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Word Segmentation with Scale Space Technique

Update 2021: installable Python package, added line clustering and word sorting

Implementation of the scale space technique for word segmentation proposed by R. Manmatha and N. Srimal. Even though the paper is from 1999, the method still achieves good results, is fast, and has a simple implementation. The algorithm takes an image containing words as input and outputs the detected words. Optionally, the words are sorted according to reading order (top to bottom, left to right).

Installation

• Go to the root level of the repository
• Execute pip install .
• Go to tests/ and execute pytest to check if installation worked

Usage

This example loads an image of a text line, prepares it for the detector (1), detects words (2), sorts them (3), and finally shows the cropped words (4).

from word_detector import prepare_img, detect, sort_line
import matplotlib.pyplot as plt
import cv2

# (1) prepare image:
# (1a) convert to grayscale
# (1b) scale to specified height because algorithm is not scale-invariant
img = prepare_img(cv2.imread('data/line/0.png'), 50)

# (2) detect words in image
detections = detect(img,
                    kernel_size=25,
                    sigma=11,
                    theta=7,
                    min_area=100)

# (3) sort words in line
line = sort_line(detections)[0]

# (4) show word images
plt.subplot(len(line), 1, 1)
plt.imshow(img, cmap='gray')
for i, word in enumerate(line):
  print(word.bbox)
  plt.subplot(len(line), 1, i + 2)
  plt.imshow(word.img, cmap='gray')
plt.show()

The repository contains some examples showing how to use the package:

• Install requirements: pip install -r requirements.txt
• Go to examples/
• Run python main.py to detect words in line images (IAM dataset)
• Or, run python main.py --data ../data/page --img_height 1000 --theta 5 to run the detector on an image of a page (also from IAM dataset)

The package contains the following functions:

• prepare_img: prepares input image for detector
• detect: detect words in image
• sort_line: sort words in a (single) line
• sort_multiline: cluster words into lines, then sort each line separately

For more details on the functions and their parameters use help(function_name), e.g. help(detect).

Algorithm

The illustration below shows how the algorithm works:

• top left: input image
• top right: apply filter to the image
• bottom left: threshold filtered image
• bottom right: compute bounding boxes

The filter kernel with size=25, sigma=5 and theta=3 is shown below on the left. It models the typical shape of a word, with the width larger than the height (in this case by a factor of 3). On the right the frequency response is shown (DFT of size 100x100). The filter is in fact a low-pass, with different cut-off frequencies in x and y direction.

How to select parameters

• The algorithm is not scale-invariant
  • The default parameters give good results for a text height of 25-50 pixels
  • If working with lines, resize the image to 50 pixels height
  • If working with pages, resize the image so that the words have a height of 25-50 pixels
• The sigma parameter controls the width of the Gaussian function (standard deviation) along the x-direction. Small values might lead to multiply detection per word (over-segmentation), while large values might lead to a detection containing multiple words (under-segmentation)
• The kernel size depends on the sigma parameter and should be chosen large enough to contain as much of the non-zero kernel values as possible
• The average aspect ratio (width/height) of the words to be detected is a good initial guess for the theta parameter

The best way to find the optimal parameters is to use a dataset (e.g. IAM) and optimize the parameters w.r.t. some evaluation metric (e.g. intersection over union).

Results

This algorithm gives good results on datasets with large inter-word-distances and small intra-word-distances like IAM. However, for historical datasets like Bentham or Ratsprotokolle results are not very good and more complex approaches should be preferred (e.g., a neural network based approach as implemented in the WordDetectorNN repository).