How to compare two contours of a binary pattern image?

Question

I'm creating a part scanner in C that pulls all possibilities for scanned parts as images in a directory. My code currently fetches all images from that directory and dumps them into a vector. I then produce groups of contours for all the images. The program then falls into a while loop where it constantly grabs images from a webcam, and generates contours for those as well. I have set up a jig for the part to rest on, so orientation and size are not a concern, however I don't want to have to calibrate the machine, so there may be movement between the template images and the part images taken.

What is the best way to compare the contours? I have tried several methods including matchTemplate without contours, but if you take a look at the two parts below, you can see that these two are very close to each other, so matchShapes and matchTemplate can't distinguish between them the way I was using them. I'm also not sure how to use cvMatchShapes. It works with just loading the images directly into match shapes, but the results are inconclusive. I think that contours is the way to go, I'm just not sure of how to go about implementing the comparison phase. Any help would be great.

You can view the templates here: http://www.cryogendesign.com/partDetection.html"

possible duplicate of [OpenCV matching images](http://stackoverflow.com/questions/8893539/opencv-matching-images) — karlphillip, Apr 27 '12 at 18:39
Or this http://stackoverflow.com/questions/9444142/comparing-images-for-similarity — karlphillip, Apr 27 '12 at 18:40

score 1 · Answer 1 · answered May 07 '12 at 10:55

If you are ready for do-it-yourself, one approach could be to compute a "distance image" (assign every pixel the smallest Euclidean distance to the contour taken as the reference). See http://en.wikipedia.org/wiki/Distance_transform.

Using this distance image, you can quickly compute the average distance of a new contour to the reference one (for every contour pixel, get the distance from the distance image). The average distance gives you an indication of the goodness-of-fit and will let you find the best match to a set of reference templates.

If the parts have some moving freedom, the situation is a bit harder: before computing the average distance, you must fit the new contour to the reference one. You will need to apply a suitable transform (translation, rotation, possibly scaling), and find the parameters that will minimize... the average distance.

score 1 · Answer 2 · answered May 03 '13 at 18:32

You can calculate the chamfer distance between the two contours:

chamfer

T and E are the set of edges of the template and the image and x is the point of reference where you start to compare the two set of edges. So for each x you get a different value.

DT is the distance transform of an image. Matlab provides the algorithm here.

If you want a more detailed version of how to calculate the chamfer distance, take a look here.

How to compare two contours of a binary pattern image?

2 Answers2

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