Alyn

Skew detection and correction in images containing text

Image with skew

Image after deskew

### Requires

• numpy
• matplotlib
• scipy
• scikit-image
• wheel (to create personalized wheel)

Techniques used

• Canny Edge Detection
• Hough Transform

Features

• Detect the skew in given images
• Display the output
• Save the output to txt file
• Batch process files in a directory
• View Hough Transform of a given image
• Set the number of peaks for Hough Transform and Sigma for Canny Edge detection
• Rotate the image to remove the skew
• Accept and return numpy grayscale images

How the skew detection works

The skew detection script takes image file as input, then performs the following steps:

• Converts the image to greyscale
• Performs Canny Edge Detection on the Image
• Calculates the Hough Transform values
• Determines the peaks
• Determines the deviation of each peaks from 45 degree angle
• Segregates the detected peaks into bins
• Chooses the probable skew angle using the value in the bins

The deskew script uses the skew angle determined using skew detection script to remove the skew from the image.

Usage

Image skew inside project using alyn and numpy image

First of all download and install the alyn wheel from the my_wheels folder.

Then integrate it into your project to de-skew a numpy array image:

from alyn import deskew

dw = deskew.Deskew(
    input_numpy=image_np,
    output_numpy=True
)
rotated_np_image = dw.run()

Read image, de-skew it and write to disk:

from alyn import deskew

dw = deskew.Deskew(
    input_file=path/to/image,
    output_file=path/to/destination
)
dw.run()

Image skew calculation using skew_detect.py

To calculate the skew angle for a given image file, use -i option followed by the path to file:

./skew_detect.py -i image.jpg

To save output in a text file add -o option followed by the output file name:

./skew_detect.py -i image.jpg -o output.txt

To display output information add -d option followed by a string Yes:

./skew_detect.py -i image.jpg -d Yes

To batch process files in a directory, use -b option followed by the path to directory:

./skew_detect.py  -b examples

To display Hough Transform plot for an image,:

./skew_detect.py -i image.jpg -p Yes

Output of the Hough Transform:

To set the value of sigma for Gaussian blurring in Canny Edge Detection, use -s option followed by the desired value:

./skew_detect.py -i image.jpg -s 3

To set the number of peaks collected from Hough Transform, use -n option followed by the desired value:

./skew_detect.py -i image.jpg -n 10

Image Deskew using deskew.py

To perform a simple deskew and display the output:

./deskew.py -i image.jpg -d Yes

To save the deskewed image, use the following:

./deskew.py -i image.jpg -o rotated.jpg

In some cases the result image might be upside down or the text may be running vertical, To fix this, use -r followed by the desired angle in int:

./deskew.py -i image.jpg -o rotated.jpg -r 90

To generate data for experimental purposes, run the test_img_gen.py in test_data folder. This will generate images containing a white line having angle between 0 to 180 degrees.