Copy shape to blank canvas (OpenCV, Python)

Question

import numpy as np
import cv2

blank_image = np.zeros((40,40,3), np.uint8)
blank_image.fill(255)

#cv2.imshow('i', blank_image)
#cv2.waitKey(0)

im = cv2.imread('img.png')
imgray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(imgray, 127, 255, 0)
image, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

cnt = contours[4]
cnts = cv2.drawContours(im,[cnt],0,(255,0,0), -1)

cv2.imshow('i', im)
cv2.waitKey(0)

for a in cnt:
    print(a) #this contour is a 3D numpy array

Source image:

I am using this code to: 1. create a white canvas of 40x40 pixels 2. found the contours of number (in this case 5) using Opencv function findContours.

What I want to do is to copy this shape (please, not the bounding box or rectangle, the blue shape) into the canvas.

After some research I learned that an opencv image is just a numpy array. This array, theoretically, should be translated in the new image (my white canvas..) and then reconstruct the shape using the values inside the array. I am right ?

Someone know how to do that ? Creating a bounding box / rectangle around the numbers would, in some cases, result inaccurate. Please, don't give that as solution. I already did this process in atleast 3-4 different ways and the results are not satisfactory enough.

So, the desired output would be something like this..

Thanks.

Answer 1

For contours image

I think want something like

---

For opened number such as 1, 2, 5 , it is easy to do: Crop from the whole image, or draw on new image. For closed number such as 0, 6, 8, 9, more steps are needed. Here is example for 5, you will get .

---

Details and description are in the code.

#!/usr/bin/python3
# 2018.01.14 09:48:15 CST
# 2018.01.14 11:39:03 CST

import numpy as np
import cv2

im = cv2.imread('test.png')
imgray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(imgray, 127, 255, 0)
contours = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)[-2]


## this contour is a 3D numpy array
cnt = contours[4]
res = cv2.drawContours(im,[cnt],0,(255,0,0), -1)
cv2.imwrite("contours.png", res)

## Method 1: crop the region
x,y,w,h = cv2.boundingRect(cnt)
croped = res[y:y+h, x:x+w]
cv2.imwrite("croped.png", croped)

## Method 2: draw on blank
# get the 0-indexed coords
offset = cnt.min(axis=0)
cnt = cnt - cnt.min(axis=0)
max_xy = cnt.max(axis=0) + 1
w,h = max_xy[0][0], max_xy[0][1]
# draw on blank
canvas = np.ones((h,w,3), np.uint8)*255
cv2.drawContours(canvas, [cnt], -1, (255,0,0), -1)
cv2.imwrite("canvas.png", canvas)