大家帮忙看下这段代码，怎么修改让缩略图质量高些

生成后有的缩略图很模糊
/// <param name="pPath">图片路径</param>
/// <param name="pSavePath">保存路径</param>
/// <param name="pPartStartPointX">目标图片开始绘制处的坐标X值(通常为)</param>
/// <param name="pPartStartPointY">目标图片开始绘制处的坐标Y值(通常为)</param>
/// <param name="pPartWidth">目标图片的宽度</param>
/// <param name="pPartHeight">目标图片的高度</param>
/// <param name="pOrigStartPointX">原始图片开始截取处的坐标X值</param>
/// <param name="pOrigStartPointY">原始图片开始截取处的坐标Y值</param>
/// <param name="imageWidth">缩放后的宽度</param>
/// <param name="imageHeight">缩放后的高度</param>
/// <param name="houzhui">图片格式</param>
public void GetPart(string pPath, string pSavedPath, int pPartStartPointX, int pPartStartPointY, int pPartWidth, int pPartHeight, int pOrigStartPointX, int pOrigStartPointY, int imageWidth, int imageHeight,string houzhui)
{
            string normalJpgPath = pSavedPath;
            using (Image originalImg = Image.FromFile(pPath))
            {
                if (originalImg.Width == imageWidth && originalImg.Height == imageHeight)
                {
                    GetPart(pPath, pSavedPath, pPartStartPointX, pPartStartPointY, pPartWidth, pPartHeight, pOrigStartPointX, pOrigStartPointY);
                    return;
                }                Image.GetThumbnailImageAbort callback = new Image.GetThumbnailImageAbort(ThumbnailCallback);
                Image zoomImg = originalImg.GetThumbnailImage(imageWidth, imageHeight, callback, IntPtr.Zero);//缩放
                Bitmap partImg = new Bitmap(pPartWidth, pPartHeight);                Graphics graphics = Graphics.FromImage(partImg);
                graphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                Rectangle destRect = new Rectangle(new Point(pPartStartPointX, pPartStartPointY), new Size(pPartWidth, pPartHeight));//目标位置
                Rectangle origRect = new Rectangle(new Point(pOrigStartPointX, pOrigStartPointY), new Size(pPartWidth, pPartHeight));//原图位置（默认从原图中截取的图片大小等于目标图片的大小）                graphics.DrawImage(zoomImg, destRect, origRect, GraphicsUnit.Pixel);                originalImg.Dispose();
                if (File.Exists(normalJpgPath))
                {
                    File.SetAttributes(normalJpgPath, FileAttributes.Normal);
                    File.Delete(normalJpgPath);
                }
                partImg.Save(normalJpgPath);
                //switch (houzhui)
                //{
                //    case "gif": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Gif); break;
                //    case "jpg": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Jpeg); break;
                //    case "bmp": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Bmp); break;
                //    case "png": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Png); break;
                //    case "GIF": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Gif); break;
                //    case "JPG": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Jpeg); break;
                //    case "BMP": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Bmp); break;
                //    case "PDG": partImg.Save(normalJpgPath, System.Drawing.Imaging.ImageFormat.Png); break;
                //}
            }
}