Opencv 3.4.6-dev
Python 3.6.8

There is a problem in seamlessClone that was produced by this pr #12512.

1. When mask is on borders only (like border line) it's just crashes. It happens because
   

   opencv/modules/photo/src/seamless_cloning.cpp

   Line 77 in 72ccb5f

   Mat mask_inner = mask(Rect(1, 1, mask.cols - 2, mask.rows - 2));

   
   this line places zeros on mask borders.
2. It produces undesirable ghost effect on borders

Here is code to reproduce:

dst = np.ones((101, 101, 3), np.uint8) * 255
src = np.ones((101, 101, 3), np.uint8) * 255
cv.circle(dst,  center=(50, 50), radius=5, color=(0, 0, 255), thickness=-1)
cv.circle(dst, center=(50, 0), radius=5, color=(0, 0, 255), thickness=-1)

mask = np.zeros((101, 101, 1), np.uint8)
mask[:LINES,:] = 255

res = cv.seamlessClone(src, dst, mask, (50, 50), cv.NORMAL_CLONE)
cv.imwrite('result.png', np.concatenate([dst, src, np.concatenate([mask, mask, mask], axis=2), res], axis=1))

That's even stranger behavior
If i set LINES to 1 or 2 i get
terminate called after throwing an instance of 'std::length_error' what(): vector::_M_default_append Aborted (core dumped)
If i set LINES to 3 i get
Traceback (most recent call last): File "seamless_test.py", line 13, in <module> res = cv.seamlessClone(src, dst, mask, (50, 50), cv.NORMAL_CLONE) cv2.error: OpenCV(3.4.6-dev) /home/vfilev/packages/opencv/modules/core/src/matrix_wrap.cpp:1658: error: (-215:Assertion failed) !fixedSize() in function 'release'

and if i set LINES to 60 i get the result below
Example:

So you can see that circle inside image is gone but circle on a border is left here.