[Python] OMR sheet scanning using OpenCV in Python

When I scan the OMR sheet to detect contours it is not detecting all rectangles. steps as below:

1. Loading color image
2. Convert to gray and resize
3. Blur the image using GaussianBlur
4. Edge detection using Canny
5. Finding contours
6. Visualizing the results

But it is not showing all rectangles. can anyone help me to resolve this issue.

Attached OMR sheet for reference:



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