Code blocks

        static void Main(string[] args)
        {
            const float rScale = 0.3f;
            const float gScale = 0.59f;
            const float bScale = 0.11f;

            const string filename = "bigImage";

            var original = Image.FromFile(string.Format("{0}.jpg", filename)) as Bitmap;
            var grayscale = new Bitmap(original.Width, original.Height, original.PixelFormat);

            grayscale.SetData(from pixel in original.GetData<Format24BppRgb>()
                              let grayValue = (byte)(pixel.R * rScale + pixel.G * gScale + pixel.B * bScale)
                              select new Format24BppRgb(grayValue));

            grayscale.Save(string.Format("{0}-grayscale.jpg", filename));
        }