Question

I would like to declare an cv::Mat object and somewhere else in my code, change its dimension (nrows and ncols). I couldn’t find any method in the documentation of OpenCV. They always suggest to include the dimension in the constuctor.

Answer 1

An easy and clean way is to use the create() method. You can call it as many times as you want, and it will reallocate the image buffer when its parameters do not match the existing buffer:

Mat frame;

for(int i=0;i<n;i++)
{
    ...
    // if width[i], height[i] or type[i] are != to those on the i-1
    // or the frame is empty(first loop)
    // it allocates new memory
    frame.create(height[i], width[i], type[i]); 
    ... 
    // do some processing
}

Docs are available at https://docs.opencv.org/3.4/d3/d63/classcv_1_1Mat.html#a55ced2c8d844d683ea9a725c60037ad0