core: parallel backends API

- allow to replace parallel_for() backend

samples/cpp/CMakeLists.txt

@@ -30,6 +30,7 @@ if(NOT HAVE_opencv_cudaarithm OR NOT HAVE_opencv_cudafilters)
   ocv_list_filterout(cpp_samples "/gpu/")
 endif()
 ocv_list_filterout(cpp_samples "real_time_pose_estimation/")
+ocv_list_filterout(cpp_samples "parallel_backend/")
 foreach(sample_filename ${cpp_samples})
   set(package "cpp")
   if(sample_filename MATCHES "tutorial_code")
@@ -57,3 +58,11 @@ foreach(sample_filename ${cpp_samples})
 endforeach()
 
 include("tutorial_code/calib3d/real_time_pose_estimation/CMakeLists.txt" OPTIONAL)
+
+# Standalone samples only
+if(OpenCV_FOUND AND NOT CMAKE_VERSION VERSION_LESS "3.1")
+  add_subdirectory("example_cmake")
+endif()
+if(OpenCV_FOUND AND NOT CMAKE_VERSION VERSION_LESS "3.9")
+  add_subdirectory("tutorial_code/core/parallel_backend")
+endif()

samples/cpp/tutorial_code/core/parallel_backend/CMakeLists.txt

@@ -0,0 +1,26 @@
+cmake_minimum_required(VERSION 3.9)
+
+find_package(OpenCV REQUIRED COMPONENTS opencv_core)
+
+find_package(OpenMP)
+if(OpenMP_FOUND)
+  project(opencv_example_openmp_backend)
+  add_executable(opencv_example_openmp_backend example-openmp.cpp)
+  target_link_libraries(opencv_example_openmp_backend PRIVATE
+      opencv_core
+      OpenMP::OpenMP_CXX
+  )
+endif()
+
+# TODO: find_package(TBB)
+find_path(TBB_INCLUDE_DIR NAMES "tbb/tbb.h")
+find_library(TBB_LIBRARY NAMES "tbb")
+if(TBB_INCLUDE_DIR AND TBB_LIBRARY AND NOT OPENCV_EXAMPLE_SKIP_TBB)
+  project(opencv_example_tbb_backend)
+  add_executable(opencv_example_tbb_backend example-tbb.cpp)
+  target_include_directories(opencv_example_tbb_backend SYSTEM PRIVATE ${TBB_INCLUDE_DIR})
+  target_link_libraries(opencv_example_tbb_backend PRIVATE
+      opencv_core
+      ${TBB_LIBRARY}
+  )
+endif()

samples/cpp/tutorial_code/core/parallel_backend/example-openmp.cpp

@@ -0,0 +1,44 @@
+#include "opencv2/core.hpp"
+#include <iostream>
+
+#include <chrono>
+#include <thread>
+
+//! [openmp_include]
+#include "opencv2/core/parallel/backend/parallel_for.openmp.hpp"
+//! [openmp_include]
+
+namespace cv { // private.hpp
+CV_EXPORTS const char* currentParallelFramework();
+}
+
+static
+std::string currentParallelFrameworkSafe()
+{
+    const char* framework = cv::currentParallelFramework();
+    if (framework)
+        return framework;
+    return std::string();
+}
+
+using namespace cv;
+int main()
+{
+    std::cout << "OpenCV builtin parallel framework: '" << currentParallelFrameworkSafe() << "' (nthreads=" << getNumThreads() << ")" << std::endl;
+
+    //! [openmp_backend]
+    //omp_set_dynamic(1);
+    cv::parallel::setParallelForBackend(std::make_shared<cv::parallel::openmp::ParallelForBackend>());
+    //! [openmp_backend]
+
+    std::cout << "New parallel backend: '" << currentParallelFrameworkSafe() << "'" << "' (nthreads=" << getNumThreads() << ")" << std::endl;
+
+    parallel_for_(Range(0, 20), [&](const Range range)
+    {
+        std::ostringstream out;
+        out << "Thread " << getThreadNum() << "(opencv=" << utils::getThreadID() << "): range " << range.start << "-" << range.end << std::endl;
+        std::cout << out.str() << std::flush;
+
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    });
+}

samples/cpp/tutorial_code/core/parallel_backend/example-tbb.cpp

@@ -0,0 +1,43 @@
+#include "opencv2/core.hpp"
+#include <iostream>
+
+#include <chrono>
+#include <thread>
+
+//! [tbb_include]
+#include "opencv2/core/parallel/backend/parallel_for.tbb.hpp"
+//! [tbb_include]
+
+namespace cv { // private.hpp
+CV_EXPORTS const char* currentParallelFramework();
+}
+
+static
+std::string currentParallelFrameworkSafe()
+{
+    const char* framework = cv::currentParallelFramework();
+    if (framework)
+        return framework;
+    return std::string();
+}
+
+using namespace cv;
+int main()
+{
+    std::cout << "OpenCV builtin parallel framework: '" << currentParallelFrameworkSafe() << "' (nthreads=" << getNumThreads() << ")" << std::endl;
+
+    //! [tbb_backend]
+    cv::parallel::setParallelForBackend(std::make_shared<cv::parallel::tbb::ParallelForBackend>());
+    //! [tbb_backend]
+
+    std::cout << "New parallel backend: '" << currentParallelFrameworkSafe() << "'" << "' (nthreads=" << getNumThreads() << ")" << std::endl;
+
+    parallel_for_(Range(0, 20), [&](const Range range)
+    {
+        std::ostringstream out;
+        out << "Thread " << getThreadNum() << "(opencv=" << utils::getThreadID() << "): range " << range.start << "-" << range.end << std::endl;
+        std::cout << out.str() << std::flush;
+
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    });
+}