Extracting and saving characters from an image

Question

I am following up from this post: How to extract only characters from image?

This solution works perfectly for me (with some tweaking) for its desired purpose. However, I am attempting to take it 1 step further by saving each of the characters. So in the example of this post, I wish to save the characters K, N, and M as their own individual images. I attempted iterating through the nested if loop with a cv2.imwrite function with the rect object although the final output is 7 images containing the overall image with just an additional rectangle to highlight the next contour each time.

Answer 1

Here's a simple approach:

1. Obtain binary image. Load image, grayscale, Otsu's threshold

2. Extract ROIs. Find contours and sort from left-to-right to ensure we have the contours in the correct order with imutils.contours.sort_contours. We filter using contour area then extract and save each ROI using Numpy slicing.

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Input

Binary image

Detected characters highlighted in green

Extracted ROIs

Code

import cv2
from imutils import contours

# Load image, grayscale, Otsu's threshold
image = cv2.imread('1.png')
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_OTSU + cv2.THRESH_BINARY_INV)[1]

# Find contours, sort from left-to-right, then crop
cnts = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
cnts, _ = contours.sort_contours(cnts, method="left-to-right")

# Filter using contour area and extract ROI
ROI_number = 0
for c in cnts:
    area = cv2.contourArea(c)
    if area > 10:
        x,y,w,h = cv2.boundingRect(c)
        ROI = image[y:y+h, x:x+w]
        cv2.imwrite('ROI_{}.png'.format(ROI_number), ROI)
        cv2.rectangle(image, (x, y), (x + w, y + h), (36,255,12), 2)
        ROI_number += 1

cv2.imshow('thresh', thresh)
cv2.imshow('image', image)
cv2.waitKey()