Creating a 3D histogram {#tutorial_histo3D}

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 ](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/viz/histo3D.cpp).

@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

@snippet histo3D.cpp command_line_parser

or without path, a synthetic image is generated with pixel values are a gaussian distribution @ref 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.

@snippet histo3D.cpp synthetic_image

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

@snippet histo3D.cpp calchist_for_histo3d

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.

@snippet histo3D.cpp slide_bar_for_thresh

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

@snippet histo3D.cpp manage_viz_imshow_window

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

@snippet histo3D.cpp get_cube_size

To get histogram value at a specific location we use @ref 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.

@snippet histo3D.cpp get_cube_values

- Callback function

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

@snippet histo3D.cpp viz_keyboard_callback

Results

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

- @subpage tutorial_histo3D

*Compatibility:* \> OpenCV 3.0.0

*Author:* Laurent Berger

You will learn how to plot a 3D histogram.

/** @brief n-dimensional dense array class \anchor CVMat_Details

#include <opencv2/opencv.hpp>

#include <iostream>

using namespace std;

using namespace cv;

#ifdef HAVE_OPENCV_VIZ

const String keys =

;

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)

{

//! [get_cube_size]

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));

//! [get_cube_size]

//! [get_cube_values]

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);

}

}

}

}

//! [get_cube_values]

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)

{

//! [command_line_parser]

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;

}

}

//! [command_line_parser]

//! [synthetic_image]

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));

}

//! [synthetic_image]

//! [calchist_for_histo3d]

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);

//! [calchist_for_histo3d]

//! [slide_bar_for_thresh]

namedWindow("Image");

imshow("Image",img);

AddSlidebar("threshold","Image",0,100,h.seuil,&h.seuil, UpdateThreshold,&h);

waitKey(30);

//! [slide_bar_for_thresh]

//! [manage_viz_imshow_window]

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);

}

//! [manage_viz_imshow_window]

return 0;

}

#else

int main(int argc, char **argv)

{

cout << " you need VIZ module\n";

return 0;

}

#endif