Converting OpenGL Image to OpenCV

/**
  * http://answers.opencv.org/question/39386/opencv-error-segmentation-fault-converting-data/
  */
#include <stdio.h>
#include <unistd.h>
#include <cstring>

#include "camera.h"
#include "graphics.h"

#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>

#define MAIN_TEXTURE_WIDTH 512
#define MAIN_TEXTURE_HEIGHT 512

//entry point
int main(int argc, const char **argv)
{

    //how many detail levels (1 = just the capture res, > 1 goes down by half each level, 4 max)
    int num_levels = 4;

    //init graphics and the camera
    InitGraphics();
    CCamera* cam = StartCamera(MAIN_TEXTURE_WIDTH, MAIN_TEXTURE_HEIGHT,30,num_levels,true);

    //create MOG background substractor model
    //BackgroundSubtractorMOG bg_model;

    //create 4 textures of decreasing size
    GfxTexture textures[4];
    for(int texidx = 0; texidx < num_levels; texidx++)
        textures[texidx].Create(MAIN_TEXTURE_WIDTH >> texidx, MAIN_TEXTURE_HEIGHT >> texidx);

    printf("Running frame loop\n");
    for(;;)
    {

        //lock the chosen frame buffer, and copy it directly into the corresponding open gl texture
        const void* frame_data; int frame_sz;
        if(cam->BeginReadFrame(1,frame_data,frame_sz))
        {
            //if doing argb conversion the frame data will be exactly the right size so just set directly
            textures[1].SetPixels(frame_data);
            char* frame_data_copy = new char[frame_sz]; // create a buffer of same size in bytes
            std::memcpy(frame_data_copy, frame_data, frame_sz); // copy data from frame_data to frame_data_copy
            cv::Mat TempMat(MAIN_TEXTURE_HEIGHT, MAIN_TEXTURE_WIDTH, CV_8UC4, frame_data_copy); //create cv::Mat
            delete [] frame_data_copy; //free allocated memory for copy, this also invalidates the data of TempMat
            imshow("Camera Feed",TempMat);
            cam->EndReadFrame(1);
        }

        //begin frame, draw the texture then end frame (the bit of maths just fits the image to the screen while maintaining aspect ratio)
        BeginFrame();
        float aspect_ratio = float(MAIN_TEXTURE_WIDTH)/float(MAIN_TEXTURE_HEIGHT);
        float screen_aspect_ratio = 1280.f/720.f;
        DrawTextureRect(&textures[1],-aspect_ratio/screen_aspect_ratio,-1.f,aspect_ratio/screen_aspect_ratio,1.f);
        EndFrame();
        int c = cv::waitKey(30);
        if (c == 'q' || c == 'Q' || (c & 255) == 27)
            break;
    }

    StopCamera();
}