c#裁剪时,图像会出现黑色边框

对不起我的英语。我试图在拖动后裁剪图像。它可以工作,但我的问题是,黑色粗体边框出现在裁剪后的图像上。我不知道怎么解决它。

这是我的代码:

using (Bitmap sourceBitmap = new Bitmap(fullSizeImage))
{
    Rectangle cropRect = new Rectangle(0, 0, sourceWidth, sourceHeight);
    using (Bitmap newBitMap = new Bitmap(cropRect.Width, cropRect.Height))
    {
       using (Graphics g = Graphics.FromImage(newBitMap))
       {
           g.SmoothingMode = SmoothingMode.HighQuality;
           g.PixelOffsetMode = PixelOffsetMode.HighQuality;
           g.CompositingQuality = CompositingQuality.HighQuality;
           g.CompositingMode = CompositingMode.SourceCopy;
           g.DrawImage(sourceBitmap, new Rectangle(0, pp, sourceWidth, sourceHeight), cropRect, GraphicsUnit.Pixel);
           g.InterpolationMode = InterpolationMode.HighQualityBicubic;
           ImageCodecInfo codec = ImageCodecInfo.GetImageEncoders()[4];
           EncoderParameters parameters = new EncoderParameters(1);
           parameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);
           newBitMap.Save(filePath);
        }
    }
}

下面是裁剪后的图像: