itextsharp split shared xObject streams - pdf

Question

I am trying to split shared XObject streams (originally flatten form fields with the same content) in the PDF.
What is the correct way to do this using itextsharp? I am trying the code below but the stream is still shared in the resulting document.
Sample pdf with shared XObject streams flatten.pdf
PdfReader pdf = new PdfReader(path);
PdfStamper stamper = new PdfStamper(pdf, new FileStream("processed.pdf", FileMode.OpenOrCreate, FileAccess.ReadWrite));
EliminateSharedStreams(stamper, 1);
stamper.Close();
virtual public void EliminateSharedXObjectStreams(PdfStamper pdfStamper, int pageNum)
{
PdfReader pdfReader = pdfStamper.Reader;
PdfDictionary page = pdfReader.GetPageN(pageNum);
PdfDictionary resources = page.GetAsDict(PdfName.RESOURCES);
PdfDictionary xObjResources = resources.GetAsDict(PdfName.XOBJECT);
List<PRIndirectReference> newRefs = new List<PRIndirectReference>();
List<PdfName> newNames = new List<PdfName>();
List<PRStream> newStreams = new List<PRStream>();
IntHashtable visited = new IntHashtable();
foreach (PdfName key in xObjResources.Keys)
{
PdfStream xObj = xObjResources.GetAsStream(key);
if (xObj is PRStream && xObj.GetAsName(PdfName.SUBTYPE) != null &&
xObj.GetAsName(PdfName.SUBTYPE).CompareTo(PdfName.FORM) == 0)
{
PRIndirectReference refi = (PRIndirectReference)xObjResources.Get(key);
PRStream xFormStream = (PRStream)xObj;
if (visited.ContainsKey(refi.Number))
{
// need to duplicate
newRefs.Add(refi);
PRStream newStream = new PRStream(xFormStream, null);
newStreams.Add(newStream);
newNames.Add(key);
}
else
visited[xFormStream.ObjNum] = 1;
}
}
if (newStreams.Count == 0)
return;
PdfContentByte canvas = pdfStamper.GetOverContent(pageNum);
PdfWriter writer = pdfStamper.Writer;
for (int k = 0; k < newStreams.Count; ++k)
{
canvas.SaveState();
//add copied stream
PdfIndirectReference newRef = writer.AddToBody(newStreams[k]).IndirectReference;
//change the ref
xObjResources.Put(newNames[k], newRef);
canvas.RestoreState();
}
}

Answer 1

First remarks without a sample document
There are numerous reasons why your code may not work as desired. As you did not supply your sample PDF, I cannot tell which are more relevant and which are not.
You only search xobjects shared on the same page; if a xobject is once used on page one and once on page two, your code cannot identify this.
If you want to be able to find such shares, you'll at least have to use the same IntHashtable instance visited across all calls of EliminateSharedXObjectStreams for the same PdfStamper pdfStamper, e.g. by creating it once outside this method and making it a parameter of your method.
You only check for shared xobjects in the immediate page resources. But form xobjects have their own resources which can contain even more form xobject declarations.
If you want to find such shares, you'll have to recurse into the resources of your page's xobjects, those xobjects' xobjects, etc. pp.
(Strictly speaking you also have to recurse into the form xobjects of patterns and Type 3 Font glyph definitions, but these are unlikely positions to flatten form fields into.)
You only check for shared xobjects with different names. But xobjects can also be shared by referencing the same name multiple times from the same content stream.
If you want to find such shares, you have to analyse the content streams in question to find duplicate usages of the form xobject with the same name.
(By the way, doing so you may also check whether declared xobjects are used at all: if a form xobject is declared in some resources, this does not mean it is used in the context of these resources, it may be an unused resource.)
You don't mark xObjResources (if it itself is indirect) or page (otherwise) as used. If your PdfStamper pdfStamper is working in append mode, your changes may be ignored.
Solution with a sample document
After you provided the information that
It's single page document containing shared streams (xobjects with different names) in the immediate page resources. pdfStamper is not in append mode.
it turned out that the problems mentioned above are not relevant in your case. As you meanwhile also have provided an example document, I could reproduce the issue.
Indeed, your code does not split the shared XObjects. The reason is that the PdfStamper is made for manipulating the PDF in the PdfReader in the state it was in when the stamper was constructed, using stamper methods only. Your code, on the other hand, manipulates objects directly retrieved from the PdfReader after the construction of the stamper. Thus, while your new streams are added to the PDF (actually up front), the changes in the pre-existing XObject resource dictionaries don't make it to the result.
If you want to manipulate objects you retrieve from the reader, you instead should do this before creating the stamper.
This actually should suite you as your code structurally is copied from a Pdfreader method anyways, EliminateSharedStreams, which you adapted to your use case.
The only problem is that that method uses a hidden member variable of the PdfReader class. But you can access that variable bei means of reflection.
Thus, the manipulated method (working on a pure PdfReader) could look like this:
virtual public void EliminateSharedXObjectStreams(PdfReader pdfReader, int pageNum)
{
PdfDictionary page = pdfReader.GetPageN(pageNum);
PdfDictionary resources = page.GetAsDict(PdfName.RESOURCES);
PdfDictionary xObjResources = resources.GetAsDict(PdfName.XOBJECT);
List<PRIndirectReference> newRefs = new List<PRIndirectReference>();
List<PRStream> newStreams = new List<PRStream>();
IntHashtable visited = new IntHashtable();
foreach (PdfName key in xObjResources.Keys)
{
PdfStream xObj = xObjResources.GetAsStream(key);
if (xObj is PRStream && xObj.GetAsName(PdfName.SUBTYPE) != null &&
xObj.GetAsName(PdfName.SUBTYPE).CompareTo(PdfName.FORM) == 0)
{
PRIndirectReference refi = (PRIndirectReference)xObjResources.Get(key);
PRStream xFormStream = (PRStream)xObj;
if (visited.ContainsKey(refi.Number))
{
// need to duplicate
newRefs.Add(refi);
PRStream newStream = new PRStream(xFormStream, null);
newStreams.Add(newStream);
}
else
visited[xFormStream.ObjNum] = 1;
}
}
if (newStreams.Count == 0)
return;
FieldInfo xrefObjField = typeof(PdfReader).GetField("xrefObj", BindingFlags.Instance | BindingFlags.NonPublic);
List<PdfObject> xrefObj = (List<PdfObject>)xrefObjField.GetValue(pdfReader);
for (int k = 0; k < newStreams.Count; ++k)
{
xrefObj.Add(newStreams[k]);
PRIndirectReference refi = newRefs[k];
refi.SetNumber(xrefObj.Count - 1, 0);
}
}
and you can use it like this:
using (PdfReader pdfReader = new PdfReader(sourcePath))
using (Stream pdfStream = new FileStream(targetPath, FileMode.Create, FileAccess.Write))
{
EliminateSharedXObjectStreams(pdfReader, 1);
PdfStamper pdfStamper = new PdfStamper(pdfReader, pdfStream);
pdfStamper.Close();
}
in particular calling EliminateSharedXObjectStreams before constructing the PdfStamper.
If you are after a generic solution, you of course will have to extend the method to remove the restrictions observed in the first part of the answer...
Solution without reflection
The OP found out:
Manipulating PdfReader works as expected. Only thing is that instead of using xrefObj private field, the stream can be add using AddPdfObject:
for (int k = 0; k < newStreams.Count; ++k)
{
PRIndirectReference newRef = pdfReader.AddPdfObject(newStreams[k]);
PRIndirectReference refi = newRefs[k];
refi.SetNumber(newRef.Number, 0);
}
Indeed, this improves the solution substantially.