samples/cpp/squares.cpp


A program using pyramid scaling, Canny, contours and contour simplification to find squares in a list of images (pic1-6.png). Returns sequence of squares detected on the image.

// The "Square Detector" program.
// It loads several images sequentially and tries to find squares in
// each image
#include " opencv2/core.hpp "
#include <iostream>
using namespace cv ;
using namespace std ;
static void help( const char * programName)
{
cout <<
"\nA program using pyramid scaling, Canny, contours and contour simplification\n"
"to find squares in a list of images (pic1-6.png)\n"
"Returns sequence of squares detected on the image.\n"
"Call:\n"
"./" << programName << " [file_name (optional)]\n"
"Using OpenCV version " << CV_VERSION << "\n" << endl;
}
int thresh = 50, N = 11;
const char * wndname = "Square Detection Demo" ;
// helper function:
// finds a cosine of angle between vectors
// from pt0->pt1 and from pt0->pt2
static double angle( Point pt1, Point pt2, Point pt0 )
{
double dx1 = pt1. x - pt0. x ;
double dy1 = pt1. y - pt0. y ;
double dx2 = pt2. x - pt0. x ;
double dy2 = pt2. y - pt0. y ;
return (dx1*dx2 + dy1*dy2)/ sqrt ((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
// returns sequence of squares detected on the image.
static void findSquares( const Mat & image, vector<vector<Point> >& squares )
{
squares.clear();
Mat pyr, timg, gray0(image. size (), CV_8U ), gray;
// down-scale and upscale the image to filter out the noise
pyrDown (image, pyr, Size (image. cols /2, image. rows /2));
pyrUp (pyr, timg, image. size ());
vector<vector<Point> > contours;
// find squares in every color plane of the image
for ( int c = 0; c < 3; c++ )
{
int ch[] = {c, 0};
mixChannels (&timg, 1, &gray0, 1, ch, 1);
// try several threshold levels
for ( int l = 0; l < N; l++ )
{
// hack: use Canny instead of zero threshold level.
// Canny helps to catch squares with gradient shading
if ( l == 0 )
{
// apply Canny. Take the upper threshold from slider
// and set the lower to 0 (which forces edges merging)
Canny (gray0, gray, 0, thresh, 5);
// dilate canny output to remove potential
// holes between edge segments
dilate (gray, gray, Mat (), Point (-1,-1));
}
else
{
// apply threshold if l!=0:
// tgray(x,y) = gray(x,y) < (l+1)*255/N ? 255 : 0
gray = gray0 >= (l+1)*255/N;
}
// find contours and store them all as a list
vector<Point> approx;
// test each contour
for ( size_t i = 0; i < contours.size(); i++ )
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP (contours[i], approx, arcLength (contours[i], true )*0.02, true );
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if ( approx.size() == 4 &&
fabs( contourArea (approx)) > 1000 &&
{
double maxCosine = 0;
for ( int j = 2; j < 5; j++ )
{
// find the maximum cosine of the angle between joint edges
double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
maxCosine = MAX (maxCosine, cosine);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if ( maxCosine < 0.3 )
squares.push_back(approx);
}
}
}
}
}
// the function draws all the squares in the image
static void drawSquares( Mat & image, const vector<vector<Point> >& squares )
{
for ( size_t i = 0; i < squares.size(); i++ )
{
const Point * p = &squares[i][0];
int n = (int)squares[i].size();
polylines (image, &p, &n, 1, true , Scalar (0,255,0), 3, LINE_AA );
}
imshow (wndname, image);
}
int main( int argc, char ** argv)
{
static const char * names[] = { "pic1.png" , "pic2.png" , "pic3.png" ,
"pic4.png" , "pic5.png" , "pic6.png" , 0 };
help(argv[0]);
if ( argc > 1)
{
names[0] = argv[1];
names[1] = "0" ;
}
vector<vector<Point> > squares;
for ( int i = 0; names[i] != 0; i++ )
{
string filename = samples::findFile (names[i]);
Mat image = imread (filename, IMREAD_COLOR );
if ( image. empty () )
{
cout << "Couldn't load " << filename << endl;
continue ;
}
findSquares(image, squares);
drawSquares(image, squares);
int c = waitKey ();
if ( c == 27 )
break ;
}
return 0;
}