OpenCV histogram of an irregular shape - c++

Question

I'm comparatively new to OpenCV. I was wondering if it is possible to get histogram of a contour (which can be a perfect rectangular or irregular in shape) found by findcontour.
Thanks in advance.
Edit:
This is what exactly I'm trying to achieve. I want to analyse area in contour to detect defects (by analyzing histogram of an area ?) and declare piece defective or good. Images attached.
Good sample. (Contour detected is outlined in gray color)
Defective sample. (defect around top left corner)

Answer 1

You may probably misuse the histogram.
Contour of an image should be a binary-valued, color-less matrix which does not represent grayscale-level of pixels, but the boundaries.
Meanwhile, histogram is a tool for analyzing how grayscale-valued of pixels distribute in your 2D image, isn't it?
Thus, why you want to profile the histogram of a binary-valued matrix which might not help you analyzing the image? Histogram is not the right t ool for the contour analysis though.
What you may get from the histogram should just be only a two-bar histogram because the contoured matrix contains only binary values. This might not be helpful for analysis.

Answer 2

Here's an other way, using morphological operations.
#include <string>
#include <iostream>
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
cv::Mat make_element(int morph_size, int elem_type)
{
cv::Size sz{2*morph_size+1, 2*morph_size+1};
cv::Point pt{morph_size, morph_size};
cv::Mat element{getStructuringElement(elem_type, sz, pt)};
return element;
}
int main(int argc, char **argv)
{
std::string fn{argv[1]};
cv::Mat src{cv::imread(fn)}, dst, mask[3];
if (!src.data) {
std::cerr << "No image data :(" << std::endl;
return -1;
}
// Clean out noise
cv::Mat elem1{make_element(5, cv::MORPH_RECT)};
cv::morphologyEx(src, dst, cv::MORPH_OPEN, elem1);
// Close the hole, then XOR with original
cv::Mat elem2{make_element(45, cv::MORPH_ELLIPSE)};
morphologyEx(dst, dst, cv::MORPH_CLOSE, elem2);
cv::bitwise_xor(src, dst, dst);
// Clean out noise (again)
cv::Mat elem3{make_element(1, cv::MORPH_RECT)};
cv::morphologyEx(dst, dst, cv::MORPH_OPEN, elem3);
// Mark the hole
cv::split(dst, mask);
cv::bitwise_xor(src, dst, dst, mask[0]);
// Overlay
cv::split(dst, mask);
cv::Mat empty{dst.size(), CV_8UC1};
std::vector<cv::Mat> v{empty, empty, mask[0]};
cv::merge(v, dst);
cv::bitwise_or(src, dst, dst);
cv::namedWindow("Defect (ESC to quit)", cv::WINDOW_NORMAL);
cv::startWindowThread();
cv::imshow("Defect (ESC to quit)", dst);
while (true) {
int k = cv::waitKey(100) & 0xff;
if (k == 27) {
break;
}
}
cv::destroyAllWindows();
return 0;
};
Some additional reading:
Shapiro/Stockman, Finding gear defects, Chapter 3
OpenCV morphology tutorial