How do I use OpenCV MatchTemplate?

Question

I\'m attempting to find an image in another.

im = cv.LoadImage(\'1.png\', cv.CV_LOAD_IMAGE_UNCHANGED)
    tmp = cv.LoadImage(\'e1.png\', cv.CV_LOAD_IMAGE_UN         


        

Answer 1

This might work for you! :)

def FindSubImage(im1, im2):
    needle = cv2.imread(im1)
    haystack = cv2.imread(im2)

    result = cv2.matchTemplate(needle,haystack,cv2.TM_CCOEFF_NORMED)
    y,x = np.unravel_index(result.argmax(), result.shape)
    return x,y

CCOEFF_NORMED is just one of many comparison methoeds. See: http://docs.opencv.org/doc/tutorials/imgproc/histograms/template_matching/template_matching.html for full list.

Not sure if this is the best method, but is fast, and works just fine for me! :)

Answer 2

MatchTemplate returns a similarity map and not a location. You can then use this map to find a location.

If you are only looking for a single match you could do something like this to get a location:

minVal,maxVal,minLoc,maxLoc = cv.MinMaxLoc(result)

Then minLoc has the location of the best match and minVal describes how well the template fits. You need to come up with a threshold for minVal to determine whether you consider this result a match or not.

If you are looking for more than one match per image you need to use algorithms like non-maximum supression.