转：设置图片透明的四种方法

参照博文：http://blog.csdn.net/lexiaoyao20/article/details/6628830

最有效的一种方法：

矩阵法：

这个方法的效率最高。

    /// <summary>
           /// 设置图片的透明度
           /// </summary>
           /// <param name="image">原图</param>
           /// <param name="alpha">透明度0-255</param>
           /// <returns></returns>
           private Bitmap SetPictureAlpha(Image image,int alpha)
           {
               //颜色矩阵
               float[][] matrixItems =
               {
                   new float[]{1,0,0,0,0},
                   new float[]{0,1,0,0,0},
                   new float[]{0,0,1,0,0},
                   new float[]{0,0,0,alpha/255f,0},
                   new float[]{0,0,0,0,1}
               };
               ColorMatrix colorMatrix = new ColorMatrix(matrixItems);
               ImageAttributes imageAtt = new ImageAttributes();
               imageAtt.SetColorMatrix(colorMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
               Bitmap bmp = new Bitmap(image.Width, image.Height);
               Graphics g = Graphics.FromImage(bmp);
               g.DrawImage(image, new Rectangle(0, 0, image.Width, image.Height),
                       0, 0, image.Width, image.Height, GraphicsUnit.Pixel, imageAtt);
               g.Dispose();  

               return bmp;
           }