Extracting geometric forms with opencv - c++

Question

I running a video in which I want to detect 3 geometric forms triangle circle and pentagon, and nothing else, here a frame from this video and the result that I get :
the correct result
One bad result from many.
and here's my code:
img =src;
cv::Moments mom;
cv::Mat result(img.size(),CV_8U,cv::Scalar(255));
cv::threshold(img,img,127,255,CV_THRESH_BINARY_INV);
cv::findContours(img,contours,/*hiararchy,*/CV_RETR_LIST, CV_CHAIN_APPROX_NONE );
for ( int i=0; i<contours.size();i++){
cv::approxPolyDP(cv::Mat(contours[i]),approx,(cv::arcLength(cv::Mat(contours[i]),true)*.02),true);
switch(approx.size()){
case 8: // should be a circle
cv::minEnclosingCircle(cv::Mat(contours[i]),center,radius);
cv::circle(src,cv::Point(center),static_cast<int> (radius),cv::Scalar(255,255,0),3,3);
mom= cv::moments(cv::Mat(contours[i]));
// draw mass center
cv::circle(result,
// position of mass center converted to integer
cv::Point(mom.m10/mom.m00,mom.m01/mom.m00),
2,cv::Scalar(0,0,255),2);// draw black dot
break;
case 3: // should be a triangle
poly.clear();
cv::approxPolyDP(cv::Mat(contours[i]),poly,
5, // accuracy of the approximation
true); // yes it is a closed shape
// Iterate over each segment and draw it
itp= poly.begin();
while (itp!=(poly.end()-1)) {
cv::line(src,*itp,*(itp+1),cv::Scalar(0,255,255),2);
++itp;
}
// last point linked to first point
cv::line(src,*(poly.begin()),*(poly.end()-1),cv::Scalar(100,255,100),2);
mom= cv::moments(cv::Mat(contours[i]));
// draw mass center
cv::circle(result,
// position of mass center converted to integer
cv::Point(mom.m10/mom.m00,mom.m01/mom.m00),
2,cv::Scalar(0,0,255),2);// draw black dot
break;
case 5 :// should be a pentagon
poly.clear();
cv::approxPolyDP(cv::Mat(contours[i]),poly,
5, // accuracy of the approximation
true); // yes it is a closed shape
// Iterate over each segment and draw it
itp= poly.begin();
while (itp!=(poly.end()-1)) {
cv::line(src,*itp,*(itp+1),cv::Scalar(0,0,255),2);
++itp;
}
// last point linked to first point
cv::line(src,*(poly.begin()),*(poly.end()-1),cv::Scalar(255,0,0),2);
mom= cv::moments(cv::Mat(contours[i]));
// draw mass center
cv::circle(result,
// position of mass center converted to integer
cv::Point(mom.m10/mom.m00,mom.m01/mom.m00),
2,cv::Scalar(0,0,255),2);// draw black dot
break;
default :
contours[i].clear();
}
// iterate over all contours
int j = 0;
for( int i = 0; i < contours.size();i++) {
if ( !contours[i].empty()){
// compute all moments
j++; // At the end j should be 3
}
if(j ==3 ){
cv::drawContours(result,contours,-1, // draw all contours
cv::Scalar(0), // in black
2); // with a thickness of 2
cv::imshow("result",result);
}
} std::cout<<j<<std::endl;
return j;
}
Any idea how to solve this ?
thanks!

Answer 1

If the geometric shapes are always located within such a checkerboard pattern and the shapes have same size and orientation within that pattern all the time, I would:
detect the checkerboard pattern.
remove the perspective distortion with a homography
scale and rotate so that the checkerboard pattern is axis aligned and at a given scale
use chamfer matching to detect the specified shapes.