public static double[] AveragePixelColour(Bitmap image, RectangularRegion region) =>
new double[]
{
MapRegions(region, (x, y) => image.GetPixel(x, y).R),
MapRegions(region, (x, y) => image.GetPixel(x, y).G),
MapRegions(region, (x, y) => image.GetPixel(x, y).B)
};
public static double MapRegions(RectangularRegion region, Func <int, int, byte> f) =>
Enumerable.Range(region.X1, region.X2 + 1 - region.X1)
.SelectMany(x =>
Enumerable.Range(region.Y1, region.Y2 + 1 - region.Y1)
.Select(y => (double)(f(x, y)))
)
.Average();