c++ - findContours, contourArea give errors with nested contours. "Assertion failed", "input array is not a valid matrix" -

i trying find largest contour in binarized image. judging this question , this tutorial think trivial, , agree. when run code on the image below though, produces errors. note 2x2 dot in upper left hand corner, should count 1 contour.

mat img = imread("problem.png", cv_load_image_grayscale); vector<vector<point>> contourvector; findcontours(img, contourvector, cv_retr_list, cv_chain_approx_none); //findcontours(img, contourvector, cv_retr_external, cv_chain_approx_none);    // alternative mode  int biggest = 0; double biggestcontourarea = contourarea(contourvector[biggest]); (int = 1; != contourvector.size(); ++i){ if ( (contourarea(contourvector[i])) > biggestcontourarea) {     biggest = i;     biggestcontourarea = contourarea(contourvector[biggest]); } } img = scalar(0,0,0); drawcontours(img, contourvector, biggest, scalar(255,255,255), cv_filled ); imshow("largest contour", img); waitkey(0); 

if mode cv_retr_list used, error @ = 3, although contourvector has size 4. why vector larger number of contours, though?

"assertion failed: (0 <= contouridx && contouridx < (int)last) in unknown function, file ..[..]contours.cpp, line 1810" 

if mode cv_retr_external used (which make more sense), error. why happen

opencv error: bad argument (input array not valid matrix)  in unknown function, file ..[..]utils.cpp, line 54 

i'd grateful if explain errors.

i wonder why result of contourarea inside loop 0 reason , contourvector[i].size() gives absurdly large number (around 4 billion).

i cannot judge cv_external case. in first case problem seems simple.

for (int = 1; != contourvector.size(); ++i){     if ( (contourarea(contourvector[i])) > biggestcontourarea) {         biggest = i;         biggestcontourarea = contourarea(contourvector[biggest]);     } } 

in c++ arrays , vectors indexed zero. first item in vector has index 0, , last 1 has contourvector.size() - 1.

to fix code change cycle to:

for (int = 1; < contourvector.size(); ++i)    ....