Crop text out of binary image OpenCV C++

Question

How can I crop the image so that only the text is included in the image using OpenCV?

Answer 1

Approach

1. Dilating the image in in the vertical and horizontal direction
2. Finding the bounding rectangle
3. Cropping the image

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Code

# reading the input image in grayscale image
image = cv2.imread('image2.png',cv2.IMREAD_GRAYSCALE)
image /= 255
if image is None:
    print 'Can not find/read the image data'
# Defining ver and hor kernel
N = 5
kernel = np.zeros((N,N), dtype=np.uint8)
kernel[2,:] = 1
dilated_image = cv2.dilate(image, kernel, iterations=2)

kernel = np.zeros((N,N), dtype=np.uint8)
kernel[:,2] = 1
dilated_image = cv2.dilate(dilated_image, kernel, iterations=2)
image *= 255

# finding contours in the dilated image
contours,a = cv2.findContours(dilated_image,cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

# finding bounding rectangle using contours data points
rect = cv2.boundingRect(contours[0])
pt1 = (rect[0],rect[1])
pt2 = (rect[0]+rect[2],rect[1]+rect[3])
cv2.rectangle(image,pt1,pt2,(100,100,100),thickness=2)

# extracting the rectangle
text = image[rect[1]:rect[1]+rect[3],rect[0]:rect[0]+rect[2]]

plt.subplot(1,2,1), plt.imshow(image,'gray')
plt.subplot(1,2,2), plt.imshow(text,'gray')

plt.show()

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Output

Answer 2

Assuming that you have pixel information you could draw a convex hull around the white text. Once that you have the hull, you can get the lowest and highest x and y coordinate and use those as your rectangle coordinates so that you could crop the rest of the image.

An example of how to obtain a convex hull in Open_CV (C++) is available here.

It might be a good idea to apply some filter on the image (maybe use the erosion filter) so that you can remove any false positives.

Answer 3

Take projection of white pixels in x-direction and y-direction. The lowest and highest values in x and y direction are bound of your rectangle's y and x co-ordinates respectively. This solution is for simple situations with low processing and in-place calculation.