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//// <summary>
/// 改变图像透明度（真透明）
/// </summary>
/// <param name="img">所要转变的图像</param>
/// <param name="alpha">透明度，最大为1，最小为0</param>
/// <returns>改变后的图像</returns>
public static Bitmap VitrificationImage(Image img, float alpha)
{
    Bitmap _newImg = new Bitmap(img.Width, img.Height);

    using (Graphics _g = Graphics.FromImage(_newImg))
    {
        using (ImageAttributes _imageAttrs = new ImageAttributes())
        {
            _imageAttrs.SetColorMatrix(new ColorMatrix(CreateAlphaMatrix(alpha)));

            _g.DrawImage(img, new Rectangle(0, 0, img.Width, img.Height), 1, 1, img.Width, img.Height, GraphicsUnit.Pixel, _imageAttrs);
        }
    }

    return _newImg;
}

//// <summary>
/// 创建用于改变图像透明度的颜色矩阵
/// </summary>
/// <param name="alpha">所要设置的透明度</param>
/// <returns>返回用于图像转换的颜色矩阵</returns>
private static float[][] CreateAlphaMatrix(float alpha)
{
    if (alpha > 1)
        alpha = 1;

    if (alpha < 0)
        alpha = 0;

    float[][] _matrix =
    {
        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, 0},
        new float[] {0, 0, 0, 0, 1}
    };

    return _matrix;
}