What's wrong with CascadeClassifier (opencv) that is called many time - c++

Question

Assume:
There are N files on disk that require face detection.
If I do it in "slow" mode all is well (faces are faces at their rects):
for(auto mat : getMyMats()) {
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
vector<Rect> faces;
facer.detectMultiScale(mat, faces);
}
But if i try to cache and reuse CascadeClassifier - near by 50% image are detected very wrong - invalid place, size, count of faces:
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
for(auto mat : getMyMats()) {
vector<Rect> faces;
facer.detectMultiScale(mat, faces);
}
What's wrong with CascadeClassifier reusing
Windows, OpenCV 3.0 both debug and release build.
SAMPLE CODE TO REPRODUCE:
// compiler - opencv/include referenced
// linker - opencv/lib referenced and
// opencv_core300.lib, opencv_imgproc300.lib, opencv_imgcodecs300.lib, opencv_objdetect300.lib (or world or debug libs) are referenced
// 1.png, 2.png, 3.png,
#include <vector>
#include <string>
#include <iostream>
#include "opencv2/opencv.hpp"
using namespace cv;
using namespace std;
//count faces in load-every-time call
int no_reuse_call() {
int cnt = 0;
for (auto n : { "1.png","2.png","3.png" }) {
vector<Rect> faces{};
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
for (auto n : { "1.png","2.png","3.png" }) {
vector<Rect> faces{};
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
return cnt;
}
//coutn faces in one-instance call
int reuse_call() {
int cnt = 0;
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
for (auto n : { "1.png","2.png","3.png" }) {
vector<Rect> faces{};
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
for (auto n : { "1.png","2.png","3.png" }) {
vector<Rect> faces{};
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
return cnt;
}
int reuse_call_reverse() {
int cnt = 0;
CascadeClassifier facer("haarcascade_frontalface_alt.xml");
for (auto n : {"3.png" ,"2.png", "1.png"}) {
vector<Rect> faces{};
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
for (auto n : { "3.png" ,"2.png", "1.png" }) {
vector<Rect> faces{};
Mat m = imread(n, CV_LOAD_IMAGE_GRAYSCALE);
facer.detectMultiScale(m, faces, 1.1, 3, CASCADE_SCALE_IMAGE, Size(50, 50));
cnt += faces.size(); //vector given to detectMultiScale cleared in detectMultiScale
cout << faces[0] << endl;
}
return cnt;
}
int main() {
cout << "Created every image 2*3 files" << endl;
cout << no_reuse_call() << endl;
cout << "Created one time and files in alphabet order 2*3 files" << endl;
cout << reuse_call() << endl;
cout << "Created one time and files in contra - alphabet order 2*3 files" << endl;
cout << reuse_call_reverse() << endl;
cout << "You can see:" << endl;
cout << " if cascade is reused - detection in 3.png file is stabily wrong if it is processed after 1.png and 2.png" << endl;
cout << " it's depend on order - if 3.png is first in row it detected right even in second call after 1.png and 2.png" << endl;
cout << " it's random and based somehow on image - 1.png and 2.png are always detected validly" << endl;
return 0;
}

Answer 1

Look at https://github.com/Itseez/opencv/issues/5475
due to some reasones ocl in opencv 3.0.0 is not (stable?)
so author recomend to call setUseOpenCL(false); in programm - with this option all work well