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 <opencv2/opencv.hpp>
#include <iostream>

using namespace std;
using namespace cv;

#ifdef HAVE_OPENCV_VIZ

const String keys =
"{Aide h usage ? help  |     | print this message   }"
"{@arg1                |     | Full path to color imag (3 channels)}"
;


struct Histo3DData {
    Mat histogram;
    int seuil;
    double threshold;
    Ptr<viz::Viz3d> fen3D;
    int nbWidget;
    bool status;
    double maxH;
    int code;
};

void DrawHistogram3D(Histo3DData &);
void AddSlidebar(String sliderName, String windowName, int sliderMin, int sliderMax, int valeurDefaut, int *sliderVal, void(*f)(int, void *), void *r);
void UpdateThreshold(int , void * r);
void  KeyboardViz3d(const viz::KeyboardEvent &w, void *t);


void DrawHistogram3D(Histo3DData &h)
{
    int planSize = (int)h.histogram.step1(0);
    int cols = (int)h.histogram.step1(1);
    int rows = (int)planSize / cols;
    int plans = (int)h.histogram.total() / planSize;
    h.fen3D->removeAllWidgets();
    h.nbWidget=0;
    if (h.nbWidget==0)
        h.fen3D->showWidget("Axis", viz::WCoordinateSystem(10));
    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<float>(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);
                }
            }
        }
    }
    h.status = false;
}
void  KeyboardViz3d(const viz::KeyboardEvent &w, void *t)
{
   Histo3DData *x=(Histo3DData *)t;
   if (w.action)
       cout << "you pressed "<< w.symbol<< " in viz window "<<x->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);
   }
}


void AddSlidebar(String sliderName, String windowName, int sliderMin, int sliderMax, int defaultSlider, int *sliderVal, void(*f)(int, void *), void *r)
{
    createTrackbar(sliderName, windowName, sliderVal, 1, f, r);
    setTrackbarMin(sliderName, windowName, sliderMin);
    setTrackbarMax(sliderName, windowName, sliderMax);
    setTrackbarPos(sliderName, windowName, defaultSlider);
}


void UpdateThreshold(int , void * r)
{
    Histo3DData *h = (Histo3DData *)r;
    h->status=true;
    h->threshold = h->seuil/1000000.0;
    cout<<"Widget : "<<h->nbWidget<<","<< h->threshold<<"\n";
}

int main (int argc,char **argv)
{
    CommandLineParser parser(argc, argv, keys);

    if (parser.has("help"))
    {
        parser.printMessage();
        return 0;
    }
    String nomFic = parser.get<String>(0);
    Mat img;
    if (nomFic.length() != 0)
    {
        img = imread(nomFic, IMREAD_COLOR);
        if (img.empty())
        {
            cout << "Image does not exist!";
            return 0;
        }
    }
    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));
    }
    Histo3DData h;
    h.status=true;
    h.seuil=90;
    h.threshold= h.seuil/1000000.0;
    float hRange[] = { 0, 256 };
    const float* etendu[] = { hRange, hRange,hRange };
    int hBins = 32;
    int histSize[] = { hBins, hBins , hBins  };
    int channel[] = { 2, 1,0 };
    calcHist(&img, 1, channel, Mat(), h.histogram, 3, histSize, etendu, true, false);
    normalize(h.histogram, h.histogram, 100.0/(img.total()), 0, NORM_MINMAX, -1, Mat());
    minMaxIdx(h.histogram,NULL,&h.maxH,NULL,NULL);
    namedWindow("Image");
    imshow("Image",img);
    AddSlidebar("threshold","Image",0,100,h.seuil,&h.seuil, UpdateThreshold,&h);
    waitKey(30);
    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);
    }
    return 0;
}
#else

int main(int argc, char **argv)
{
cout << " you need VIZ module\n";
return 0;
}
#endif

Explanation

Here is the general structure of the program:

• You can give full path to an image in command line

      CommandLineParser parser(argc, argv, keys);
  
      if (parser.has("help"))
      {
          parser.printMessage();
          return 0;
      }
      String nomFic = parser.get<String>(0);
      Mat img;
      if (nomFic.length() != 0)
      {
          img = imread(nomFic, IMREAD_COLOR);
          if (img.empty())
          {
              cout << "Image does not exist!";
              return 0;
          }
      }

  or without path, a synthetic image is generated with pixel values are a gaussian distribution cv::RNG::fill 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.

      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));
      }

  Image tridimensional histogram is calculated using opencv cv::calcHist and cv::normalize between 0 and 100.

      Histo3DData h;
      h.status=true;
      h.seuil=90;
      h.threshold= h.seuil/1000000.0;
      float hRange[] = { 0, 256 };
      const float* etendu[] = { hRange, hRange,hRange };
      int hBins = 32;
      int histSize[] = { hBins, hBins , hBins  };
      int channel[] = { 2, 1,0 };
      calcHist(&img, 1, channel, Mat(), h.histogram, 3, histSize, etendu, true, false);
      normalize(h.histogram, h.histogram, 100.0/(img.total()), 0, NORM_MINMAX, -1, Mat());
      minMaxIdx(h.histogram,NULL,&h.maxH,NULL,NULL);

  channel are 2, 1 and 0 to synchronise color with Viz axis color in objetc cv::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.

      namedWindow("Image");
      imshow("Image",img);
      AddSlidebar("threshold","Image",0,100,h.seuil,&h.seuil, UpdateThreshold,&h);
      waitKey(30);

  We are ready to open a viz window with a callback function to capture keyboard event in viz window. Using cv::viz::Viz3d::spinOnce enable keyboard event to be capture in cv::imshow window too.

      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);
      }

  The function DrawHistogram3D processes histogram Mat to display it in a Viz window. Number of plan, row and column in three dimensional Mat can be found using this code :

      int planSize = (int)h.histogram.step1(0);
      int cols = (int)h.histogram.step1(1);
      int rows = (int)planSize / cols;
      int plans = (int)h.histogram.total() / planSize;
      h.fen3D->removeAllWidgets();
      h.nbWidget=0;
      if (h.nbWidget==0)
          h.fen3D->showWidget("Axis", viz::WCoordinateSystem(10));

  To get histogram value at a specific location we use cv::Mat::at(int i0,int i1, int i2) method with three arguments k, i and j where k is plane number, i row number and j column number.

      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<float>(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);
                  }
              }
          }
      }

• Callback function Principle are as mouse callback function. Key code pressed is in field code of class cv::viz::KeyboardEvent.

  void  KeyboardViz3d(const viz::KeyboardEvent &w, void *t)
  {
     Histo3DData *x=(Histo3DData *)t;
     if (w.action)
         cout << "you pressed "<< w.symbol<< " in viz window "<<x->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);
     }
  }

  Results

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