ImageProcessor.Processor.GrayscaleA C# (CSharp) Method

        public unsafe Image GrayscaleA()
        {
            Bitmap b = new Bitmap(_image);

            BitmapData bData = b.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, b.PixelFormat);

            byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);

            int size = bData.Stride * bData.Height;

            byte* scan0 = (byte*)bData.Scan0.ToPointer();

            int colWidth = bData.Width * bitsPerPixel / 8;

            for (int i = 0; i < bData.Height; ++i)
            {
                for (int j = 0; j < bData.Width; ++j)
                {
                    byte* data = scan0 + i * bData.Stride + j * bitsPerPixel / 8;

                    byte magnitude = (byte)(1 / 3d * (data[0] + data[1] + data[2]));
                    data[0] = (byte)magnitude;
                    data[1] = (byte)magnitude;
                    data[2] = (byte)magnitude;

                }
            }

            b.UnlockBits(bData);

            return b;
        }