Assertion failed with accumulateWeighted in OpenCV

Question

I am using openCV and trying to calculate a moving average of the background, then taking the current frame and subtracting the background to determine movement (of some sort).

However, when running the program I get:

OpenCV Error: Assertion failed (func != 0) in accumulateWeighted, file /home/sebbe/projekt/opencv/trunk/opencv/modules/imgproc/src/accum.cpp, line 431
terminate called after throwing an instance of 'cv::Exception'
what():  /home/sebbe/projekt/opencv/trunk/opencv/modules/imgproc/src/accum.cpp:431: error: (-215) func != 0 in function accumulateWeighted

I cant possibly see what arguments are wrong to accumulateWeighted.

Code inserted below:

#include <stdio.h>
#include <stdlib.h>
#include "cv.h"
#include "highgui.h"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "cxcore.h"

using namespace cv;

int main( int argc, char **argv )
{

    Mat  colourFrame;
Mat  frame;
Mat greyFrame;
Mat movingAverage;
Mat difference;
Mat temp;

    int       key = 0;
VideoCapture cap(0);


/* always check */
    if ( !cap.isOpened() ) {
        fprintf( stderr, "Cannot open initialize webcam!\n" );
        return 1;
    }

namedWindow("Camera Window", 0);

// Initialize
cap >> movingAverage;

    while( key != 'q' ) {
      /* get a frame */

  cap >> colourFrame;

  /* Create a running average of the motion and convert the scale */

  accumulateWeighted(colourFrame, movingAverage, 0.02, Mat() );

  /* Take the difference from the current frame to the moving average */
  absdiff(colourFrame, movingAverage, difference);

  /* Convert the image to grayscale */
  cvtColor(difference, greyFrame, CV_BGR2GRAY);

  /* Convert the image to black and white */
  threshold(greyFrame, greyFrame, 70, 255, CV_THRESH_BINARY);

        /* display current frame */
    imshow("Camera Window",greyFrame);

        /* exit if user press 'q' */
        key = cvWaitKey( 1 );
    }

    return 0;
}

Answer 1

Looking at the OpenCV sources, specifically at modules/imgproc/src/accum.cpp line 431, the lines that precede this assertion are:

void cv::accumulateWeighted( InputArray _src, CV_IN_OUT InputOutputArray _dst,
                             double alpha, InputArray _mask )
{
    Mat src = _src.getMat(), dst = _dst.getMat(), mask = _mask.getMat();
    int sdepth = src.depth(), ddepth = dst.depth(), cn = src.channels();

    CV_Assert( dst.size == src.size && dst.channels() == cn );
    CV_Assert( mask.empty() || (mask.size == src.size && mask.type() == CV_8U) );

    intfidx = getAccTabIdx(sdepth, ddepth);
    AccWFunc func = fidx >= 0 ? accWTab[fidx] : 0;
    CV_Assert( func != 0 ); // line 431

What's happening in your case is that getAccTabIdx() is returning -1, which in turn makes func be ZERO.

For accumulateWeighted() to work properly, the depth of colourFrame and movingAverage must be one of the following options:

colourFrame.depth() == CV_8U && movingAverage.depth() == CV_32F
colourFrame.depth() == CV_8U && movingAverage.depth() == CV_64F 
colourFrame.depth() == CV_16U && movingAverage.depth() == CV_32F
colourFrame.depth() == CV_16U && movingAverage.depth() == CV_64F
colourFrame.depth() == CV_32F && movingAverage.depth() == CV_32F
colourFrame.depth() == CV_32F && movingAverage.depth() == CV_64F
colourFrame.depth() == CV_64F && movingAverage.depth() == CV_64F

Anything different than that will make getAccTabIdx() return -1 and trigger the exception at line 431.

Answer 2

From the documentation on OpenCV API you can see that the output image from accumulateWeighted is

• dst – Accumulator image with the same number of channels as input image, 32-bit or 64-bit floating-point.

So your initialization is wrong. You should retrieve the colourFrame size first and then do this:

cv::Mat movingAverage = cv::Mat::zeros(colourFrame.size(), CV_32FC3);