图片上传并转成灰白图片-白红宇

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();
         }