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VideoCapture open starts many threads

asked 2015-11-18 12:22:43 -0600

genzm gravatar image

Hy,

I've noticed that when opening a video file using the VideoCapture class (in C++), about eight new threads are created. As I need 7 video files simultaneously this results in about 50 threads running. I was wondering why this happens and what the threads do (because I can't find anything about this online).

Thanks

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os ? opencv version ?

berak gravatar imageberak ( 2015-11-18 22:33:44 -0600 )edit

I'm sorry, ubuntu 14.04 and opencv 2.4.11

genzm gravatar imagegenzm ( 2015-11-19 01:19:32 -0600 )edit

Could you add your CMAKE configuration? I am interested in what build options you have used (might give a clue to where to look for the solution)

StevenPuttemans gravatar imageStevenPuttemans ( 2015-11-19 07:52:42 -0600 )edit

Maybe I don't undestand your question but I think this is expected. On my win7, VideoCapture from cam opens 5 threads. In opencv many operations are in parallel. If you use cv::cvtColor you will see 4 threads more. In addiction the codec could have internal threads.

pklab gravatar imagepklab ( 2015-11-20 04:06:32 -0600 )edit
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Yes, I understand that it is to make the decoding process faster. But decoding one video creates many threads. I can imagine quite a few use cases where you do not want to use your entire computational power to decode the video. It just seemed a little bit strange to have this as a default behaviour.

genzm gravatar imagegenzm ( 2015-11-26 11:38:36 -0600 )edit

@genzm you should be able to force your application only to use a single thread. If you built with OpenMP for example you can do it like this. Exactly the reason why I am asking your CMAKE output.

StevenPuttemans gravatar imageStevenPuttemans ( 2015-11-27 04:09:40 -0600 )edit

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answered 2015-11-26 11:40:45 -0600

genzm gravatar image

I found this line of code in the github repositories.

https://github.com/Itseez/opencv/blob...

Changing this line will determine how many threads get created when using ffmpeg. I changed this to two in my application. It is surprising that this is no parameter.

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You might want to supply the functionality to parametrize this through a PR?

StevenPuttemans gravatar imageStevenPuttemans ( 2015-11-27 04:10:16 -0600 )edit

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Asked: 2015-11-18 12:22:43 -0600

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Last updated: Nov 26 '15