处理图片透明操作

// <summary>
/// 处理图片透明操作
/// </summary>
/// <param name="srcImage">原始图片</param>
/// <param name="opacity">透明度(0.0---1.0)</param>
/// <returns></returns>
private Image TransparentImage(Image srcImage, float opacity)
{
    float[][] nArray ={ 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, opacity, 0},
                        new float[] {0, 0, 0, 0, 1}};
    ColorMatrix matrix = new ColorMatrix(nArray);
    ImageAttributes attributes = new ImageAttributes();
    attributes.SetColorMatrix(matrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
    Bitmap resultImage = new Bitmap(srcImage.Width, srcImage.Height);
    Graphics g = Graphics.FromImage(resultImage);
    g.DrawImage(srcImage, new Rectangle(0, 0, srcImage.Width, srcImage.Height), 0, 0, srcImage.Width, srcImage.Height, GraphicsUnit.Pixel, attributes);  

    return resultImage;
}