OpenCV : Remove background of an image

Question

I am using Opencv and python to detect shapes and then crop them. I have succeeded to do that, however now I am trying to take the cropped images and remove their backgroun

Answer 1

At least in for this image, there is no need to detect the circle use houghCircle). I think threshold it and find the inner contour , then make mask and do bitwise-op is OK!

My steps:

(1) Read and convert to gray

(2) findContours

(3) find contour that smaller, create a mask

(4) do bitwise_and to crop

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Here is my result:

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#!/usr/bin/python3
# 2018.01.20 20:58:12 CST
# 2018.01.20 21:24:29 CST
import cv2
import numpy as np

## (1) Read
img = cv2.imread("img04.png")
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

## (2) Threshold
th, threshed = cv2.threshold(gray, 127, 255, cv2.THRESH_BINARY_INV|cv2.THRESH_OTSU)

## (3) Find the min-area contour
_cnts = cv2.findContours(threshed, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)[-2]
cnts = sorted(cnts, key=cv2.contourArea)
for cnt in cnts:
    if cv2.contourArea(cnt) > 100:
        break

## (4) Create mask and do bitwise-op
mask = np.zeros(img.shape[:2],np.uint8)
cv2.drawContours(mask, [cnt],-1, 255, -1)
dst = cv2.bitwise_and(img, img, mask=mask)

## Save it
cv2.imwrite("dst.png", dst)