设置图像透明度

public static Bitmap ChangeOpacity(Image img, float opacityvalue)
        {
            Bitmap bmp = new Bitmap(img.Width,img.Height); // Determining Width and Height of Source Image
            Graphics graphics = Graphics.FromImage(bmp);
            ColorMatrix colormatrix = new ColorMatrix();
            colormatrix.Matrix33 = opacityvalue;
            ImageAttributes imgAttribute = new ImageAttributes();
            imgAttribute.SetColorMatrix(colormatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
            graphics.DrawImage(img, new Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, imgAttribute);
            graphics.Dispose();   // Releasing all resource used by graphics 
            return bmp;
        }