1// The "Square Detector" program.
2// It loads several images sequentially and tries to find squares in
3// each image
4
5#include "opencv2/core/core.hpp"
6#include "opencv2/imgproc/imgproc.hpp"
7#include "opencv2/imgcodecs.hpp"
8#include "opencv2/highgui/highgui.hpp"
9
10#include <iostream>
11#include <math.h>
12#include <string.h>
13
14using namespace cv;
15using namespace std;
16
17static void help()
18{
19    cout <<
20    "\nA program using pyramid scaling, Canny, contours, contour simpification and\n"
21    "memory storage (it's got it all folks) to find\n"
22    "squares in a list of images pic1-6.png\n"
23    "Returns sequence of squares detected on the image.\n"
24    "the sequence is stored in the specified memory storage\n"
25    "Call:\n"
26    "./squares\n"
27    "Using OpenCV version %s\n" << CV_VERSION << "\n" << endl;
28}
29
30
31int thresh = 50, N = 11;
32const char* wndname = "Square Detection Demo";
33
34// helper function:
35// finds a cosine of angle between vectors
36// from pt0->pt1 and from pt0->pt2
37static double angle( Point pt1, Point pt2, Point pt0 )
38{
39    double dx1 = pt1.x - pt0.x;
40    double dy1 = pt1.y - pt0.y;
41    double dx2 = pt2.x - pt0.x;
42    double dy2 = pt2.y - pt0.y;
43    return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
44}
45
46// returns sequence of squares detected on the image.
47// the sequence is stored in the specified memory storage
48static void findSquares( const Mat& image, vector<vector<Point> >& squares )
49{
50    squares.clear();
51
52    Mat pyr, timg, gray0(image.size(), CV_8U), gray;
53
54    // down-scale and upscale the image to filter out the noise
55    pyrDown(image, pyr, Size(image.cols/2, image.rows/2));
56    pyrUp(pyr, timg, image.size());
57    vector<vector<Point> > contours;
58
59    // find squares in every color plane of the image
60    for( int c = 0; c < 3; c++ )
61    {
62        int ch[] = {c, 0};
63        mixChannels(&timg, 1, &gray0, 1, ch, 1);
64
65        // try several threshold levels
66        for( int l = 0; l < N; l++ )
67        {
68            // hack: use Canny instead of zero threshold level.
69            // Canny helps to catch squares with gradient shading
70            if( l == 0 )
71            {
72                // apply Canny. Take the upper threshold from slider
73                // and set the lower to 0 (which forces edges merging)
74                Canny(gray0, gray, 0, thresh, 5);
75                // dilate canny output to remove potential
76                // holes between edge segments
77                dilate(gray, gray, Mat(), Point(-1,-1));
78            }
79            else
80            {
81                // apply threshold if l!=0:
82                //     tgray(x,y) = gray(x,y) < (l+1)*255/N ? 255 : 0
83                gray = gray0 >= (l+1)*255/N;
84            }
85
86            // find contours and store them all as a list
87            findContours(gray, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
88
89            vector<Point> approx;
90
91            // test each contour
92            for( size_t i = 0; i < contours.size(); i++ )
93            {
94                // approximate contour with accuracy proportional
95                // to the contour perimeter
96                approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
97
98                // square contours should have 4 vertices after approximation
99                // relatively large area (to filter out noisy contours)
100                // and be convex.
101                // Note: absolute value of an area is used because
102                // area may be positive or negative - in accordance with the
103                // contour orientation
104                if( approx.size() == 4 &&
105                    fabs(contourArea(Mat(approx))) > 1000 &&
106                    isContourConvex(Mat(approx)) )
107                {
108                    double maxCosine = 0;
109
110                    for( int j = 2; j < 5; j++ )
111                    {
112                        // find the maximum cosine of the angle between joint edges
113                        double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
114                        maxCosine = MAX(maxCosine, cosine);
115                    }
116
117                    // if cosines of all angles are small
118                    // (all angles are ~90 degree) then write quandrange
119                    // vertices to resultant sequence
120                    if( maxCosine < 0.3 )
121                        squares.push_back(approx);
122                }
123            }
124        }
125    }
126}
127
128
129// the function draws all the squares in the image
130static void drawSquares( Mat& image, const vector<vector<Point> >& squares )
131{
132    for( size_t i = 0; i < squares.size(); i++ )
133    {
134        const Point* p = &squares[i][0];
135        int n = (int)squares[i].size();
136        polylines(image, &p, &n, 1, true, Scalar(0,255,0), 3, LINE_AA);
137    }
138
139    imshow(wndname, image);
140}
141
142
143int main(int /*argc*/, char** /*argv*/)
144{
145    static const char* names[] = { "../data/pic1.png", "../data/pic2.png", "../data/pic3.png",
146        "../data/pic4.png", "../data/pic5.png", "../data/pic6.png", 0 };
147    help();
148    namedWindow( wndname, 1 );
149    vector<vector<Point> > squares;
150
151    for( int i = 0; names[i] != 0; i++ )
152    {
153        Mat image = imread(names[i], 1);
154        if( image.empty() )
155        {
156            cout << "Couldn't load " << names[i] << endl;
157            continue;
158        }
159
160        findSquares(image, squares);
161        drawSquares(image, squares);
162
163        int c = waitKey();
164        if( (char)c == 27 )
165            break;
166    }
167
168    return 0;
169}
170