为什么即使将Bitmap设置为Graphics.Clear（Color.Transparent），也会在调整大小时在图像周围显示黑色背景

我正在构建一个图像上传工具，可以调整图像大小以适应固定大小，但它会在图像周围的填充空间中添加黑色背景而不是透明背景。

I'm building an Image upload tool that resizes an image to fit a fixed size but it's adding a black background instead of a transparent one to the filler space around the image.

我读过Bitmap需要设置为带有Alpha层的PixelFormat，我可以将Graphics clear颜色设置为透明但我仍然遇到同样的问题。

I've read that the Bitmap needs to be set to a PixelFormat with an Alpha layer and that I can set the Graphics clear colour to transparent but I'm still getting the same problem.

我的图片主要是jpeg。下面是代码：

My images are mostly jpegs. Here is the code:

private void ResizeImage(Image Original, Int32 newWidth, Int32 newHeight, String pathToSave)
    {
        int sourceX = 0;
        int sourceY = 0;
        int destX = 0;
        int destY = 0;

        int originalWidth = Original.Width;
        int originalHeight = Original.Height;
        float nPercent = 0;
        float nPercentW = 0;
        float nPercentH = 0;

        nPercentW = ((float)newWidth / (float)originalWidth);
        nPercentH = ((float)newHeight / (float)originalHeight);

        if (nPercentH < nPercentW)
        {
            nPercent = nPercentH;
            destX = System.Convert.ToInt16((newWidth -
                          (originalWidth * nPercent)) / 2);
        }
        else
        {
            nPercent = nPercentW;
            destY = System.Convert.ToInt16((newHeight -
                          (originalHeight * nPercent)) / 2);
        }

        int destWidth = (int)(originalWidth * nPercent);
        int destHeight = (int)(originalHeight * nPercent);


        Bitmap bmp = new Bitmap(newWidth, newHeight, PixelFormat.Format32bppArgb);

            bmp.SetResolution(Original.HorizontalResolution, Original.VerticalResolution);
            using (Graphics Graphic = Graphics.FromImage(bmp))
            {
                Graphic.CompositingQuality = CompositingQuality.HighQuality;
                Graphic.Clear(Color.Red);
                Graphic.CompositingMode = CompositingMode.SourceCopy;
                Graphic.SmoothingMode = SmoothingMode.AntiAlias;
                Graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
                Graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;

                Graphic.DrawImage(
                    Original,
                    new Rectangle(destX, destY, destWidth, destHeight),
                    new Rectangle(sourceX, sourceY, originalWidth, originalHeight),
                    GraphicsUnit.Pixel
                    );

                bmp.Save(pathToSave,Original.RawFormat);

            }

}