I agree with you, it's not safe to have a rule saying "non-black means invisible".
I should have made it clear in the blog post, but the first thing I do when processing a PDF is I identify the document_type. Then I run special rules for that document_type.
So the "non-black means invisible" rule only applies when processing this document_type.
I should have made it clear in the blog post, but the first thing I do when processing a PDF is I identify the document_type. Then I run special rules for that document_type.
So the "non-black means invisible" rule only applies when processing this document_type.