iText 7.1.0 flattening signature field with appearance stretched - itext

Question

I have a PDF (https://github.com/giacgbj/stackOverflow/blob/master/xxx.pdf), somehow digitally signed by a third party, whose signature has an appearance
When I flatten the PDF (https://github.com/giacgbj/stackOverflow/blob/master/xxxFlattened.pdf) using the following code (iText 7.1.0)
try (PdfWriter output = new PdfWriter("output.pdf");
PdfDocument input = new PdfDocument(new PdfReader("input.pdf"), output)) {
PdfAcroForm.getAcroForm(input, true).flattenFields();
}
the appearance of the signature is stretched like this:
Flattening the same PDF using PDFBox or command line arguments like GhostScript or convert (imagemagick) or pdf2ps/ps2pdf works.
What's the reason of this behavior?

Answer 1

The problem is that the signature annotation has this Rect value:
[35.0 115.0 215.0 155.0]
which is a rectangle with its lower left at (35,115) and the upper left at (215,155), i.e. a rectangle 180 wide and 40 high.
Its appearance, though, has this BBox value:
[100.0 50.0 0.0 0.0]
which is a rectangle 100 units wide and 50 high. (Strictly speaking this is not completely valid: generally in PDFs rectangles are written as an array of four numbers giving the coordinates of a pair of diagonally opposite corners. For BBox values, though, the order is fixed: An array of four numbers giving the left, bottom, right, and top coordinates, respectively. But the problem discussed here also turns up if the correct order is used.)
According to the PDF specification the appearance will be stretched to match the annotation rectangle.
During flattening, though, the former signature appearance is added to the page using a transformation matrix of:
1 0 0 1 35 115
which correctly positions its lower left at (35,115) but maps the rectangle using the identity matrix; thus, it incorrectly is not stretched to a size of 180x40 but remains a 100x50 which is the stretch (actually the missing stretch!) you observe.
In short, you appear to have found a bug in iText, a missing transformation...