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我对图像尺寸有疑问。当我上传 500*500 尺寸或其他尺寸的图像时，在这种情况下，我想要 1024*768 尺寸的图像输出而不压缩。意味着，在图像背景上自动添加其他空间（作为黑色背景或其他）。我该怎么做，任何建议。

第一次我尝试在固定大小的图像控件上显示图像。

.aspx

<asp:Image ID="ImagePreview1" BorderColor="Aqua" 
      BorderWidth="1px" runat="server"  Height="768px" Width="1024px" />

然后单击按钮保存特定文件夹的图像..

.aspx.cs

protected void btnsave_Click(object sender, EventArgs e)
{
    string filepath = ImagePreview1.ImageUrl;
    using (WebClient client = new WebClient())
    {
        client.DownloadFile(filepath, Server.MapPath("~/imgprev/pic.jpg"));
    }

}

在这两者之间，我也使用此代码。此代码用于压缩图像大小..

System.Drawing.Image fullSizeImg = System.Drawing.Image.FromFile(imagePath);
            var thumbnailImg = new Bitmap(1024, 768);
            var thumbGraph = Graphics.FromImage(thumbnailImg);
            thumbGraph.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            thumbGraph.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            thumbGraph.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            var imageRectangle = new Rectangle(0, 0, 1024, 768);
            thumbGraph.DrawImage(fullSizeImg, imageRectangle);
            string targetPath = imagePath.Replace(Path.GetFileNameWithoutExtension(imagePath), Path.GetFileNameWithoutExtension(imagePath) + "-resize");
            string SName = Path.GetFileName(targetPath);
            string SImgName = ("~/imgprev/") + SName;
            //string st = targetPath.ToString();
            //string stArr = st.Split('\\')[8];        

            thumbnailImg.Save(targetPath, System.Drawing.Imaging.ImageFormat.Jpeg); //(A generic error occurred in GDI+) Error occur here !
            Label1.Text = SImgName.ToString();

            thumbnailImg.Dispose();
            return targetPath;
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