Gradient mask blending in opencv python

Question

I have an image and circle zone. I need to blur all, except for circle zone. Also i need to make border of circle smooth.
The input:

The output(made it in image

Answer 1

I think maybe you want something like that.

This is the source image:

The source-blured-pair :

The mask-alphablened-pair:

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The code with description in the code comment.

#!/usr/bin/python3
# 2018.01.16 13:07:05 CST
# 2018.01.16 13:54:39 CST
import cv2
import numpy as np

def alphaBlend(img1, img2, mask):
    """ alphaBlend img1 and img 2 (of CV_8UC3) with mask (CV_8UC1 or CV_8UC3)
    """
    if mask.ndim==3 and mask.shape[-1] == 3:
        alpha = mask/255.0
    else:
        alpha = cv2.cvtColor(mask, cv2.COLOR_GRAY2BGR)/255.0
    blended = cv2.convertScaleAbs(img1*(1-alpha) + img2*alpha)
    return blended

img = cv2.imread("test.png")

H,W = img.shape[:2]
mask = np.zeros((H,W), np.uint8)
cv2.circle(mask, (325, 350), 40, (255,255,255), -1, cv2.LINE_AA)
mask = cv2.GaussianBlur(mask, (21,21),11 )

blured = cv2.GaussianBlur(img, (21,21), 11)

blended1 = alphaBlend(img, blured, mask)
blended2 = alphaBlend(img, blured, 255- mask)

cv2.imshow("blened1", blended1);
cv2.imshow("blened2", blended2);
cv2.waitKey();cv2.destroyAllWindows()

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Some useful links:

1. Alpha Blending in OpenCV C++ : Combining 2 images with transparent mask in opencv

2. Alpha Blending in OpenCV Python: Gradient mask blending in opencv python