIMP (on the Red channel) gave me a Mean of 191.4 and a St. Dev. of 54.9. IM gives a mean of 203.1, st. dev. of 30908 and variance of 1.277 * 10^-313. These are just wrong.

I've written my own quick and dirty class to generate statistics from the PixelCollection that IM gives me and I get the same values as GIMP. Not sure this actually makes a difference, but its worth noting.

var imStats = img.Statistics();
var myStats = new Statistics(img.GetReadOnlyPixels());

    public class Statistics
    {
        public double StDev = 0.0;
        public double Variance = 0.0;
        public double Mean = 0;
        public Int32 PixelCount = 0;
        public Statistics(ImageMagick.PixelCollection pixels) 
        {
            var pixelEnumerator = pixels.GetEnumerator();
            double mean = 0.0;
            double variance = 0.0;
            int n = 0;
            int total = 0;

            while (pixelEnumerator.MoveNext())
            {
                total += Convert.ToInt32(pixelEnumerator.Current.ToColor().R);
                n++;
            }

            mean = total / n;
            pixelEnumerator.Reset();

            while (pixelEnumerator.MoveNext())
            {
                variance += Math.Pow((Convert.ToDouble(pixelEnumerator.Current.ToColor().R) - Convert.ToDouble(mean)), 2);
            }

            Variance = variance / total;
            StDev = Math.Sqrt(variance);
            PixelCount = total;
            Mean = mean;
        }
    }