在Visual C ++上，如何处理带有线程的图像数组（并行性）

Question

I want to ask how can use multiple threads to works on array of OpenCV Mat images...

In simple words: using suggestion give me from user of this site, I have packed two array of six images in a struct to pass it at thread:

struct Args
{
     Mat in[6];
     Mat out[6];
};

In main code I populate the input "in" array with six images, with this code, and assing it to struct in array:

    Mat inn[6],ou[6];

    inn[0]=imread("C:/OPENCV/Test/imgtest/bird1.jpg",1);
inn[1]=imread("C:/OPENCV/Test/imgtest/bird2.jpg",1);
inn[2]=imread("C:/OPENCV/Test/imgtest/bird3.jpg",1);
inn[3]=imread("C:/OPENCV/Test/imgtest/pig1.jpg",1);
inn[4]=imread("C:/OPENCV/Test/imgtest/pig2.jpg",1);
inn[5]=imread("C:/OPENCV/Test/imgtest/pig3.jpg",1);

Args dati;
*dati.in = *inn;
*dati.out = *ou;

Now I want to use multiple threads to process this images...all six images, and store them in output array to visualize them.

the functions are:

 //greyscale funct
 void grey (void *param){
while (TRUE)
{
WaitForSingleObject(mutex,INFINITE);
Args* arg = (Args*)param;
cvtColor(*arg->in,*arg->out,CV_BGR2GRAY);
ReleaseMutex(mutex);
}
_endthread();
 }
  //threshold funct
 void soglia(void *param){
while (TRUE)
{
WaitForSingleObject(mutex,INFINITE);
Args* arg = (Args*)param;
threshold(*arg->out,*arg->out,128,255,THRESH_BINARY);
ReleaseMutex(mutex);
}
_endthread();
 }
 //output
void visualizza(void *param){
while (TRUE)
{
WaitForSingleObject(mutex,INFINITE);
Args* arg = (Args*)param;
imshow("Immagine",*arg->out);
waitKey(10);
ReleaseMutex(mutex);
}
//_endthread();
  }

using a mutex object to make them thread safe. These functions using cast to make conversion from void to Args...but, if I want to process all 6 images of input array, and I think to use for cicle, how can I modify these functions to accept and use array with "i" position? Because I use for cicle without use threads and works...but with threads and for have a parallelism, how can I modify these functions for elaborate every images of input array?

I mean: while second thread soglia works on first image of array, the first thread grey start works on secondo images..and so on....

How can I do this?

Thanks in advance for your attention and time.

Answer 1

I am assuming that the windows threading code you have shown isn't set in stone. I couldn't work out what you are trying to do with them. SInce you only have one mutex, (at least that what it looks like), all your threads will be blocked most of the time.

To quote from Intel TBB run a function in a parallel thread? :

From the TBB tutorial

Intel® Threading Building Blocks targets threading for performance. Most general-purpose threading packages support many different kinds of threading, such as threading for asynchronous events in graphical user interfaces. As a result, general-purpose packages tend to be low-level tools that provide a foundation, not a solution. Instead, Intel® Threading Building Blocks focuses on the particular goal of parallelizing computationally intensive work, delivering higher-level, simpler solutions.

OpenCV uses TBB internally (if you build it with TBB) so you look at the source to see how things are done. Since 2.4.3 OpenCV has an inbuilt parallel_for_ construct (which uses TBB). You can find out some more about it at http://answers.opencv.org/question/3730/how-to-use-parallel_for/