Annotation(stamp) missed after flattened - pdf

Question

Annotation(PdfName.STAMP) missed after flattened by itextsharp5.5.13.1.
I have two pdfs. One does work, the other does not work.
Any ideas will be appreciated.
The code is following
string outFile = inputFile + "_f.pdf";
using (PdfReader pdfReader = new PdfReader(inputFileName))
{
PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(outFile, FileMode.Create, FileAccess.Write, FileShare.None))
//pdfStamper.FormFlattening = true;
//pdfStamper.FreeTextFlattening = true;
pdfStamper.AnnotationFlattening = true;
//pdfStamper.AcroFields.GenerateAppearances = true;
}

Answer 1

The cause is a bug in iTextSharp, annotation flattening does not correctly calculate the position for the flattened annotation if its bounding box does not use the origin as lower left corner.
If you look at the code of PdfStamperImp.FlattenAnnotations(bool), you'll quickly realize that the calculations in the if (app != null) block only make sense if the bounding box is located at the origin or no scaling is necessary for fitting the appearance bounding box into the annotation rectangle.
(As often the lower left corner of the bounding box is the origin, this does not pop up often.)
Thus, for flattening such annotations you have to use a different method for flattening, e.g. like this:
using (PdfReader pdfReader = new PdfReader(inputFileName))
{
PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(outFile, FileMode.Create, FileAccess.Write, FileShare.None));
ImprovedAnnotationFlattening(pdfStamper);
pdfStamper.Close();
}
with these helper methods:
void ImprovedAnnotationFlattening(PdfStamper pdfStamper)
{
double[] DEFAULT_MATRIX = { 1, 0, 0, 1, 0, 0 };
PdfReader reader = pdfStamper.Reader;
for (int page = 1; page <= reader.NumberOfPages; ++page)
{
PdfDictionary pageDic = reader.GetPageN(page);
PdfArray annots = pageDic.GetAsArray(PdfName.ANNOTS);
if (annots == null)
continue;
for (int idx = 0; idx < annots.Size; ++idx)
{
PdfObject annoto = annots.GetDirectObject(idx);
if (!(annoto is PdfDictionary))
continue;
PdfDictionary annDic = (PdfDictionary)annoto;
PdfNumber ff = annDic.GetAsNumber(PdfName.F);
int flags = ff != null ? ff.IntValue : 0;
if ((flags & PdfFormField.FLAGS_PRINT) == 0 || (flags & PdfFormField.FLAGS_HIDDEN) != 0)
continue;
PdfObject obj1 = annDic.Get(PdfName.AP);
if (obj1 == null)
continue;
PdfDictionary appDic = obj1 is PdfIndirectReference
? (PdfDictionary)PdfReader.GetPdfObject(obj1)
: (PdfDictionary)obj1;
PdfObject obj = appDic.Get(PdfName.N);
PdfStream objDict = appDic.GetAsStream(PdfName.N);
if (objDict != null)
{
Rectangle rect = PdfReader.GetNormalizedRectangle(annDic.GetAsArray(PdfName.RECT));
Rectangle bbox = PdfReader.GetNormalizedRectangle(objDict.GetAsArray(PdfName.BBOX));
PdfContentByte cb = pdfStamper.GetOverContent(page);
cb.SetLiteral("Q ");
PdfArray matrixArray = objDict.GetAsArray(PdfName.MATRIX);
double[] matrix = matrixArray != null ? matrixArray.AsDoubleArray() : DEFAULT_MATRIX;
AffineTransform transform = new AffineTransform(matrix);
double[] bboxCorners = { bbox.Left, bbox.Bottom, bbox.Right, bbox.Bottom, bbox.Right, bbox.Top, bbox.Left, bbox.Top };
transform.Transform(bboxCorners, 0, bboxCorners, 0, 4);
double minX = Min(bboxCorners, 0, 2);
double maxX = Max(bboxCorners, 0, 2);
double minY = Min(bboxCorners, 1, 2);
double maxY = Max(bboxCorners, 1, 2);
transform.preConcatenate(AffineTransform.GetTranslateInstance(-minX, -minY));
transform.preConcatenate(AffineTransform.GetScaleInstance(rect.Width/(maxX-minX), rect.Height/(maxY-minY)));
transform.preConcatenate(AffineTransform.GetTranslateInstance(rect.Left, rect.Bottom));
transform.GetMatrix(matrix);
cb.AddFormXObj(objDict, GenerateName(), matrix[0], matrix[1], matrix[2], matrix[3], matrix[4], matrix[5]);
cb.SetLiteral("q ");
annots.Remove(idx);
--idx;
}
}
}
}
double Min(double[] array, int start, int step)
{
double result = array[start];
for (int i = start + step; i < array.Length; i+=step)
{
result = Math.Min(result, array[i]);
}
return result;
}
double Max(double[] array, int start, int step)
{
double result = array[start];
for (int i = start + step; i < array.Length; i += step)
{
result = Math.Max(result, array[i]);
}
return result;
}
PdfName GenerateName()
{
PdfName name = new PdfName("XXX" + formXObjectsCounter);
++formXObjectsCounter;
return name;
}
int formXObjectsCounter = 4711;
Beware: I just wrote these methods (copying as much as possible from the original flattening code) and only tested with your example files. Some border conditions might still have to be considered for general use. In particular I did not do all relevant null or 0 tests. Also I did not attempt to support proper tagging.