A working solution

integral_image.h

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+#pragma once
+
+#include <opencv2/opencv.hpp>
+
+
+/*! \file integral_image.h
+    \brief The file provides functions that calculate an integral image.
+*/
+
+
+
+namespace integral_image {
+
+
+using Mat = cv::Mat_<double>;
+
+//! A reference serial implementation of the integral image algorithm.
+/*!
+  \param image an input 1-channel image.
+  \return The integral image of the input
+  \sa integral_image_openmp
+*/
+Mat integral_image_serial(const Mat &image);
+
+//! An OpenMP-accelerated function that calculates an integral image.
+/*!
+  \param image an input 1-channel image.
+  \param thread_number number of worker threads. If \p thread_number is equal to 0, the threads are created dynamically. Defaults to 0.
+  \return The integral image of the input
+  \sa integral_image_serial
+*/
+Mat integral_image_openmp(const Mat &image, int thread_number = 0);
+
+}