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_UNCHANGED)
    w,h = cv.GetSize(im)
    W,H = cv.GetSize(tmp)
    width = w-W+1
    height = h-H+1
    result = cv.CreateImage((width, height), 32, 1)
    cv.MatchTemplate(im, tmp, result, cv.CV_TM_SQDIFF)
    print result

When I run this, everything executes just fine, no errors get thrown. But I'm unsure what to do from here. The doc says that result stores "A map of comparison results". I tried printing it, but it gives me width, height, and step.

How do I use this information to find whether or not one image is in another/where it is located?

So you don't struggle with OpenCV as much, try SimpleCV. It's a wrapper around the OpenCV modules that makes them easy to use: http://simplecv.org/ — Blender, Mar 14 '12 at 20:21

score 13 · Answer 1 · answered Jan 19 '13 at 18:56

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! :)

Note that this will always return x, y irregardless of whether the template image is actually present, I recommend using a threshold instead. — Alexis Drakopoulos, Dec 27 '20 at 13:42

score 12 · Accepted Answer · answered Mar 15 '12 at 08:59

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.

So if I was using a method that records the best match at maxLoc I would look at maxVal? — Lightsout, Apr 22 '16 at 23:42

How do I use OpenCV MatchTemplate?

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