With iText i can't fill signature field using base64 string image on setValue

With iText i can't fill signature field using base64 string image on setValue

There is no signature filled so what's wrong with this code?

private byte[] Sign(byte[] src, X509Certificate[] chain, ICipherParameters pk,
   String digestAlgorithm, PdfSigner.CryptoStandard subfilter, string from, string to, string subject, string business, List<PDFExistingFieldsInfos> pdfExistingFieldsInfos)
{

  byte[] filledPDF = src;

  using (MemoryStream srcMemStream = new MemoryStream(src))
  {
    using (MemoryStream memStream = new MemoryStream())
    {
      PdfReader r = new PdfReader(srcMemStream);
      r.SetUnethicalReading(true); // todo: à utilisé pour bypass l'encryption du document
      PdfDocument pdfDoc = new PdfDocument(r, new PdfWriter(memStream));

      PdfAcroForm form = PdfAcroForm.GetAcroForm(pdfDoc, true);
      form.SetGenerateAppearance(true); // a tester!!

      foreach (var pdfFieldInfos in pdfExistingFieldsInfos)
      {
        switch (pdfFieldInfos.FieldType)
        {
          case "sign":
            PdfSignatureFormField myPdfSignatureFormField = (PdfSignatureFormField)form.GetField(pdfFieldInfos.FieldName);
            myPdfSignatureFormField.SetValue(pdfFieldInfos.FieldValue);
            break;

          case "text":
            form.GetField(pdfFieldInfos.FieldName).SetValue(pdfFieldInfos.FieldValue ?? "", null);
            break;
        }
      }
      form.FlattenFields();
      pdfDoc.Close();

      filledPDF = memStream.ToArray();
    }
  }
  return filledPDF;
}

pdfFieldInfos.FieldValue is a base64 image string