What is CvMatchShapes returning?

Question

I have these two images.

small size

And... larger size

I made them into graysclae images and then passed the two images to cvMatchImageslike so

Double someValue = cvMatchShapes(grayImage, grayImage2, CV_CONTOURS_MATCH_I2, 0);
    System.out.println(someValue);

The value that was printed by sysout was 1.465496453484283. What is this value representing? How can i use it to match other images and also how can I reject those that do not?

score 2 · Accepted Answer · answered Mar 23 '12 at 06:26

2

the value means how different the two objects are,it represents the Hu moment feature distance between the two shapes. the smaller it is ,more similar the two objects are. A threshold can be used to perform the match like this: if output value is smaller than the threshold, two objects match, or the two objects differ. As to how to choose a threshold, it depends . you can use some instances to train the threshold. I strongly advise you to normalize objects in images to similar size.this will give you better accuracy

answered Mar 23 '12 at 06:26

zhijie

642
11
17

So the value that is being returned is a factor of translation? – prometheuspk Mar 23 '12 at 07:16
if these images were of same size what value will be returned? 0? – prometheuspk Mar 23 '12 at 07:40
You can say that, but it is a distance between features of the two objects, the distance may be Euclidian distance or something similar – zhijie Mar 23 '12 at 07:42
if input images were same , 0 should be returned , which means absolutely similar, i.e. the same – zhijie Mar 23 '12 at 07:44
I have tested the function,if two images were exactly the same , 0 will be returned.By the way, taken the two images you posted as input, cvMatchShapes outputs 0.001815 in my demo. – zhijie Mar 23 '12 at 07:53
did you use grayscale images or these exact images? – prometheuspk Mar 23 '12 at 10:10
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/9226/discussion-between-prometheus87-and-zhijie) – prometheuspk Mar 23 '12 at 10:10
the input image should be grayscale, in fact it will be binary during the process of cvMatchShapes – zhijie Mar 28 '12 at 08:18
1

It's interesting why OpenCV docs miss even this short description of cvMatchShapes function and its return value. – Valeriy Van Apr 21 '13 at 16:14

What is CvMatchShapes returning?

1 Answers1