Merge pull request #7960 from catree:tutorial_parallel_for_

Add OpenCV parallel_for_ tutorial.

doc/tutorials/core/table_of_content_core.markdown

@@ -106,3 +106,10 @@ understanding how to manipulate the images on a pixel level.
     *Author:* Elena Gvozdeva
 
     You will see how to use the IPP Async with OpenCV.
+
+
+-   @subpage tutorial_how_to_use_OpenCV_parallel_for_
+
+    *Compatibility:* \>= OpenCV 2.4.3
+
+    You will see how to use the OpenCV parallel_for_ to easily parallelize your code.

samples/cpp/tutorial_code/core/how_to_use_OpenCV_parallel_for_/how_to_use_OpenCV_parallel_for_.cpp

+#include <iostream>
+#include <opencv2/core.hpp>
+#include <opencv2/imgcodecs.hpp>
+
+using namespace std;
+using namespace cv;
+
+namespace
+{
+//! [mandelbrot-escape-time-algorithm]
+int mandelbrot(const complex<float> &z0, const int max)
+{
+    complex<float> z = z0;
+    for (int t = 0; t < max; t++)
+    {
+        if (z.real()*z.real() + z.imag()*z.imag() > 4.0f) return t;
+        z = z*z + z0;
+    }
+
+    return max;
+}
+//! [mandelbrot-escape-time-algorithm]
+
+//! [mandelbrot-grayscale-value]
+int mandelbrotFormula(const complex<float> &z0, const int maxIter=500) {
+    int value = mandelbrot(z0, maxIter);
+    if(maxIter - value == 0)
+    {
+        return 0;
+    }
+
+    return cvRound(sqrt(value / (float) maxIter) * 255);
+}
+//! [mandelbrot-grayscale-value]
+
+//! [mandelbrot-parallel]
+class ParallelMandelbrot : public ParallelLoopBody
+{
+public:
+    ParallelMandelbrot (Mat &img, const float x1, const float y1, const float scaleX, const float scaleY)
+        : m_img(img), m_x1(x1), m_y1(y1), m_scaleX(scaleX), m_scaleY(scaleY)
+    {
+    }
+
+    virtual void operator ()(const Range& range) const
+    {
+        for (int r = range.start; r < range.end; r++)
+        {
+            int i = r / m_img.cols;
+            int j = r % m_img.cols;
+
+            float x0 = j / m_scaleX + m_x1;
+            float y0 = i / m_scaleY + m_y1;
+
+            complex<float> z0(x0, y0);
+            uchar value = (uchar) mandelbrotFormula(z0);
+            m_img.ptr<uchar>(i)[j] = value;
+        }
+    }
+
+    ParallelMandelbrot& operator=(const ParallelMandelbrot &) {
+        return *this;
+    };
+
+private:
+    Mat &m_img;
+    float m_x1;
+    float m_y1;
+    float m_scaleX;
+    float m_scaleY;
+};
+//! [mandelbrot-parallel]
+
+//! [mandelbrot-sequential]
+void sequentialMandelbrot(Mat &img, const float x1, const float y1, const float scaleX, const float scaleY)
+{
+    for (int i = 0; i < img.rows; i++)
+    {
+        for (int j = 0; j < img.cols; j++)
+        {
+            float x0 = j / scaleX + x1;
+            float y0 = i / scaleY + y1;
+
+            complex<float> z0(x0, y0);
+            uchar value = (uchar) mandelbrotFormula(z0);
+            img.ptr<uchar>(i)[j] = value;
+        }
+    }
+}
+//! [mandelbrot-sequential]
+}
+
+int main()
+{
+    //! [mandelbrot-transformation]
+    Mat mandelbrotImg(4800, 5400, CV_8U);
+    float x1 = -2.1f, x2 = 0.6f;
+    float y1 = -1.2f, y2 = 1.2f;
+    float scaleX = mandelbrotImg.cols / (x2 - x1);
+    float scaleY = mandelbrotImg.rows / (y2 - y1);
+    //! [mandelbrot-transformation]
+
+    double t1 = (double) getTickCount();
+    //! [mandelbrot-parallel-call]
+    ParallelMandelbrot parallelMandelbrot(mandelbrotImg, x1, y1, scaleX, scaleY);
+    parallel_for_(Range(0, mandelbrotImg.rows*mandelbrotImg.cols), parallelMandelbrot);
+    //! [mandelbrot-parallel-call]
+    t1 = ((double) getTickCount() - t1) / getTickFrequency();
+    cout << "Parallel Mandelbrot: " << t1 << " s" << endl;
+
+    Mat mandelbrotImgSequential(4800, 5400, CV_8U);
+    double t2 = (double) getTickCount();
+    sequentialMandelbrot(mandelbrotImgSequential, x1, y1, scaleX, scaleY);
+    t2 = ((double) getTickCount() - t2) / getTickFrequency();
+    cout << "Sequential Mandelbrot: " << t2 << " s" << endl;
+    cout << "Speed-up: " << t2/t1 << " X" << endl;
+
+    imwrite("Mandelbrot_parallel.png", mandelbrotImg);
+    imwrite("Mandelbrot_sequential.png", mandelbrotImgSequential);
+
+    return EXIT_SUCCESS;
+}