Blank frames when explicitly copying pixel values in OpenCV

Question

I have been porting some video processing code to C++ using OpenCV 2.4.3. The following test program closely mimics how my code will read each frame from a video, operate on its contents, and then write new frames to a new video file.

Strangely, the output frames are entirely black when the pixels are set individually, but are written correctly when the entire frame is cloned.

In practice, I'd use the two macros to access and assign desired values, but the sequential scan used in the example shows the idea more clearly.

Does anyone know where I'm going wrong?

test.cpp:

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <string>
using namespace std;
using namespace cv;

#define RGB_REF(PR,NC,R,C,CH) (*((PR) + ((3*(NC)*(R)+(C))+(CH))))
#define GRAY_REF(PR,NC,R,C) (*((PR) + (NC)*(R)+(C)))

int main(int argc, char* argv[])
{
    string video_path(argv[1]);
    cerr << "Video path is " + video_path + "\n";
    VideoCapture capture(video_path);
    if ( !capture.isOpened() )
    {
        cerr << "Input file could not be opened\n";
        return 1;
    } else
    {
        string output_path(argv[2]);
        VideoWriter output;
        int ex = (int)capture.get(CV_CAP_PROP_FOURCC);
        Size S = Size((int) capture.get(CV_CAP_PROP_FRAME_WIDTH),
                (int) capture.get(CV_CAP_PROP_FRAME_HEIGHT));
        output.open(output_path,ex,capture.get(CV_CAP_PROP_FPS),S,true);
        if ( !output.isOpened() )
        {
            cerr << "Output file could not be opened\n";
            return 1;
        }
        unsigned int numFrames = (unsigned int) capture.get(CV_CAP_PROP_FRAME_COUNT);
        unsigned int m = (unsigned int) capture.get(CV_CAP_PROP_FRAME_HEIGHT);
        unsigned int n = (unsigned int) capture.get(CV_CAP_PROP_FRAME_WIDTH);
        unsigned char* im = (unsigned char*) malloc(m*n*3*sizeof(unsigned char));
        unsigned char* bw = (unsigned char*) malloc(m*n*3*sizeof(unsigned char));
        Mat frame(m,n,CV_8UC3,im);
        Mat outputFrame(m,n,CV_8UC3,bw);
        for (size_t i=0; i<numFrames; i++)
        {
            capture >> frame;
            for (size_t x=0;x<(3*m*n);x++)
            {
                bw[x] = im[x];
            }
            output << outputFrame; // blank frames
//            output << frame;  // works
//            output << (outputFrame = frame); // works
        }
    }
}

Answer 1

When you query a frame from VideoCapture as capture >> frame;, frame is modified. Say, it has a new data buffer. So im no longer points to the buffer of frame.
Try
bm[x] = frame.ptr()[x];