Patents certainly don't reveal everything about an invention. The basic premise of a patent is that in exchange for revealing the invention you get a monopoly on that invention for a specified period of time.
However, when I was a patent agent most of my clients wanted to reveal nothing and get a monopoly on everything. It was like pulling teeth to get them to tell me anything. I've never met a more paranoid group of people than inventors.
I do know that they got a patent on their process, but generally left out a crucial ingredient, or step in the process. The examiners at the patent office don't have the technical knowledge to know that a small, but vital piece was missing. It won't help future generations to make film if all they have to rely on is patents. Of course, as PE has alluded to, many of the ingredients or processes are kept internal as trade secrets and never published.