OpenCV matchShapes() output value - ios

Question

How do I use value from OpenCV matchShapes output? We implemented OpenCV matchShapes function to compare two images, particularly, shapes. But when we obtained the answer we are confused how to use these values?
The code is
- (bool) someMethod:(UIImage *)image :(UIImage *)temp {
RNG rng(12345);
cv::Mat src_base, hsv_base;
cv::Mat src_test1, hsv_test1;
src_base = [self cvMatWithImage:image];
src_test1 = [self cvMatWithImage:temp];
int thresh=150;
double ans=0, result=0;
Mat imageresult1, imageresult2;
cv::cvtColor(src_base, hsv_base, cv::COLOR_BGR2HSV);
cv::cvtColor(src_test1, hsv_test1, cv::COLOR_BGR2HSV);
std::vector<std::vector<cv::Point>>contours1, contours2;
std::vector<Vec4i>hierarchy1, hierarchy2;
Canny(hsv_base, imageresult1, thresh, thresh*2);
Canny(hsv_test1, imageresult2, thresh, thresh*2);
findContours(imageresult1,contours1,hierarchy1,CV_RETR_TREE,CV_CHAIN_APPROX_SIMPLE,cvPoint(0,0));
for(int i=0;i<contours1.size();i++)
{
//cout<<contours1[i]<<endl;
Scalar color=Scalar(rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255));
drawContours(imageresult1,contours1,i,color,1,8,hierarchy1,0,cv::Point());
}
findContours(imageresult2,contours2,hierarchy2,CV_RETR_TREE,CV_CHAIN_APPROX_SIMPLE,cvPoint(0,0));
for(int i=0;i<contours2.size();i++)
{
Scalar color=Scalar(rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255));
drawContours(imageresult2,contours2,i,color,1,8,hierarchy2,0,cv::Point());
}
for(int i=0;i<contours1.size();i++)
{
ans = matchShapes(contours1[i],contours2[i],CV_CONTOURS_MATCH_I1,0);
cout<<" "<<ans<<endl;
}
std::cout<<"The answer is "<<ans<<endl;
if (ans<=20) {
return true;
}
return false;
}
The output values are
0.225069
0.234417
0
7.63599
0
7.06392
0.335966
0.211358
0.327552
0.842969
0.761659
0.614039
The image is

Answer 1

See my comment on imoutidi's answer. Here is a visual explanation:
The first col are the two original images,the second the canny edges. The 3. col are an arbitrary selection of detected shapes with the same index in both images. As you see, it is not even guaranteed that they correspond to the same image parts as a human would see them. What you end up comparing are different triangles in this case, which say little about the overall shape similarity. The two shape arrays are not even of the same size, since there are more structures in the bottom drawing for example(like small shapes between a thick line). in The 4. col is the last shape in the array. This is the best bet you can make to compare the images. In this example, I get a value of 0.0920794532771 for their similarity.

Answer 2

If I understand correctly your question, you want to know what the return value of matchShapes() stands for.
In your case given the two contours (shapes) the function returns a similarity metric (value). A small value indicates that the two shapes are similar and a big value that they are not.
A good explanation is here: http://docs.opencv.org/3.1.0/d5/d45/tutorial_py_contours_more_functions.html (check the third paragraph).
Also check out the documentation: http://docs.opencv.org/3.1.0/d3/dc0/group__imgproc__shape.html#gaadc90cb16e2362c9bd6e7363e6e4c317