All fprintf() calls should be replaced with CV_LOG_ macros (perhaps CV_LOG_WARNING / CV_LOG_INFO).

This patch should go into 3.4 branch first. We will merge changes from 3.4 into master regularly (weekly/bi-weekly).

So, please:

• change "base" branch of this PR: master => 3.4 (use "Edit" button near PR title)
• rebase your commits from master onto 3.4 branch. For example:
  git rebase -i --onto upstream/3.4 upstream/master
  (check list of your commits, save and quit (Esc + "wq" + Enter)
  where upstream is configured by following this GitHub guide and fetched (git fetch upstream).
• push rebased commits into source branch of your fork (with --force option)

Note: no needs to re-open PR, apply changes "inplace".

@alalek implemented your suggestions. Although in hindsight, i'm not sure whether keeping the log message. It's really not one which should be handed by OpenCV, more by the downstream user.

I'm reasonably sure WARNING is the right level, but that's generally the default log level, so this likely won't actually solve my original issue!

this likely won't actually solve my original issue!

The message becomes avoidable. Set environment variable OPENCV_LOG_LEVEL=ERROR or OPENCV_LOG_LEVEL=FATAL.