Question

I have got OpenCV installed on vs2010/win7 however I’m seeing some behaviour that I can’t figure out.

I am new to OpenCV so just have a basic program to pull frames from an avi file – it then splits that frame into single channel images and generates histograms for each of those (taken from an internet example). It actually all works fine, it’s just extremely slow. It turns out that cvFillConvexPoly is actually taking 10-15 seconds (sometimes longer) to complete – but when it eventually returns it is correct.

This is the code snippet where I call the culprit function and as you can see I also tried cvFillPoly which took the same amount of time to complete.

IplImage* DrawHistogram(CvHistogram *hist, float sX)
{
    float histMax = 0;
    cvGetMinMaxHistValue(hist, 0, &histMax, 0, 0);

    IplImage *imgHist = cvCreateImage(cvSize(256, 64), IPL_DEPTH_8U, 1);
    cvZero(imgHist);

    float histValue = 0;
    float nextValue = 0;
    for (int i = 0; i < ((BINS - 1)*sX); i++)
    {
        histValue = cvQueryHistValue_1D(hist, i);
        nextValue = cvQueryHistValue_1D(hist, i + 1);

        CvPoint p1 = cvPoint(i * sX, 64);
        CvPoint p2 = cvPoint((i + 1) * sX, 64);
        CvPoint p3 = cvPoint((i + 1) * sX, 64 - histValue*(64/histMax));
        CvPoint p4 = cvPoint(i * sX, 64 - histValue*(64/histMax));

        int n = 5;
        CvPoint pts[] = {p1, p2, p3, p4};

        cvFillConvexPoly(imgHist, pts, n, cvScalar(255));
        //cvFillPoly(imgHist, pts, &n, 1,cvScalar(255));
    }

    return imgHist;
}

Any help is appreciated.

Compiled on Win7 x64 with CMake 2.8.2/VS2010 as 32 bit. Same behaviour when debugging and when running as a standalone.

Also have it running on Ubuntu 10.10 32 bit, compiled with gcc 4.4.5 where there isn’t a problem.

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Edit

I’ve tried recompiling with VS2008 and it still does the same thing. I don’t understand what would cause it to run so slowly – unless it’s the way 64bit windows “emulates” 32 bit which is causing the problem.

Answer 1

I can spot 2 possible bugs in your code, both of which have to do with bounds. Reading/writing outside array boundaries may result in all kinds of unexpected behavior, so it’s a wonder your programs don’t crash. Maybe GCC and/or the OpenCV library behaves differently on Ubuntu and Windows causing it not to crash on Ubuntu, but you should definitely take a look at the following 2 points.

1. I assume sX is a scaling factor? Your for loop should run from 0 to (BINS-1) no matter this scaling, since you’re using i to index your histogram and there are BINS bins, not BINS*sX. As long as sX == 1 you won’t run into trouble but any other value will invalidate your histogram drawing code. You are already using sX in its correct way too in the cvPoint declarations.

2. According to the docs of the cvFillConvexPoly function, n should be the number of points, which is 4 in your case, not 5.