problem playing avi video, cvcreateFileCapture returning NULL pointer

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I did have the same problem, when trying to run this code with some videos, because I did not have the right codecs to play it. I do not see any problems with your code. Try more videos with different codecs.

And to complement my suggestion made, try this code in C++, you can already see how much easier it is :)

#include "opencv2/highgui/highgui.hpp" 
#include "stdio.h"

int main( int argc, char** argv ) { 
     cv::VideoCaptuee capture ( argv[1] );
     cv::Mat frame;
     for(;;) {
         capture >> frame;
         if( frame.empty() )  { 
             printf("EMPTY OR NULL");
             break;//ends HERE!!!!!
         }
         cv::imshow( "Example2", frame );  
         char c = cv::waitKey(33);
         if( c == 27 ) break;
    }    
}

your codes are correct. the only reason is you did not put the dll file of opencv_ffmpeg240.dll in your project.