C# 我保存的图像显示为黑色-更改图像的不透明度

我试图在更改图像不透明度后保存图像这是我的代码：

protected void bntChangeOpacity_Click(object sender, EventArgs e) {
        String saveDir = mydir;
        Image watermarkImage = Image.FromFile(Server.MapPath(mydir + "imgname.jpg"));


        Graphics gr = Graphics.FromImage(watermarkImage);

        Rectangle r2 = new Rectangle(new Point(0, 0), new Size(watermarkImage.Width, watermarkImage.Height));

        float opacityvalue = 0.5f;

        ImageUtils.ImageTransparency.ChangeOpacity(watermarkImage, opacityvalue);

        Bitmap b1 = new Bitmap(watermarkImage.Width, watermarkImage.Height);
        gr.DrawImage(watermarkImage, r2);

        b1.Save(Server.MapPath(saveDir + "sasf.jpg"));


} 

课程代码为：

public class ImageTransparency {
    public static Bitmap ChangeOpacity(Image img, float opacityvalue)
    {

       Bitmap bmp = new Bitmap(img.Width,img.Height);
        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();
        return bmp;

    }
}

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有什么问题吗？？请帮助

您可以这样做：

private void button1_Click(object sender, EventArgs e)
{
    float opacityvalue = 0.5f;
    var img= ImageTransparency.ChangeOpacity(Image.FromFile(@"PathToYourImage.png"), opacityvalue);
    img.Save(@"PathToYourImage-Opacity.png");
}



class ImageTransparency
{
    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;
    }
}

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基于

i，我使用浮点数opacityvalue=0.5f；所以colormatrix.Matrix33将采用该值。。错了吗@CommuSofti收到以下错误：System.Runtime.InteropServices.ExternalException:GDI+中发生一般错误。打开：img.Save（@“mydir\img.jpg”）；完成了，非常感谢我更改了img.Save（@“mydir\img.jpg”）；保存（Server.MapPath（“mydir/imgnew.jpg”）@H.A.M很高兴听到你成功了。不客气：）