extension method definition for converting image to grayscale

bitmap-grayscale-converter.cs

using System.Drawing;
using System.Drawing.Imaging;

namespace BitmapConversion
{
    static class GrayScale
    {
        public static Bitmap Transform(Bitmap bmp, float[][] matrix)
        {
            ColorMatrix mat = new ColorMatrix(matrix);

            ImageAttributes attr = new ImageAttributes();
            attr.SetColorMatrix(mat);

            int w = bmp.Width;
            int h = bmp.Height;

            Bitmap newbmp = new Bitmap(w, h);
            using (Graphics graphics = Graphics.FromImage(newbmp))
            {
                Rectangle rect = new Rectangle(0, 0, w, h);
                graphics.DrawImage(bmp, rect, 0, 0, w, h, GraphicsUnit.Pixel, attr);
            }

            return newbmp;
        }

        // (r + g + b) / 3
        public static Bitmap SimpleGrayscale(this Bitmap bmp)
        {
            const float r = 0.333333F;
            const float g = 0.333333F;
            const float b = 0.333333F;

            float[][] matrix = {
                new float[]{r, r, r, 0, 0},
                new float[]{g, g, g, 0, 0},
                new float[]{b, b, b, 0, 0},
                new float[]{0, 0, 0, 1, 0},
                new float[]{0, 0, 0, 0, 1}
            };
            return Transform(bmp, matrix);
        }

        // 0.298912r + 0.586611g + 0.114478b
        public static Bitmap NTSCWeightedAverage(this Bitmap bmp)
        {
            const float r = 0.298912F;
            const float g = 0.586611F;
            const float b = 0.114478F;

            float[][] matrix = {
                new float[]{r, r, r, 0, 0},
                new float[]{g, g, g, 0, 0},
                new float[]{b, b, b, 0, 0},
                new float[]{0, 0, 0, 1, 0},
                new float[]{0, 0, 0, 0, 1}
            };
            return Transform(bmp, matrix);
        }
    }
}