使用Itextsharp更改pdf图像背景

Question

我正在尝试使用Itextshap更改pdf的所有图像的背景颜色。 如何循环浏览所有图像并更改图像的背景颜色

我使用下面的代码来提取pdf图像

 public static void ExtractImagesFromPDF(string sourcePdf, string outputPath)
{
    // NOTE:  This will only get the first image it finds per page.
    PdfReader pdf = new PdfReader(sourcePdf);
    RandomAccessFileOrArray raf = new iTextSharp.text.pdf.RandomAccessFileOrArray(sourcePdf);

    try
    {
        for (int pageNumber = 1; pageNumber <= pdf.NumberOfPages; pageNumber++)
        {
            PdfDictionary pg = pdf.GetPageN(pageNumber);

            // recursively search pages, forms and groups for images.
            PdfObject obj = FindImageInPDFDictionary(pg);
            if (obj != null)
            {

                int XrefIndex = Convert.ToInt32(((PRIndirectReference)obj).Number.ToString(System.Globalization.CultureInfo.InvariantCulture));
                PdfObject pdfObj = pdf.GetPdfObject(XrefIndex);
                PdfStream pdfStrem = (PdfStream)pdfObj;
                byte[] bytes = PdfReader.GetStreamBytesRaw((PRStream)pdfStrem);
                if ((bytes != null))
                {
                    using (System.IO.MemoryStream memStream = new System.IO.MemoryStream(bytes))
                    {
                        memStream.Position = 0;
                        System.Drawing.Image img = System.Drawing.Image.FromStream(memStream);
                        // must save the file while stream is open.
                        if (!Directory.Exists(outputPath))
                            Directory.CreateDirectory(outputPath);

                        string path = Path.Combine(outputPath, String.Format(@"{0}.jpg", pageNumber));
                        System.Drawing.Imaging.EncoderParameters parms = new System.Drawing.Imaging.EncoderParameters(1);
                        parms.Param[0] = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Compression, 0);
                        System.Drawing.Imaging.ImageCodecInfo jpegEncoder = Utilities.GetImageEncoder("JPEG");
                        img.Save(path, jpegEncoder, parms);
                    }
                }
            }
        }
    }
    catch
    {
        throw;
    }
    finally
    {
        pdf.Close();
        raf.Close();
    }


}

 private static PdfObject FindImageInPDFDictionary(PdfDictionary pg)
{
    PdfDictionary res =
        (PdfDictionary)PdfReader.GetPdfObject(pg.Get(PdfName.RESOURCES));


    PdfDictionary xobj =
      (PdfDictionary)PdfReader.GetPdfObject(res.Get(PdfName.XOBJECT));
    if (xobj != null)
    {
        foreach (PdfName name in xobj.Keys)
        {

            PdfObject obj = xobj.Get(name);
            if (obj.IsIndirect())
            {
                PdfDictionary tg = (PdfDictionary)PdfReader.GetPdfObject(obj);

                PdfName type =
                  (PdfName)PdfReader.GetPdfObject(tg.Get(PdfName.SUBTYPE));

                //image at the root of the pdf
                if (PdfName.IMAGE.Equals(type))
                {
                    return obj;
                }// image inside a form
                else if (PdfName.FORM.Equals(type))
                {
                    return FindImageInPDFDictionary(tg);
                } //image inside a group
                else if (PdfName.GROUP.Equals(type))
                {
                    return FindImageInPDFDictionary(tg);
                }

            }
        }
    }

    return null;

}

非常感谢提前