What is the correct way of creating a small list of cv::Mat and iterating over it? - c++

Question

I am new to C++ and I am trying to create a list of cv::Mat.
This could allocate quite a lot of memory, but I only have around ten small Mat's to load in the list.
I made this code, but not sure why it is not working.
void Utils::getFramesFromVideo(std::string file,std::list<cv::Mat>& listMatOutput) {
cv::VideoCapture* videoCapture = new cv::VideoCapture(file);
bool hasImage = false;
cvNamedWindow("WhileReading", 1);
do {
cv::Mat frame;
hasImage = videoCapture->read(frame);
if (hasImage) {
listMatOutput.push_back(frame);
imshow("WhileReading", frame);
waitKey(0);//give some time to see the frame (debug)
}
} while (hasImage);
cvNamedWindow("AfterReading", 2);
for (std::list<Mat>::const_iterator iterator = listMatOutput.begin();
iterator != listMatOutput.end(); iterator++) {
imshow("AfterReading", *iterator);
waitKey(0);//give some time to see the frame (debug)
}
videoCapture->release();
}
The first loading is displaying the frames correctly, but in the second window (AfterReading) the image is black with red stripes.
Could someone please give some advice?

Answer 1

The list format is an STL container, meaning that you've got some things to keep in mind to work with it. push_back() is the preferred method of adding instances to the container, much like using an iterator is the preferred method of accessing the elements of the list. If you try to directly set an element of the list to a cv::Mat() you're working on, then a) you need to know exactly what sort of wrapping the list is doing to each instance, so you can do it properly yourself, and b) you're defeating the purpose of using an STL container in the first place, which is to abstract away the details of the list.
You don't necessarily need to use the frame.clone() call in your code, as this creates a deep copy of the image and takes up precious resources. I know that I've used std::vector<cv::Mat> in the past without having to make deep copies for each element in the vector, so I assume that you should be able to pass the Mat itself to a list. Consult the C++ documentation on STL lists.
Another thing you might consider, if low memory usage and speed of access through the list is a concern, and your number of images is low, is a list of image pointers. Keep your images stored as individual cv::Mat instances, and make your list of type std::list<cv::Mat*>, passing the handle of the image to the push_back() call. Of course, with this method, your images will not be 'thread safe' because they will be stored in one place, but called and worked on from another. I hope this shed a little light on your inquiry.

Answer 2

I tried all sort of things, like changing to pointers or vectors and checking for code optimizations on GCC. Then after trying to clone cv::Mat it worked:
listMatOutput.push_back(frame.clone());
Would be glad if anyone could tell me why and suggest a better way of doing, so I can choose a better answer then my own.