Contour width measurement along its entire lendth

Question

I am working on a project to measure width of contour. I have detected the contour from the image (see image 1). The next step is to measure the width of the coutour along its length (like shown in image 2). Please suggest me any ideas. I truly appreciate your help!

Thank you!

masked contour image

required width measurement from contour, green lines indicate width

I have a function that calculates contours and then next step is to measure the width from selected contours along their length. Below is the sample code.

...
// image is read, thresholded and canny edges are detected. That image is input to a function that computes contours from the image. 
///Below is the code in the contour function

cv::Mat src_contour= inputImage.clone(); // input image is cloned for contour detection
cv::Mat maskContour = cv::Mat::zeros(src_contour.size(), CV_8UC3);
std::vector<std::vector<cv::Point> > contours; // stores contours points. Each contour is stored in a vector and there are number of vectors for number of contours
cv::RNG rng(12345); // random number used for random colours of contours
cv::findContours( src_contour, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );

int nc=contours.size();// nc: total number of countours detected  

std::vector<int>areas(nc); // stores area of each contour in a vector
std::vector<double>arclens(nc); // stores arc length (perimeter) of each contour in a vector
std::vector<double>aspect_ratio(nc); // Apect ratio of the contour , width/height 
std::vector<cv::Rect> r(nc) ; // Vector of rectangles,

int min_area=15000 ; // Minimum area for contour selection 
int min_arclen=1000; // Minimum Arc length used for filtering contours 
double min_aspRatio= 2; // Minimum Aspect ratio used for filtering , deafutl 2.0
double max_aspRatio=4; // Maximum aspect rario used for filtering , default 4.0

for (int i=0; i< nc; i++) // Loop iterates through contours , calculates properties and draws selected contours 
{  
    areas[i]=cv::contourArea(contours[i],false); // Area of each contour is stored in a vector, false: any contour, true: closed contour
    arclens[i]=cv::arcLength(contours[i],false); // Arclength of each contour is stored in a vector 
    r[i]=cv::boundingRect(contours[i]); // Stores bounding rect for each contour in a vector r
    aspect_ratio[i]=float(r[i].width)/r[i].height; // Aspect ratio of each contour is stored in a vector 

    if ((areas[i] > min_area) && (arclens[i] > min_arclen) && (aspect_ratio[i] > min_aspRatio && aspect_ratio[i] < max_aspRatio)) 
    {   
        cv::drawContours(maskContour, contours, i, cv::Scalar(255,255,255), CV_FILLED); // creates mask from contours (filterd by criteria), fills them
    }
}

// maskContour image is the image of selected contours filled , I have access to all the points on the contour. From the selected contours
// need to compute width of contours 

.....
// Now contour width measurement is required 

Answer 1

I think distanceTransform(of OpenCV) and skeleton(maybe by yourself) will work.

Main idea:

1. Threshold you gray image, then do distanceTransform to get dist-map
2. Find the skeleton of the dist-map, the width is twice of the skeleton value.

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The distance map is as follow.

Then you try to find the skeleton, double the dist value to get the width.

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Update with C++ code:

int main() {
    // read as gray and threshold 
    Mat gray, threshed, dist;
    gray = imread("img01.png", 0);
    threshold(gray, threshed, 100, 255, THRESH_BINARY);
    imshow("threshed", threshed);

    //distanceTransform
    distanceTransform(threshed, dist, DIST_L2, 3);

    // normalize for display
    Mat dst;
    normalize(dist, dst, 255, 0, NORM_MINMAX,CV_8UC1);
    imshow("dst", dst);
    waitKey();

    return 0;
}