Morphological Reconstruction in OpenCV

Question

When processing an image with text in OpenCV, my opening operation does not result in proper output data. The issue is quite similar to the one described in this article: ht

Answer 1

This answer arrives late, but here is the basic algorithm for under-reconstruction:

1. Inputs are two images: ImReference and ImMarker, with marker <= reference
2. Intermediate image: ImRec
3. Output image: ImResult
4. Copy ImMarker into ImRec
5. copy ImRec into ImResult
6. ImDilated = Dilation(ImResult)
7. ImRec = Minimum(ImDilated, ImReference)
8. If ImRec != ImResult then return to step 5.

It's not the most optimal algorithm, but it uses only basic operations.

Answer 2

Here's my Python3 implementation in analogy to MatLab's imreconstruct algorithm:

import cv2
import numpy as np


def imreconstruct(marker: np.ndarray, mask: np.ndarray, radius: int = 1):
    """Iteratively expand the markers white keeping them limited by the mask during each iteration.

    :param marker: Grayscale image where initial seed is white on black background.
    :param mask: Grayscale mask where the valid area is white on black background.
    :param radius Can be increased to improve expansion speed while causing decreased isolation from nearby areas.
    :returns A copy of the last expansion.
    Written By Semnodime.
    """
    kernel = np.ones(shape=(radius * 2 + 1,) * 2, dtype=np.uint8)
    while True:
        expanded = cv2.dilate(src=marker, kernel=kernel)
        cv2.bitwise_and(src1=expanded, src2=mask, dst=expanded)

        # Termination criterion: Expansion didn't change the image at all
        if (marker == expanded).all():
            return expanded
        marker = expanded