Access value with a pointer in Mat [closed]

asked 2018-12-30 03:22:34 -0600

happy__ gravatar image

A Mat element is stored in Mat data pointer described in OpenCV document, and the address is mat.data + i0 * mat.step[0] + i1 * mat.step[1] +... + (id-1) * mat.step[id - 1] . I want to access and re-assign an element with data pointer like this:

cv::Mat test = (cv::Mat_<uchar>(3,3) << 1,2,3,4,5,6,7,8,9);
*(test.data + 2 * test.step[1]) = *test.data + 10;

if the element type of Mat is uchar, the code above works well, and the third element is re-assigned as 11, but if the type is not uchar, such as double, the code above does not work, I'm confused about this, please help me.

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Closed for the following reason the question is answered, right answer was accepted by LBerger
close date 2019-01-02 07:35:01.620423

Comments

For one pixel you can use pointer access : 

double *x=(double *)test.data
*(x+ 2 * test.step[1])=*test.data + 10;

it's better llike this

double *ptr1=test.ptr<double>(2)
double *ptr2=test.ptr<double>(0)
*ptr1 = *ptr2 +10
LBerger gravatar imageLBerger ( 2018-12-30 03:51:27 -0600 )edit

what are you trying to achieve here ? what is the purpose of your program ?

(in general, you probably should avoid raw pointers, and use higher level opencv functions instead)

berak gravatar imageberak ( 2018-12-30 06:37:31 -0600 )edit
2

Thank you very much. I'm a beginner in OpenCV. I just want to test the pointer data, I will follow your advice to use ptr pointer later.

happy__ gravatar imagehappy__ ( 2019-01-02 07:05:41 -0600 )edit

I forgot this tutorials https://docs.opencv.org/trunk/db/da5/...

LBerger gravatar imageLBerger ( 2019-01-02 07:34:44 -0600 )edit
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