Different method to calculate blob area in OpenCv

Question

I'm trying to use a different method to calculate a blob area from contourArea, using the countNonZero function. I want to see if there is significant approximation introduced by contourArea function, and also be able to calculate area of contour with auto-intersection.

cv::findContours(drawingIn, contoursIn, CV_RETR_TREE, CV_CHAIN_APPROX_NONE);
double max=0;
double currentarea=0;
int currentareaCounted = 0;
int maxAreaCounted = 0;
for (unsigned int i=0;i<contoursIn.size();i++)
{
 currentarea=cv::contourArea(contoursIn[i]);
 cv::Mat drawingMatCount = cv::Mat::zeros(ROI_INTERACTION,ROI_INTERACTION, CV_8UC1);
 cv::drawContours(drawingMatCount, contoursIn, i, 255, -1, 8);
 currentareaCounted = cv::countNonZero(drawingMatCount);
 if (currentarea>max)
 max=currentarea; 
 if (currentareaCounted>maxAreaCounted)
 maxAreaCounted=currentareaCounted;
}

Is there something that could be improved? This code works, but my doubt is about declaring a new matrix every iteration. Could this lead to some performance issue?

Answer 1

• Declare drawingMatCount once before the loop
• cv::drawContours(drawingMatCount, contoursIn, i, 0, -1, 8); at the end of the loop

BTW: this is not the blob area, its the contour area, including the holes.