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Cascade Classifier

Next Tutorial: Cascade Classifier Training

Original author | Ana Huamán
Compatibility | OpenCV >= 3.0

# Goal

In this tutorial,

- We will learn how the Haar cascade object detection works.
- We will see the basics of face detection and eye detection using the Haar Feature-based Cascade Classifiers
- We will 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 classifier
  - cv::CascadeClassifier::detectMultiScale to perform the detection.

# Theory

Object Detection using Haar feature-based cascade classifiers is an effective object detection method proposed by Paul Viola and Michael Jones in their paper, "Rapid Object Detection using a Boosted Cascade of Simple Features" in 2001. It is a machine learning based approach where a cascade function is trained from a lot of positive and negative images. It is then used to detect objects in other images.

Here we will work with face detection. Initially, the algorithm needs a lot of positive images (images of faces) and negative images (images without faces) to train the classifier. Then we need to extract features from it. For this, Haar features shown in the below image are used. They are just like our convolutional kernel. Each feature is a single value obtained by subtracting sum of pixels under the white rectangle from sum of pixels under the black rectangle.

image

Now, all possible sizes and locations of each kernel are used to calculate lots of features. (Just imagine how much computation it needs? Even a 24x24 window results over 160000 features). For each feature calculation, we need to find the sum of the pixels under white and black rectangles. To solve this, they introduced the integral image. However large your image, it reduces the calculations for a given pixel to an operation involving just four pixels. Nice, isn't it? It makes things super-fast.

But among all these features we calculated, most of them are irrelevant. For example, consider the image below. The top row shows two good features. The first feature selected seems to focus on the property that the region of the eyes is often darker than the region of the nose and cheeks. The second feature selected relies on the property that the eyes are darker than the bridge of the nose. But the same windows applied to cheeks or any other place is irrelevant. So how do we select the best features out of 160000+ features? It is achieved by Adaboost.

image

For this, we apply each and every feature on all the training images. For each feature, it finds the best threshold which will classify the faces to positive and negative. Obviously, there will be errors or misclassifications. We select the features with minimum error rate, which means they are the features that most accurately classify the face and non-face images. (The process is not as simple as this. Each image is given an equal weight in the beginning. After each classification, weights of misclassified images are increased. Then the same process is done. New error rates are calculated. Also new weights. The process is continued until the required accuracy or error rate is achieved or the required number of features are found).

The final classifier is a weighted sum of these weak classifiers. It is called weak because it alone can't classify the image, but together with others forms a strong classifier. The paper says even 200 features provide detection with 95% accuracy. Their final setup had around 6000 features. (Imagine a reduction from 160000+ features to 6000 features. That is a big gain).

So now you take an image. Take each 24x24 window. Apply 6000 features to it. Check if it is face or not. Wow.. Isn't it a little inefficient and time consuming? Yes, it is. The authors have a good solution for that.

In an image, most of the image is non-face region. So it is a better idea to have a simple method to check if a window is not a face region. If it is not, discard it in a single shot, and don't process it again. Instead, focus on regions where there can be a face. This way, we spend more time checking possible face regions.

For this they introduced the concept of Cascade of Classifiers. Instead of applying all 6000 features on a window, the features are grouped into different stages of classifiers and applied one-by-one. (Normally the first few stages will contain very many fewer features). If a window fails the first stage, discard it. We don't consider the remaining features on it. If it passes, apply the second stage of features and continue the process. The window which passes all stages is a face region. How is that plan!

The authors' detector had 6000+ features with 38 stages with 1, 10, 25, 25 and 50 features in the first five stages. (The two features in the above image are actually obtained as the best two features from Adaboost). According to the authors, on average 10 features out of 6000+ are evaluated per sub-window.

So this is a simple intuitive explanation of how Viola-Jones face detection works. Read the paper for more details or check out the references in the Additional Resources section.

# Haar-cascade Detection in OpenCV

OpenCV provides a training method (see Cascade Classifier Training) or pretrained models, that can be read using the cv::CascadeClassifier::load method. The pretrained models are located in the data folder in the OpenCV installation or can be found here.

The following code example will use pretrained Haar cascade models to detect faces and eyes in an image. First, a cv::CascadeClassifier is created and the necessary XML file is loaded using the cv::CascadeClassifier::load method. Afterwards, the detection is done using the cv::CascadeClassifier::detectMultiScale method, which returns boundary rectangles for the detected faces or eyes.

