如何使用itextsharp将彩色pdf或图像转换为黑白或灰度pdf？

Question

当我尝试在C＃WinForm应用程序中将彩色pdf或图像转换为黑白或灰度pdf iTextSharp时，它不起作用，并且透明图像也有问题。转换时，它将背景变为黑色，以获得透明图像。我正在使用以下代码首先将其转换为pdf

string urlpdf = output.Replace(".png", ".pdf").Replace(".jpeg", ".pdf").Replace(".jpg", ".pdf").Replace(".PNG", ".pdf").Replace(".JPEG", ".pdf").Replace(".JPG", ".pdf");
                    using (FileStream fs = File.Open(urlpdf, FileMode.OpenOrCreate))
                    {
                        PdfWriter PdfIns = PdfWriter.GetInstance(docPNG, fs);
                        docPNG.Open();
                        iTextSharp.text.Image pdfImage = iTextSharp.text.Image.GetInstance(image, System.Drawing.Imaging.ImageFormat.Jpeg);
                        if (comboBox8.SelectedIndex == 1)
                        {
                            pdfImage.ScaleToFit(rec);                               
                            pdfImage.SetAbsolutePosition((docPNG.PageSize.Width - pdfImage.ScaledWidth) / 2, (docPNG.PageSize.Height - pdfImage.ScaledHeight) / 2);                              
                        }
                        else
                        {
                            pdfImage.ScalePercent((float)numericUpDown4.Value);
                        }                       

                        docPNG.Add(pdfImage);
                        docPNG.CloseDocument();
                        docPNG.Close();
                    }

，然后按如下所示灰度或黑白-

iTextSharp.text.pdf.converter.PdfContentToGrayscaleConverter grayscaleConverter = new iTextSharp.text.pdf.converter.PdfContentToGrayscaleConverter();

            using (PdfReader reader = new PdfReader(inputFile))
            {
                using (FileStream fsOutput = new FileStream(outputFile, FileMode.Create, FileAccess.Write, FileShare.ReadWrite))
                {
                    iTextSharp.text.Document document = new iTextSharp.text.Document(reader.GetPageSizeWithRotation(1));
                    PdfWriter writer = PdfWriter.GetInstance(document, fsOutput);

                    document.Open();
                    PdfContentByte cb = writer.DirectContent;

                    int numberOfPages = reader.NumberOfPages;

                    try
                    {
                        for (int pageNumber = 1; pageNumber <= numberOfPages; pageNumber++)
                        {
                            iTextSharp.text.Rectangle pageSizeWithRotation = reader.GetPageSizeWithRotation(pageNumber);

                            document.SetPageSize(pageSizeWithRotation);
                            document.NewPage();

                            int SecondChance = 0;
                        conti:  ///continue again
                            try
                            {
                                // >>> CONVERT CURRENT PAGE TO GRAYSCALE
                                grayscaleConverter.Convert(reader, pageNumber);
                                // <<<<
                            }
                            catch
                            {
                                if (SecondChance == 0)
                                {
                                    SecondChance++;
                                    goto conti;   ///junmp to conti
                                }
                            }

                            PdfImportedPage pip = writer.GetImportedPage(reader, pageNumber);

                            if (pageSizeWithRotation.Rotation == 90 || pageSizeWithRotation.Rotation == 270)
                            {
                                cb.AddTemplate(pip, 0, -1f, 1f, 0, 0, pageSizeWithRotation.Height);
                            }
                            else
                            {
                                cb.AddTemplate(pip, 1f, 0, 0, 1f, 0, 0);
                            }
                        }
                    }
                    catch (Exception ex) { MessageBox.Show(ex.Message.ToString()); }

                    cb.PdfDocument.Close();
                    document.Close(); document.Dispose(); document = null;
                    writer.Close(); writer.Dispose(); writer = null;
                    reader.Close(); reader.Dispose();
                }
            }