Problem with getting information on contours

edit

I've got no error, no seg faults and average area of 7400pixels in my test image using this:

for (size_t i = 0; i < contours.size(); i++)
    {
        const std::vector<cv::Point>& c = contours[i];
        double a = std::fabs(cv::contourArea(cv::Mat(c)));
        area += a;
    }
    double avg_area = area / contours.size();

Have a careful look at the last line! There's an extra = in your code

I also got successful results with this (no need to use an extra variable):

for (size_t i = 0; i < contours.size(); i++)
    {
        double a = std::fabs(cv::contourArea(contours[i]));
        area += a;
    }
    double avg_area = area / contours.size();

/= is not an error. However, in my 640x480 image, I get 71 contours. As I go through the contours, the first one has 4 points and an area of 56 (reasonable), the second one has 516 points and an area of 5.69e+19 (not right) and the third one has 120195045 points and issues an exception when computing area. I am perplexed.

I'll be happy to test your image and see if I can find anything wrong with it. Btw, which version are you using?

I have the image here. I am using OpenCV 3.0 on Visual Studio 2013.

can you please post your image? (and maybe also a plot of the unfilled contours detected by OpenCV)

As per the documentation, contourArea does not actually count pixels, it uses a green formula which may give nonsensical results for contours that are not well behaved.

Incidentally, you don't need the std::fabs, contourArea has an optional flag for returning signed or unsigned area. The default is unsigned.

guy

This is what I use and I've never had a problem with weird areas:

vector<cv::Point> contour_poly;
approxPolyDP( Mat(contours[i]), contour_poly, 3, true );
Area=contourArea(contour_poly);

guy

@unxnut: using the provided image I get 2 contours corresponding to the 2 white blobs. But, important thing is that your image is in .jpg format, so due to compression issues your image is not longer binary - some pixels which should be 255 are now 254 or 253, those which should be 0 can take values of 1, 2 or 5 maybe. Therefore, findContours doesn't run as expected. Just put threshold(image, image, 128, 255, CV_THRESH_BINARY); to binarize image before calling your function.

@unxnut: extra comment, if you just want to compute average area, you might find useful the new 3.0 function connectedComponentsWithStats:

Mat image = imread("img.jpg",0);
threshold(image, image, 128, 255, CV_THRESH_BINARY);
Mat labels, stats, centroids;
connectedComponentsWithStats(image, labels, stats, centroids);
double area = 0;
for (int i = 1; i < stats.rows; i++){       //Start from i=1 to avoid background label
    area += stats.at<__int32>(i, CC_STAT_AREA);
}
double avg_area = area /= stats.rows;

approxPoly didn't help either :-(