How to use parallel_for?

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Hi,

As shown by Vladislav, you only need to derivate the cv::ParallelLoopBody class to make your own.

To complete, and answer Q3 (previous Qx may be related to includes, you should give more details about encountered errors). Then, related to Q3 : if you need to process in parallel local memory buffers or other data, you need a constructor that will point to the buffers to process when operator() is called.

Here is a sample code i use that may help you : It is a simple loop that clips buffer values to max and min values. I consider here classical tables of any type using templates. You can change this using std::vectors, cv::Mat or any other, only keep in mind that you have to create private identifiers that points to the beginning of data buffer that you want to manage.

In constructor, you show which buffer to process and eventually what constants to take into account. Once done everything is prepared to run parallel. In the operator() method, create new local pointers to the target block range

Hope it helps.

Regards

Alex

template <class type>

class Parallel_clipBufferValues: public cv::ParallelLoopBody
{   
private:
  type *bufferToClip;
  type minValue, maxValue;

public:
  Parallel_clipBufferValues(type* bufferToProcess, const type min, const type max)
    : bufferToClip(bufferToProcess), minValue(min), maxValue(max){}

  virtual void operator()( const cv::Range &r ) const {
    register type *inputOutputBufferPTR=bufferToClip+r.start;
    for (register int jf = r.start; jf != r.end; ++jf, ++inputOutputBufferPTR)
    {
        if (*inputOutputBufferPTR>maxValue)
            *inputOutputBufferPTR=maxValue;
        else if (*inputOutputBufferPTR<minValue)
            *inputOutputBufferPTR=minValue;
    }
  }
};

Finally, how to use it :

const int SIZE=10;
int myTab[SIZE];
int minVal=0, maxVal=255;
parallel_for_(cv::Range(0,SIZE-1), Parallel_clipBufferValues<int>(myTab, minVal, maxVal));