图片上传并转成灰白图片

这是以前做webim时写的上传用户头像时要生成离线状态的图片，即黑白图片的代码

函数代码如下：

public void MakeSmallImg(string fileName, string saveImg, Rectangle OutputArea,bool toBlackWhite)
        {
            System.Drawing.Image ImageDemo = System.Drawing.Image.FromFile(fileName, true);

            System.Drawing.Bitmap OutputImage = new System.Drawing.Bitmap(OutputArea.Width, OutputArea.Height);

            System.Drawing.Graphics MapGraphy = System.Drawing.Graphics.FromImage(OutputImage);

            MapGraphy.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

            MapGraphy.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            MapGraphy.Clear(System.Drawing.Color.White);
            if (toBlackWhite)
            {
                ColorMatrix _matrix = new ColorMatrix();
                _matrix[0, 0] = 1 / 3f;
                _matrix[0, 1] = 1 / 3f;
                _matrix[0, 2] = 1 / 3f;
                _matrix[1, 0] = 1 / 3f;
                _matrix[1, 1] = 1 / 3f;
                _matrix[1, 2] = 1 / 3f;
                _matrix[2, 0] = 1 / 3f;
                _matrix[2, 1] = 1 / 3f;
                _matrix[2, 2] = 1 / 3f;
                ImageAttributes _attributes = new ImageAttributes();
                _attributes.SetColorMatrix(_matrix);
                MapGraphy.DrawImage(ImageDemo, OutputArea, 0, 0, ImageDemo.Width, ImageDemo.Height, GraphicsUnit.Pixel, _attributes);
            }
            else
            {
                MapGraphy.DrawImage(ImageDemo, OutputArea);
            }
            OutputImage.Save(saveImg, System.Drawing.Imaging.ImageFormat.Bmp);

            MapGraphy.Dispose();
            OutputImage.Dispose();
            ImageDemo.Dispose();
        }