OpenCV matrix element-wise division gives all-zero result - c++

Question

I use OpenCV by cocoapod on iOS C++ code. When running my app, I find it works abnormally. Finally, after digging down, I can give the following reproducible sample:
{
Mat a = (Mat_<uchar>({10, 20, 30, 40, 50, 60, 70, 80})).reshape(1, 1);
Mat b = (Mat_<uchar>({4, 5, 6, 7, 8, 9, 10, 11})).reshape(1, 1);
Mat c;
divide(a, b, c);
std::cout << "hello naive " << "a=" << a << endl << "b=" << b << endl << "a/b=" << (a / b) << "c=" << c << endl;
}
{
Mat a = (Mat_<uchar>({10, 20, 30, 40, 50, 60, 70})).reshape(1, 1);
Mat b = (Mat_<uchar>({4, 5, 6, 7, 8, 9, 10})).reshape(1, 1);
Mat c;
divide(a, b, c);
std::cout << "hello naive " << "a=" << a << endl << "b=" << b << endl << "a/b=" << (a / b) << "c=" << c << endl;
}
And the result:
im_sta[a]: {size=1 x 8, type=0, empty=0, min=10.000000, max=80.000000, miu=[45], sigma=[22.9]}
im_sta[b]: {size=1 x 8, type=0, empty=0, min=4.000000, max=11.000000, miu=[7.5], sigma=[2.29]}
im_sta[a / b]: {size=1 x 8, type=0, empty=0, min=0.000000, max=0.000000, miu=[0], sigma=[0]}
hello naive a=[ 10, 20, 30, 40, 50, 60, 70, 80]
b=[ 4, 5, 6, 7, 8, 9, 10, 11]
a/b=[ 0, 0, 0, 0, 0, 0, 0, 0]c=[ 0, 0, 0, 0, 0, 0, 0, 0]
im_sta[a]: {size=1 x 7, type=0, empty=0, min=10.000000, max=70.000000, miu=[40], sigma=[20]}
im_sta[b]: {size=1 x 7, type=0, empty=0, min=4.000000, max=10.000000, miu=[7], sigma=[2]}
im_sta[a / b]: {size=1 x 7, type=0, empty=0, min=2.000000, max=7.000000, miu=[5.29], sigma=[1.67]}
hello naive a=[ 10, 20, 30, 40, 50, 60, 70]
b=[ 4, 5, 6, 7, 8, 9, 10]
a/b=[ 2, 4, 5, 6, 6, 7, 7]c=[ 2, 4, 5, 6, 6, 7, 7]
(where im_sta is some extra statistics that I printed for more details, you can ignore it as well).
As you can see, in some cases (seems that when >=8 elements), that division results in ALL-ZERO!
How can I fix it? Thanks!

Answer 1

I find the answer later. Thus I post it here as a self-QA, in order to help people who have the same problem as me.
Solution: Upgrade OpenCV from 4.1 to 4.3! Then it is ok!
Actually, as you can see from the photo below, OpenCV >=4.2.0 is only available as cocoapod since 3 months ago (even if now it is 4.5.0 already)... So this is a quite new fix, if you do not want to compile opencv from source code by yourself... (When I started my project (of course before 3 months ago) there was only 4.1.0)