python - cv2.matchTemplate finds wrong template in image

Question

I'm trying to create a program that knows what number is on an image with the following function:

def img_in_img(big_picture, small_picture, tamper):
    big_picture = str(big_picture)
    small_picture = str(small_picture)
    if os.path.isfile(big_picture) == False or os.path.isfile(small_picture) == False:
        return "Image does not exist"

    img = cv2.imread(big_picture,0)
    templace_loc = small_picture
    template = cv2.imread(templace_loc,0)
    w, h = template.shape[::-1]
    method = cv2.TM_CCOEFF

    tamper = int(tamper)

    res = cv2.matchTemplate(img,template,method)
    min_val, max_val, min_loc, max_loc = cv2.minMaxLoc(res)

    top_left = max_loc
    bottom_right = (top_left[0] + w, top_left[1] + h)

    height, width, channels = cv2.imread(templace_loc).shape

    if int(top_left[0]) < int(height + tamper) and int(top_left[0]) > int(height - tamper) and int(top_left[1]) < int(width + tamper) and int(top_left[1]) > int(width - tamper):
        return True
    else:
        return False

But then when I check if 7.png is in img.png with the code

nur = "7"
if img_in_img("2020-01-14-17-36-08.537043/verification_image2.png", "verifynr/" + "old_orange" + "/" + nur + ".png", 25):
    print(Style.BRIGHT + Fore.GREEN + "Color: " + "old_orange" + ", Num: " + nur + Fore.RESET)
else:
    print(Style.BRIGHT + Fore.RED + "Color: " + "old_orange" + ", Num: " + nur + Fore.RESET)

it gives me in RED: Color: old_orange, Num: 7

but then if I check if 6.png is in img.png by changing nur from 7 to 6 it gives me in Green: Color: old_orange, Num: 6, but thats the wrong image.

I've also tried the following code:

img_rgb = cv2.imread("2020-01-14-17-36-08.537043/verification_image2.png")
img_gray = cv2.cvtColor(img_rgb, cv2.COLOR_BGR2GRAY)
template = cv2.imread('verifynr/old_orange/7.png',0)
w, h = template.shape[::-1]

res = cv2.matchTemplate(img_gray,template,cv2.TM_SQDIFF)
threshold = 0.8
loc = np.where( res >= threshold)
pt = list(zip(*loc[::-1]))

if len(pt) >= 1:
    print("True")

wich prints True, but it does that for every number png I saved.

How do I make my program recognize the 7.png in the img.png without recognizing every single number png?

img.png:

6.png:

7.png:

Answer 1

Template matching not for object detection or pattern recognition, it's function is to find the most similar patch position. For detection use detectors (haar, dnn based ones, ...), for recognition use classifiers (descriptor based or nn based). '