opencv python code equivalent in C++

Question

I have this piece of code that works fine on Python. I want to do the same thing in C/C++ but i do not understand how works kernel in C++:

kernel = np.ones((3,3),np.uint8)
opening = cv2.morphologyEx(thresh,cv2.MORPH_OPEN,kernel, iterations = 2)

kernel and opening are Mat objects.

I have also

mat1=np.uint8(mat1)

I do not understand what is this np prefix.

The np prefix is the numpy library. OpenCV uses that library for all numeric operations, because Python arrays are very inefficient for numeric computations. — MariusSiuram
– MariusSiuram, Commented Dec 1, 2015 at 8:28

GPPK · Accepted Answer · 2015-12-01 09:58:33Z

4

This line:

kernal = np.ones((3,3)), np.uint8)

is the same as doing this in C++:

Mat m = Mat(3, 3, CV_8UC1, cv::Scalar(1));

As MaruisSiuram said the np prefix is for the numpy library, this is not used in C++ you can just use the OpenCV Matrix container.

This line:

mat1=np.uint8(mat1)

is casting mat1 to the type uint8 which can be done like so:

mat1.convertTo(mat1, CV_8UC1);

edited Dec 1, 2015 at 9:58

answered Dec 1, 2015 at 8:54

GPPK

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4 Comments

Micka Over a year ago

Two problems that I see: 1. kernel should be 8UC1, 2. 1 might be interpreted as data address. So Mat m = Mat(3, 3, CV_8UC1, cv::Scalar(1)); or Mat m = Mat::ones(3, 3, CV_8UC1);

GPPK Over a year ago

I know it defaults to "1" so ive updated with the more obvious scalar solution. Thanks!

testpresta Over a year ago

Thanks. Another question: In python we can do this: mat1[mat2==255] = 0. What is the equivalent en C++ ?

Miki Over a year ago

You may want also to mention getStructuringElement(). @testpresta mat1.setTo(0, mat2 == 255);

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