How to get the UserUnit from a PDF file?

What do I need to do to get the user unit for each page so I can then get the page dimensions in pixels?

I have a bunch of PDF files that I read into a byte array one by one. I then pass these byte arrays to a PdfReader instance. Now I want know the dimensions of each page in pixels. From what I've read so far it seems by PDF files work in points, a point being a configurable unit stored in some kind of dictionary in an element called /UserUnit.

Loading my PDF file into a PdfReader, what do I need to do to get this user unit for each page (apparently it can vary from page to page) so I can then get the page dimensions in pixels.

At present I have this code, which grabs the dimensions for each page in "points". I guess I just need the /UserUnit value, and can then multiply these dimensions by that to get pixels or something similar.

PdfReader reader = new iTextSharp.text.pdf.PdfReader(file_content);
for (int i = 1; i  reader.NumberOfPages; i++) {
    Rectangle dim = reader.GetPageSize(i);
    int[] xy = new int[] { (int)dim.Width, (int)dim.Height };
    page_data[objectid + '-' + i] = xy;
}

Posted on StackOverflow on Jan 29, 2013 by Shawson

Allow me to quote from my book, iText in Action - Second Edition, page 9:

What is the measurement unit in PDF documents?

Most of the measurements in PDFs are expressed in user space units. ISO-32000-1 (section 8.3.2.3) tells us "the default for the size of the unit in default user space (1/72 inch) is approximately the same as a point (pt), a unit widely used in the printing industry. It is not exactly the same; there is no universal definition of a point."

In short, 1 in. = 25.4 mm = 72 user units (which roughly corresponds to 72 pt).

On the next page, I explain that it's possible to change the default value of the user unit, and I add an example on how to create a document with pages that have a different user unit.

Now for your question: suppose you have an existing PDF, how do you find which user unit was used? Before we answer this, we need to take a look at ISO-32000-1.

In section 7.7.3.3, entitled "Page Objects", you'll find the description of the /UserUnit entry in Table 30, "Entries in a page object":

(Optional; PDF 1.6) A positive number that shall give the size of default user space units, in multiples of 1/72 inch. The range of supported values shall be implementation-dependent. Default value: 1.0 (user space unit is 1/72 inch).

This key was introduced in PDF 1.6; you won't find it in older files. It's optional, so you won't always find it in every page dictionary. In my book, I also explain that the maximum value of the UserUnit key is 75,000.

Now how to retrieve this value with iText for C#?

You already have Rectangle dim = reader.GetPageSize(i); which returns the MediaBox. This may not be the size of the visual part of the page. If there's a CropBox defined for the page, viewers will show a much smaller size than what you have in xy (but you probably knew that already).

What you need now in iText 7 is the page dictionary, so that you can retrieve the value of the UserUnit key. The code is written in Java, but it can be easily converted to C#:

PdfDictionary pageDict = pdfDoc.getPage(i).getPdfObject();
PdfNumber userUnit = pageDict.getAsNumber(PdfName.UserUnit);

Most of the times userUnit will be null, but if it isn't you can use userUnit.floatValue().

Click How to get the UserUnit from a PDF file? if you want to see how to answer this question in iText 5.