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Issue with storing Mat objects in QList



  • Hello All,
    I have created QList of Mat (Memory Allocator for Image in Opencv) object and i am storing the camera frame in it.
    My problem is that after storing the camera frames when i am trying to save the content of list.
    All the images are replaced by the last image.
    Please find the code below for more clarification:

    Code:

      QList<Mat> images;  //Declaration of list of mat object
      VideoCapture camera(0);
    
    
    //Appending the image to list
    for(int m = 0; m < 12; m++){
            
            // Move the controller to the desired location
             camera >> controllerFrame;   // Capture the frame
             images.append(controllerFrame); // Appending the image frames
             qDebug()<<"Size of the image List:"<<images.size();
    
    		 //Saving the current frame which have been captured
             String saveImage;
             saveImage = "C:\\Users\\demo\\ImageAppendedToList"+to_string(m)+".jpeg";
             qDebug()<<saveImage.c_str();
             imwrite(saveImage.c_str(), controllerFrame);
    		 
    		 }
    // Display the image frames capture in the list
    for (int xy = 0 ; xy<images.size(); xy++) {
              String saveImageList;
              saveImageList="C:\\Users\\demo\\ImagesFrom the list"+to_string(xy)+".jpeg";
              qDebug()<<saveImageList.c_str();
              imwrite(saveImageList.c_str(), (images.at(xy)));
    
              }
    

    When i am trying to display the image stored in the list. I always get the last frame.
    Could not identify the probable reason of the issue.



  • replace images.append(controllerFrame); with images.append(controllerFrame.clone());

    The reason is that the copy constructor of cv::Mat does not perform any copy at all, all the images in the list point to the same data that you overwrite each time


  • Lifetime Qt Champion

    Hi,

    You are appending always the same object to the list so you likely have 12 copies of it that share their underlying data.

    As an experiment, move the instantiation of controllerFrame in your for loop.



  • @VRonin : Thanks for the help it works. You always highlight this thing :)



  • @SGaist : Nope i am taking snaps of different location which i have mentioned in the comment.


  • Lifetime Qt Champion

    I wasn't referring to what you were taking pictures of but the objects that contain it. Basically the same thing that @VRonin described.



  • @SGaist : Ohh k. Thanks for your help. :)


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