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Try :

1. Blur image with a 13x13 window size
2. otsu threshold
3. use threshold image as mask to clear region

Try :

1. Blur image with a 13x13 window size
2. otsu threshold
3. use threshold image as mask to clear region
4. bitwise_not of threshold image
5. erode image (5 iterations)
6. bitwise_and eride image and original image