opencv/doc/tutorials/viz/histo3D/histo3D.markdown

Creating a 3D histogram

Goal

In this tutorial you will learn how to

• Create your own callback keyboard function for viz window.
• Show your 3D histogram in a viz window.

Code

You can download the code from here . @include samples/cpp/tutorial_code/viz/histo3D.cpp

Explanation

Here is the general structure of the program:

• You can give full path to an image in command line @code{.cpp} CommandLineParser parser(argc, argv, keys);

  if (parser.has("help")) { parser.printMessage(); return 0; } String nomFic = parser.get(0); Mat img; if (nomFic.length() != 0) { img = imread(nomFic, IMREAD_COLOR); if (img.empty()) { cout << "Image does not exist!"; return 0; } } @endcode or without path, a synthetic image is generated with pixel values are a gaussian distribution center(60+/-10,40+/-5,50+/-20) in first quadrant, (160+/-20,10+/-5,50+/-10) in second quadrant, (90+/-10,100+/-20,50+/-20) in third quadrant, (100+/-10,10+/-5,150+/-40) in last quadrant. @code{.cpp} else { img = Mat(512,512,CV_8UC3); parser.printMessage(); RNG r; r.fill(img(Rect(0, 0, 256, 256)), RNG::NORMAL, Vec3b(60, 40, 50), Vec3b(10, 5, 20)); r.fill(img(Rect(256, 0, 256, 256)), RNG::NORMAL, Vec3b(160, 10, 50), Vec3b(20, 5, 10)); r.fill(img(Rect(0, 256, 256, 256)), RNG::NORMAL, Vec3b(90, 100, 50), Vec3b(10, 20, 20)); r.fill(img(Rect(256, 256, 256, 256)), RNG::NORMAL, Vec3b(100, 10, 150), Vec3b(10, 5, 40)); } @endcode Image tridimensional histogram is calculated using opencv calcHist and normalize between 0 and 100. @code{.cpp} float hRange[] = { 0, 256 }; const float* etendu[] = { hRange, hRange,hRange }; int hBins = 32; int tailleHist[] = { hBins, hBins , hBins }; int canaux[] = { 2, 1,0 }; calcHist(&img, 1, canaux, Mat(), h.histogram, 3, tailleHist, etendu, true, false); normalize(h.histogram, h.histogram, 100.0/(img.total()), 0, cv::NormTypes::NORM_MINMAX, -1, cv::Mat()); minMaxIdx(h.histogram,NULL,&h.maxH,NULL,NULL); @endcode channel are 2, 1 and 0 to synchronise color with Viz axis color in objetc viz::WCoordinateSystem.

  A slidebar is inserted in image window. Init slidebar value is 90, it means that only histogram cell greater than 9/100000.0 (23 pixels for an 512X512 pixels) will be display. @code{.cpp} namedWindow("Image"); imshow("Image",img); AddSlidebar("threshold","Image",0,100,h.seuil,&h.seuil, UpdateThreshold,&h); waitKey(30); @endcode We are ready to open a viz window with a callback function to capture keyboard event in viz window. Using Viz::spinonce enable keyboard event to be capture in imshow window too. @code{.cpp} h.fen3D = new viz::Viz3d("3D Histogram"); h.nbWidget=0; h.fen3D->registerKeyboardCallback(KeyboardViz3d,&h); DrawHistogram3D(h); while (h.code!=27) { h.fen3D->spinOnce(1); if (h.status) DrawHistogram3D(h); if (h.code!=27) h.code= waitKey(30); } @endcode The function DrawHistogram3D processes histogram Mat to display it in a Viz window. Number of plan, row and column in three dimensional Mat ca be found using this code : @code{.cpp} int planSize = h.histogram.step1(0); int cols = h.histogram.step1(1); int rows = planSize / cols; int plans = h.histogram.total() / planSize; h.fen3D->removeAllWidgets(); h.nbWidget=0; if (h.nbWidget==0) h.fen3D->showWidget("Axis", viz::WCoordinateSystem(10)); @endcode To get histogram value at a specific location we use at method with three arguments k, i and j where k is plane number, i row number and j column number. @code{.cpp} for (int k = 0; k < plans; k++) { for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) { double x = h.histogram.at(k, i, j); if (x >= h.threshold) { double r=std::max(x/h.maxH,0.1); viz::WCube s(Point3d(k - r / 2, i - r / 2, j - r / 2), Point3d(k + r / 2, i + r / 2, j + r / 2), false, viz::Color(j / double(plans) * 255, i / double(rows) * 255, k / double(cols) * 255)); h.fen3D->showWidget(format("I3d%d", h.nbWidget++), s); } } } } @endcode

• Callback function Principle are as mouse callback function. Key code pressed is in field code of class viz::KeyboardEvent. @code{.cpp} void KeyboardViz3d(const viz::KeyboardEvent &w, void *t) { Histo3DData *x=(Histo3DData *)t; if (w.action) cout << "you pressed "<< w.symbol<< " in viz window "<fen3D->getWindowName()<<"\n"; x->code= w.code; switch (w.code) { case '/': x->status=true; x->threshold = 0.9; break; case '': x->status = true; x->threshold *= 1.1; break;

  }
  

  if (x->status) { cout << x->threshold << "\n"; DrawHistogram3D(*x); } } @endcode

Results

Here is the result of the program with no argument and threshold equal to 50.