FastImageAverage.cs

// http://stackoverflow.com/questions/2545796/how-to-get-a-majority-color-in-an-image/2546506#2546506

namespace FastImageAverage
{
    using System;
    using System.Diagnostics;
    using System.Drawing;
    using System.Drawing.Imaging;

    class Program
    {
        static void Main(string[] args)
        {
            var watch1 = Stopwatch.StartNew();
            Console.WriteLine(UnsafeGetColorAverage(args[0]));
            watch1.Stop();
            Console.WriteLine(watch1.Elapsed);

            var watch2 = Stopwatch.StartNew();
            Console.WriteLine(GetColorAverage(args[0]));
            watch2.Stop();
            Console.WriteLine(watch2.Elapsed);
        }

        unsafe static Color UnsafeGetColorAverage(string filename)
        {
            using (var image = (Bitmap)Bitmap.FromFile(filename))
            {
                if (image.PixelFormat != PixelFormat.Format24bppRgb) throw new NotSupportedException(String.Format("Unsupported pixel format: {0}", image.PixelFormat));

                var pixelSize = 3;
                var bounds = new Rectangle(0, 0, image.Width, image.Height);
                var data = image.LockBits(bounds, ImageLockMode.ReadOnly, image.PixelFormat);

                long r = 0;
                long g = 0;
                long b = 0;

                for (int y = 0; y < data.Height; ++y)
                {
                    byte* row = (byte*)data.Scan0 + (y * data.Stride);
                    for (int x = 0; x < data.Width; ++x)
                    {
                        var pos = x * pixelSize;
                        b += row[pos];
                        g += row[pos + 1];
                        r += row[pos + 2];
                    }
                }

                r = r / (data.Width * data.Height);
                g = g / (data.Width * data.Height);
                b = b / (data.Width * data.Height);
                image.UnlockBits(data);

                return Color.FromArgb((int)r, (int)g, (int)b);
            }
        }

        static Color GetColorAverage(string filename)
        {
            using (var bmp = new Bitmap(filename))
            {
                int width = bmp.Width;
                int height = bmp.Height;
                long red = 0;
                long green = 0;
                long blue = 0;
                long alpha = 0;
                for (int x = 0; x < width; x++)
                    for (int y = 0; y < height; y++)
                    {
                        var pixel = bmp.GetPixel(x, y);
                        red += pixel.R;
                        green += pixel.G;
                        blue += pixel.B;
                        alpha += pixel.A;
                    }

                Func<long, long> avg = c => (c / (width * height));

                red = avg(red);
                green = avg(green);
                blue = avg(blue);
                alpha = avg(alpha);

                var color = Color.FromArgb((int)alpha, (int)red, (int)green, (int)blue);

                return color;
            }
        }
    }
}