Detect And Track Multiple Faces - opencv3.0 c++ - c++

Question

I'm trying to detect multiple faces from video file according to user input.
The code detects the number of the desired faces but sometimes the detection "jumps" to someone else - different from the first frame.
The video file contains 5-6 faces and the user can select a number from 1 to 4.
The program should to detect the first X faces and tracked throughout the video, in addition, the program opens another window with the detected faces.
Here is my algorithm code:
capture >> cap_img;
waitKey(2);
cvtColor(cap_img, gray_img, CV_BGR2GRAY);
equalizeHist(gray_img, gray_img);
while (1)
{
capture >> cap_img;
waitKey(2);
cvtColor(cap_img, gray_img, CV_BGR2GRAY);
equalizeHist(gray_img, gray_img);
// Detect faces
face_cascade.detectMultiScale(gray_img, faces, 1.1, 5,0 | CV_HAAR_SCALE_IMAGE | CV_HAAR_DO_CANNY_PRUNING | CASCADE_SCALE_IMAGE, Size(1, 1));
// Set Region of Interest
cv::Rect reg_b;
cv::Rect reg_c;
int i = 0; // i is index of current element
int ac = 0; // ac is area of current element
int ib = 0; // ib is index of biggest element
int ab = 0; // ab is area of biggest element
if (numOfFaces > faces.size())
{
numOfFaces = faces.size();
}
for (i = 0; i < numOfFaces; i++) // Iterate through all current elements (detected faces)
{
reg_c.x = faces[i].x;
reg_c.y = faces[i].y;
reg_c.width = (faces[i].width);
reg_c.height = (faces[i].height);
// Get the area of current element (detected face), at beginning it is same as "current" element
ac = reg_c.width * reg_c.height;
reg_b.x = faces[ib].x;
reg_b.y = faces[ib].y;
reg_b.width = (faces[ib].width);
reg_b.height = (faces[ib].height);
ab = reg_b.width * reg_b.height;
// Get the area of biggest element, at beginning it is same as "current" element
if (ac > ab)
{
ib = i;
reg_b.x = faces[ib].x;
reg_b.y = faces[ib].y;
reg_b.width = (faces[ib].width);
reg_b.height = (faces[ib].height);
}
crop = cap_img(reg_b);
resize(crop, res, Size(128, 128), 0, 0, INTER_LINEAR); // This will be needed later while saving images
cvtColor(crop, gray, CV_BGR2GRAY); // Convert cropped image to Grayscale
Point pt1(faces[i].x, faces[i].y); // Display detected faces on main window - live stream from camera
Point pt2((faces[i].x + faces[i].height), (faces[i].y + faces[i].width));
rectangle(cap_img, pt1, pt2, Scalar(0, 255, 0), 2, 8, 0);
}
// Show image
resize(cap_img, cap_img, Size(cap_img.cols / 2, cap_img.rows / 2)); // to half size or even smaller
imshow("original", cap_img);
if (!crop.empty())
{
imshow("detected", crop);
}
else
destroyWindow("detected");
}
"numOfFaces" is the number of faces to detect.
What am I doing wrong?