How to get bounding boxes of elements in EPS files - pdf

Question

I need to check if a EPS/PDF file contains any vector elements
First I convert the PDF to EPS and remove all text elements and images from the file like this
pdftocairo -f $page_number -l $page_number -eps $input - | sed '/BT/,/ET/ d' | sed '/^8 dict dup begin$/,/^Q$/ c Q' > $output
But how can I then check if any elements are written to the canvas?

Answer 1

What do you mean, exactly, by 'vector elements' ? Anything except an actual bitmap image ? Why do you care ? Perhaps if you explained what you want to achieve it would be easier to help you.
Note that the approach you are using is by no means guaranteed to work, there can easily be 'elements' in the file which won't be removed by your rather basic approach to finding image.
You could use Ghostscript; run the file to a bitmap and specify -dFILTERTEXT and -dFILTERIMAGES. Then examine the pixels fo the bitmap to see if any are non-white. If they are, then there was vector content i the file. You could probably use something like ImageMagick to count the colours and see if there's more than 1.
Or run the file to bitmap twice, once normally, and once with -dFILTERVECTOR. Compare the two bitmaps (MD5 on them would be sufficient). If there are no differences then there was no vector content.

Answer 2

Any PDF that has vector elements will use at least one of the path painting operators. According to chapter 8 of the PDF standard, those are:
S, s, f, F, f*, B, B*, b, b*, n
Of course, since PDF files can be complex, you'll also need it in a standard form. You can do that using the qpdf program's QDF format. (apt install qpdf if you don't have it).
qpdf -qdf schedule.pdf - | egrep -m1 -q '\b[SsfFBbn]\*?$' && echo Yup
That'll print "Yup" if the file schedule.pdf has vector graphics in it.
Note: I think this will do the job for you, but it is not fool proof. It's possible to get false negatives if your PDF is loading vectors from an external file, embedding raw postscript, or doing some other trickiness. And, of course it can have false positives (for example, a file that draws a completely transparent, 0pt dot in white ink on a white background).

Answer 3

Other answers have addressed identifying the drawing operators in a plain text stream. For the other question,
But how can I then check if any elements are written to the canvas?
For this, the elements need to be part of a content stream that is referred to
in the /Contents member of the Page object.
If you read in all of the pdf objects, there will be a tree connecting all the content streams to the Root object declared in the trailer.
Trailer : /Root is a reference to the Document Catalog object
Document Catalog : /Pages is an array of Page objects or Pages nodes
Page : /Contents is an array of references to Content Stream objects that draw the elements of the page
It is possible for there to be stray content stream objects that are not referenced in the Document tree. By traversing the Pages tree you could collect any and all actual content and then feed that result to one of the solutions from the other answers.