How to put the contours in a image numbers? (OCR)

Question

I've been studying CV for a few months now but I ran into a problem on my second project, I needed to remove the noise from a sequence of numbers, in order to apply ocr. I managed to clean it up, but the numbers lost some internal pixels.

See the initial and current final image.

Initial

Final

Code used:

blur = cv2.GaussianBlur(img, (15, 15), 2)
hsv = cv2.cvtColor(blur, cv2.COLOR_BGR2HSV)
lower_gray = np.array([1, 1, 1])
upper_gray = np.array([102, 102, 102])
mask = cv2.inRange(hsv, lower_gray, upper_gray)

kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (3, 3))
opened_mask = cv2.morphologyEx(mask, cv2.MORPH_OPEN, kernel)
masked_img = cv2.bitwise_and(img, img, mask=opened_mask)
coloured = masked_img.copy()
coloured[mask == 0] = (255, 255, 255)

gray = cv2.cvtColor(coloured, cv2.COLOR_BGR2GRAY)

des = cv2.bitwise_not(gray)

contour, hier = cv2.findContours(des, cv2.RETR_CCOMP, cv2.CHAIN_APPROX_SIMPLE)

for cnt in contour:
    cv2.drawContours(des, [cnt], 0, 255, -1)

#des is the final image

Is there a better way to clean the background for OCR, or maybe close the lost pixels in the characters?

Answer 1

I managed to solve it, I didn't use the method you mentioned, but it was a good way, I was apprehensive that it would cause an expansion in the characters and wouldn't be good for OCR reading.

This is my final result:

for mrz in mrz_list:
    try:
        thresh = cv2.threshold(mrz, 0, 255, cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]

        dist = cv2.distanceTransform(thresh, cv2.DIST_L2, 5)
        dist = cv2.normalize(dist, dist, 0, 1.0, cv2.NORM_MINMAX)
        dist = (dist * 255).astype("uint8")

        thresh = cv2.threshold(dist, 0, 255, cv2.THRESH_BINARY | cv2.THRESH_OTSU)[1]

        kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (3, 3))
        opening = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel)

        cnts = cv2.findContours(opening.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
        cnts = imutils.grab_contours(cnts)
        chars = []
        for c in cnts:
            (x, y, w, h) = cv2.boundingRect(c)
            if w >= 20 and h >= 25:
                chars.append(c)

        chars = np.vstack([chars[i] for i in range(0, len(chars))])
        hull = cv2.convexHull(chars)

        mask = np.zeros(mrz.shape[:2], dtype="uint8")
        cv2.drawContours(mask, [hull], -1, 255, -1)
        mask = cv2.dilate(mask, None, iterations=2)

        final = cv2.bitwise_and(opening, opening, mask=mask)`

Thanks everyone.