How to use optimal parallelization for multi template matching with OpenCvSharp?

Question

For a multi Template Matching with OpenCvSharp I want to use Parallel.ForEach() and limit the maximal number of threads used to n-1 so at least one thread remains free for other possible occuring tasks (n is the total nr of CPU-level threads available).

E.g.: My system has a 4 Core CPU with 2 Threads per Core. So the max amount here should be 7.

How can I do this without hardcoding it? (It should work also for other PCs with another number of threads)

   // Use a maximum of n-1 threads
   int maxNrOfThreads = 7;

   Parallel.ForEach(this.Symbols.Keys, new ParallelOptions { MaxDegreeOfParallelism = maxNrOfThreads }, key  => {
      //EXECUTION (template match every symbol to a given image)
   });

Here is the execution part for who may be interessted:

Mat grayTemplate = this.Symbols[key].GrayscaledSymbol;

Mat res = new Mat(grayImage.Rows - grayTemplate.Rows + 1,grayImage.Cols - grayTemplate.Cols + 1, MatType.CV_32FC1);

Cv2.MatchTemplate(grayImage, grayTemplate, res, TemplateMatchModes.CCoeffNormed);

Cv2.Threshold(res, res, MIN_ACCURACY, 1.0, ThresholdTypes.Tozero);

while (true)
{
   double minval, maxval, threshold = MIN_ACCURACY;
   OpenCvSharp.Point minloc, maxloc;
   Cv2.MinMaxLoc(res, out minval, out maxval, out minloc, out maxloc);

   if (maxval >= threshold)
   {
       // Add bounding box to result object
       ret.AddResult(key,maxloc.X,maxloc.Y,grayTemplate.Width,grayTemplate.Height);

      // Fill in the res Mat so you don't find the same area again in the MinMaxLoc
      Rect outRect;
      Cv2.FloodFill(res, maxloc, new Scalar(0), out outRect, new Scalar(0.1), new Scalar(1.0));
   }
   else
   {
      break;
   }
}

Answer 1

You're looking for Environment.ProcessorCount.