Cascade Classifier

Goal

In this tutorial you will learn how to:

• Use the cv::CascadeClassifier class to detect objects in a video stream. Particularly, we will use the functions:
  • cv::CascadeClassifier::load to load a .xml classifier file. It can be either a Haar or a LBP classifer
  • cv::CascadeClassifier::detectMultiScale to perform the detection.

Theory

Code

This tutorial code's is shown lines below. You can also download it from here . The second version (using LBP for face detection) can be found here

#include "opencv2/objdetect.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"

#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;

/* Function Headers */
void detectAndDisplay( Mat frame );

/* Global variables */
String face_cascade_name = "haarcascade_frontalface_alt.xml";
String eyes_cascade_name = "haarcascade_eye_tree_eyeglasses.xml";
CascadeClassifier face_cascade;
CascadeClassifier eyes_cascade;
String window_name = "Capture - Face detection";

/* @function main */
int main( void )
{
 VideoCapture capture;
 Mat frame;

 //-- 1. Load the cascades
 if( !face_cascade.load( face_cascade_name ) ){ printf("--(!)Error loading face cascade\n"); return -1; };
 if( !eyes_cascade.load( eyes_cascade_name ) ){ printf("--(!)Error loading eyes cascade\n"); return -1; };

 //-- 2. Read the video stream
 capture.open( -1 );
 if ( ! capture.isOpened() ) { printf("--(!)Error opening video capture\n"); return -1; }

 while ( capture.read(frame) )
 {
 if( frame.empty() )
 {
 printf(" --(!) No captured frame -- Break!");
 break;
 }

 //-- 3. Apply the classifier to the frame
 detectAndDisplay( frame );

 int c = waitKey(10);
 if( (char)c == 27 ) { break; } // escape
 }
 return 0;
}

/* @function detectAndDisplay */
void detectAndDisplay( Mat frame )
{
 std::vector<Rect> faces;
 Mat frame_gray;

 cvtColor( frame, frame_gray, COLOR_BGR2GRAY );
 equalizeHist( frame_gray, frame_gray );

 //-- Detect faces
 face_cascade.detectMultiScale( frame_gray, faces, 1.1, 2, 0|CASCADE_SCALE_IMAGE, Size(30, 30) );

 for( size_t i = 0; i < faces.size(); i++ )
 {
 Point center( faces[i].x + faces[i].width/2, faces[i].y + faces[i].height/2 );
 ellipse( frame, center, Size( faces[i].width/2, faces[i].height/2), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );

 Mat faceROI = frame_gray( faces[i] );
 std::vector<Rect> eyes;

 //-- In each face, detect eyes
 eyes_cascade.detectMultiScale( faceROI, eyes, 1.1, 2, 0 |CASCADE_SCALE_IMAGE, Size(30, 30) );

 for( size_t j = 0; j < eyes.size(); j++ )
 {
 Point eye_center( faces[i].x + eyes[j].x + eyes[j].width/2, faces[i].y + eyes[j].y + eyes[j].height/2 );
 int radius = cvRound( (eyes[j].width + eyes[j].height)*0.25 );
 circle( frame, eye_center, radius, Scalar( 255, 0, 0 ), 4, 8, 0 );
 }
 }
 //-- Show what you got
 imshow( window_name, frame );
}

Explanation

Result

1. Here is the result of running the code above and using as input the video stream of a build-in webcam:

   

   Remember to copy the files haarcascade_frontalface_alt.xml and haarcascade_eye_tree_eyeglasses.xml in your current directory. They are located in opencv/data/haarcascades

2. This is the result of using the file lbpcascade_frontalface.xml (LBP trained) for the face detection. For the eyes we keep using the file used in the tutorial.