itext7: Check for visible content in a specific area

问题

I want to check specific parts of a pdf file for visible content. With the PdfCanvasProcessor and a custom eventlistener it is possible to check for text and images but for other kind of content this does not work. So what would be the best approach to check fot any visible content?

Regards, Marco

来源：https://stackoverflow.com/questions/46104097/itext7-check-for-visible-content-in-a-specific-area