Exception thrown writing to InputOutputArray - c++

Question

I'm trying to test some motion estimation in Visual Studio 2013, using OpenCV v3.0 (which is probably my 1st mistake!). I got an unhandled exception trying to use createOptFlow_DualTVL1() and createOptFlow_Farneback(), and then, for testing, tried cv::accumulate(), which threw the same exception.
It seems that OpenCV can't write to the Mat object that I'm passing these functions. I can't read the actual cvException because I don't have the PDB files, because I didn't compile this version myself. That might be my next stop, but before I do I figured I'd see if anyone's seen this behaviour before.
Here's a minimal working example:
#include <opencv2/opencv.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/video/tracking.hpp>
#include <opencv2/videoio/videoio.hpp>
#include <iostream>
#include <stdio.h>
#include <fstream>
#include <string>
#include <regex>
using namespace std;
using namespace cv;
int main(int argc, const char** argv)
{
VideoCapture captureDevice;
std::string videoname = "example.mp4";
captureDevice.open(videoname);
//setup image files used in the capture process
Mat currFrame, dst;
captureDevice >> currFrame;
accumulate(currFrame, dst, cv::noArray());
imshow("outputCapture", dst);
//pause for 33ms
waitKey(33);
return 0;
}

Answer 1

dst should be of same size as that currFrame and of type CV_32FC3.
So, add this line of code before calling accumulate -
dst.create(currFrame.size(), CV_32FC3);
Since dst is of float type, you will need to convert it to uchar to display it. For that, convert as shown below -
Mat dst_disp;
dst.convertTo(dst_disp, CV_8UC3);
imshow("outputCapture",dst_disp );
Additionally, as you accumulate more frames in dst, you will need to normalize by number of frames(let's say N) cached in dst. Simply, divide dst by the N, then convert the result into CV_8UC3 and display. For example, if you accumulated 1000 frames in dst do as shown below,
// Accumulate 1000 frames
for(int i = 0; i < 1000; i++)
accumulate(currFrame, dst, cv::noArray());
// Normalize
dst = dst/ 1000;
// Display the frame
Mat dst_disp;
dst.convertTo(dst_disp, CV_8UC3);
imshow("outputCapture",dst_disp );
else, you might get an all white image.
UPDATE
From #berak's comment below.
For normalization, simply use
dst.convert(dst_disp, CV_8UC3, 1.0/N);
where N in example above will be 1000.