实际使用 PDF Clown 裁剪 PDF

Question

My objective is actually cropping a PDF file with PdfClown. There are a lot of tools/library that allow cropping PDF, changing the PDF cropBox. This permits hiding contents outside a rectangular area, but content is still there, it might be accessed through a PDF parser and PDF size does not change.

On the contrary what I need is creating a new page containing only the contents inside the rectangular area.

So far I've tried scanning contents and selectively cloning them. But I didn't succeed yet. Any suggestions on using PdfClown for that?

I've seen someone is trying something similar with PdfBox Cropping a region from a PDF page with PDFBox not succeeding yet.

Answer 1

A bit late, but maybe it helps someone; I am sucessfully doing what you are asking for - but with other libraries. Required libraries : iText 4 or 5 and Ghostscript

Step 1 with pseudo code

Using iText, Create a PDFWRITER instance with a blank Doc. Open a PDFREADER object to the original file you want to crop. Import the Page, get a PDFTemplate Object from the source, set its .boundingBox property to the desired cropbox, wrap the template into an iText Image object and paste it onto the new page at an absolute position.

Dim reader As New PdfReader(sourcefile)
Dim doc As New Document()
Dim writer As PdfWriter = PdfWriter.GetInstance(doc, New System.IO.FileStream(outputfilename, System.IO.FileMode.Create))

//get the source page as an Imported Page
Dim page As PdfImportedPage = writer.GetImportedPage(reader, indexOfPageToGet) page

//create PDFTemplate Object at original size from source - see iText in Action book Page 91 for full details
Dim pdftemp As PdfTemplate = page.CreateTemplate(page.Width, page.Height) 
//paste the original page onto the template object, see iText documentation what those parameters do (scaling, mirroring)
pdftemp.AddTemplate(page, 1, 0, 0, 1, 0, 0)
//now the critical part - set .boundingBox property on the template. This makes all objects outside the rectangle invisible
pdftemp.boundingBox = {iText Rectangle Structure with new Cropbox}
//template not needed anymore
writer.ReleaseTemplate(pdftemp) 
//create an iText IMAGE object as wrapper to the template - with this img object absolute positionion on the final page is much easier
dim img as iTextSharp.Text.Image = Image.GetInstance(pdftemp)
// set img position
img.SetAbsolutePosition(x, y)
//set optional Rotation if needed
img.RotationDegrees = 0
//finally, this adds the actual content to the new document
doc.Add(img) 
//cleanup
doc.Close()
reader.Close()
writer.Close()

The output file will visually look cropped. But the objects are still present in the PDF Stream. Filesize will probably remain very little changed yet.

Step 2:

Using Ghostscript and output device pdfwrite, combined with the correct command line parameters you can re-process the PDF from Step 1. This will give you a much smaller PDF. See Ghostscript documentation for the arguments https://www.ghostscript.com/doc/9.52/Use.htm This steps actually gets rid of objects that are outside the bounding box - the requirement you asked for in your OP, at least for files that I deal with.

Optional Step 3: Using MUTOOL with the -g option you can clean up unused XREF objects. Your original PDF probably had a lot of Xrefs, which increase filesize. After cropping some of them may not be needed anymore. https://mupdf.com/docs/manual-mutool-clean.html

PDF Format is a tricky thing, normally I would agree with @Tilman Hausherr , my suggestion may not work for all files and covers the 'almost impossible' scenario, but it works for all cases that I deal with.