[OpenCV Tutorial] $11 - Creating Bounding boxes and circles for contours

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::boundingRect
• Use the OpenCV function cv::minEnclosingCircle

Theory

Code

This tutorial code's is shown lines below. You can also download it from here

#include "opencv2/imgcodecs.hpp"

#include "opencv2/highgui.hpp"

#include "opencv2/imgproc.hpp"

#include <iostream>

using namespace cv;

using namespace std;

Mat src; Mat src_gray;

int thresh = 100;

int max_thresh = 255;

RNG rng(12345);

void thresh_callback(int, void* );

int main( int, char** argv )

{

src = imread( argv[1], IMREAD_COLOR );

cvtColor( src, src_gray, COLOR_BGR2GRAY );

blur( src_gray, src_gray, Size(3,3) );

const char* source_window = "Source";

namedWindow( source_window, WINDOW_AUTOSIZE );

imshow( source_window, src );

createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );

thresh_callback( 0, 0 );

waitKey(0);

return(0);

}

void thresh_callback(int, void* )

{

Mat threshold_output;

vector<vector<Point> > contours;

vector<Vec4i> hierarchy;

threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );

findContours( threshold_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );

vector<vector<Point> > contours_poly( contours.size() );

vector<Rect> boundRect( contours.size() );

vector<Point2f>center( contours.size() );

vector<float>radius( contours.size() );

for( size_t i = 0; i < contours.size(); i++ )

{

approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );

boundRect[i] = boundingRect( Mat(contours_poly[i]) );

minEnclosingCircle( contours_poly[i], center[i], radius[i] );

}

Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );

for( size_t i = 0; i< contours.size(); i++ )

{

Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );

drawContours( drawing, contours_poly, (int)i, color, 1, 8, vector<Vec4i>(), 0, Point() );

rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );

circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );

}

namedWindow( "Contours", WINDOW_AUTOSIZE );

imshow( "Contours", drawing );

}

Explanation

The main function is rather simple, as follows from the comments we do the following:

1. Open the image, convert it into grayscale and blur it to get rid of the noise.
   src = imread( argv[1], IMREAD_COLOR );

   cvtColor( src, src_gray, COLOR_BGR2GRAY );

   blur( src_gray, src_gray, Size(3,3) );
2. Create a window with header "Source" and display the source file in it.
   const char* source_window = "Source";

   namedWindow( source_window, WINDOW_AUTOSIZE );

   imshow( source_window, src );
3. Create a trackbar on the source_window and assign a callback function to it In general callback functions are used to react to some kind of signal, in our case it's trackbar's state change.
   createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );
4. Explicit one-time call of thresh_callback is necessary to display the "Contours" window simultaniously with the "Source" window.
   thresh_callback( 0, 0 );
5. Wait for user to close the windows.
   waitKey(0);

The callback function thresh_callback does all the interesting job.

1. Writes to threshold_output the threshold of the grayscale picture (you can check out about thresholding here).
   threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
2. Finds contours and saves them to the vectors contour and hierarchy.
   findContours( threshold_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );
3. For every found contour we now apply approximation to polygons with accuracy +-3 and stating that the curve must me closed.
   After that we find a bounding rect for every polygon and save it to boundRect.
   At last we find a minimum enclosing circle for every polygon and save it to center and radius vectors.
   for( size_t i = 0; i < contours.size(); i++ )

   {

   approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );

   boundRect[i] = boundingRect( Mat(contours_poly[i]) );

   minEnclosingCircle( contours_poly[i], center[i], radius[i] );

   }
   We found everything we need, all we have to do is to draw.
4. Create new Mat of unsigned 8-bit chars, filled with zeros. It will contain all the drawings we are going to make (rects and circles).
   Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
5. For every contour: pick a random color, draw the contour, the bounding rectangle and the minimal enclosing circle with it,
   for( size_t i = 0; i< contours.size(); i++ )

   {

   Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );

   drawContours( drawing, contours_poly, (int)i, color, 1, 8, vector<Vec4i>(), 0, Point() );

   rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );

   circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );

   }
6. Display the results: create a new window "Contours" and show everything we added to drawings on it.
   namedWindow( "Contours", WINDOW_AUTOSIZE );

   imshow( "Contours", drawing );

   Result

Here it is: