Is setNumThreads(x) parallelizing my OpenCV code?

Question

I really wonder if using OpenCV's setNumThreads(); really allows my code to run in parallel. I've searched a lot on the internet without finding any answer for my question.

Someone there have any answer for my question?

Answer 1

The effect depends greatly on the configuration options you select on cmake configure, see for example CMakeLists.txt, plus the catches of the different configuration options:

/* IMPORTANT: always use the same order of defines
   1. HAVE_TBB         - 3rdparty library, should be explicitly enabled
   2. HAVE_CSTRIPES    - 3rdparty library, should be explicitly enabled
   3. HAVE_OPENMP      - integrated to compiler, should be explicitly enabled
   4. HAVE_GCD         - system wide, used automatically        (APPLE only)
   5. HAVE_CONCURRENCY - part of runtime, used automatically    (Windows only - MSVS 10, MSVS 11)
*/

And with those, you can understand the code itself. All that said, the parallelising engine won't do much if you're running an inherently sequential algorithm, which is practically everything under OpenCV... My guess is that if you would have several OpenCV programs running in parallel, you could see a meaningful difference.