C++

This tutorial code's is shown lines below. You can also download it from here

#include "opencv2/xobjdetect.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/videoio.hpp"
#include <iostream>
using namespace std;
using namespace cv;
void detectAndDisplay( Mat frame );
CascadeClassifier face_cascade;
CascadeClassifier eyes_cascade;
int main( int argc, const char** argv )
{
CommandLineParser parser(argc, argv,
"{help h||}"
"{face_cascade|data/haarcascades/haarcascade_frontalface_alt.xml|Path to face cascade.}"
"{eyes_cascade|data/haarcascades/haarcascade_eye_tree_eyeglasses.xml|Path to eyes cascade.}"
"{camera|0|Camera device number.}");
parser.about( "\nThis program demonstrates using the cv::CascadeClassifier class to detect objects (Face + eyes) in a video stream.\n"
"You can use Haar or LBP features.\n\n" );
parser.printMessage();
String face_cascade_name = samples::findFile( parser.get<String>("face_cascade") );
String eyes_cascade_name = samples::findFile( parser.get<String>("eyes_cascade") );
//-- 1. Load the cascades
if( !face_cascade.load( face_cascade_name ) )
{
cout << "--(!)Error loading face cascade\n";
return -1;
};
if( !eyes_cascade.load( eyes_cascade_name ) )
{
cout << "--(!)Error loading eyes cascade\n";
return -1;
};
int camera_device = parser.get<int>("camera");
VideoCapture capture;
//-- 2. Read the video stream
capture.open( camera_device );
if ( ! capture.isOpened() )
{
cout << "--(!)Error opening video capture\n";
return -1;
}
Mat frame;
while ( capture.read(frame) )
{
if( frame.empty() )
{
cout << "--(!) No captured frame -- Break!\n";
break;
}
//-- 3. Apply the classifier to the frame
detectAndDisplay( frame );
if( waitKey(10) == 27 )
{
break; // escape
}
}
return 0;
}
void detectAndDisplay( Mat frame )
{
Mat frame_gray;
cvtColor( frame, frame_gray, COLOR_BGR2GRAY );
equalizeHist( frame_gray, frame_gray );
//-- Detect faces
std::vector<Rect> faces;
face_cascade.detectMultiScale( frame_gray, faces );
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 );
Mat faceROI = frame_gray( faces[i] );
//-- In each face, detect eyes
std::vector<Rect> eyes;
eyes_cascade.detectMultiScale( faceROI, eyes );
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 );
}
}
//-- Show what you got
imshow( "Capture - Face detection", frame );
}
cv::CascadeClassifier
Cascade classifier class for object detection.
Definition xobjdetect.hpp:230
cv::CascadeClassifier::load
bool load(const String &filename)
Loads a classifier from a file.
cv::CascadeClassifier::detectMultiScale
void detectMultiScale(InputArray image, std::vector< Rect > &objects, double scaleFactor=1.1, int minNeighbors=3, int flags=0, Size minSize=Size(), Size maxSize=Size())
Detects objects of different sizes in the input image. The detected objects are returned as a list of...
cv::CommandLineParser
Designed for command line parsing.
Definition utility.hpp:890
cv::Mat
n-dimensional dense array class
Definition mat.hpp:951
cv::Mat::empty
bool empty() const
Returns true if the array has no elements.
cv::Point_< int >
cv::Scalar_< double >
cv::Size_
Template class for specifying the size of an image or rectangle.
Definition types.hpp:338
cv::VideoCapture
Class for video capturing from video files, image sequences or cameras.
Definition videoio.hpp:727
cv::VideoCapture::open
virtual bool open(const String &filename, int apiPreference=CAP_ANY)
Opens a video file or a capturing device or an IP video stream for video capturing.
cv::String
std::string String
Definition cvstd.hpp:151
cvRound
int cvRound(double value)
Rounds floating-point number to the nearest integer.
Definition fast_math.hpp:200
cv::datasets::circle
@ circle
Definition gr_skig.hpp:62
cv::imshow
void imshow(const String &winname, InputArray mat)
Displays an image in the specified window.
cv::waitKey
int waitKey(int delay=0)
Waits for a pressed key.
cv::cvtColor
void cvtColor(InputArray src, OutputArray dst, int code, int dstCn=0, AlgorithmHint hint=cv::ALGO_HINT_DEFAULT)
Converts an image from one color space to another.
cv::ellipse
void ellipse(InputOutputArray img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws a simple or thick elliptic arc or fills an ellipse sector.
cv::equalizeHist
void equalizeHist(InputArray src, OutputArray dst)
Equalizes the histogram of a grayscale image.
highgui.hpp
main
int main(int argc, char *argv[])
Definition highgui_qt.cpp:3
imgproc.hpp
cv
Definition core.hpp:107
std
STL namespace.
videoio.hpp
xobjdetect.hpp
Java

This tutorial code's is shown lines below. You can also download it from here

import java.util.List;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.MatOfRect;
import org.opencv.core.Point;
import org.opencv.core.Rect;
import org.opencv.core.Scalar;
import org.opencv.core.Size;
import org.opencv.highgui.HighGui;
import org.opencv.imgproc.Imgproc;
import org.opencv.xobjdetect.CascadeClassifier;
import org.opencv.videoio.VideoCapture;
class ObjectDetection {
public void detectAndDisplay(Mat frame, CascadeClassifier faceCascade, CascadeClassifier eyesCascade) {
Mat frameGray = new Mat();
Imgproc.cvtColor(frame, frameGray, Imgproc.COLOR_BGR2GRAY);
Imgproc.equalizeHist(frameGray, frameGray);
// -- Detect faces
MatOfRect faces = new MatOfRect();
faceCascade.detectMultiScale(frameGray, faces);
List<Rect> listOfFaces = faces.toList();
for (Rect face : listOfFaces) {
Point center = new Point(face.x + face.width / 2, face.y + face.height / 2);
Imgproc.ellipse(frame, center, new Size(face.width / 2, face.height / 2), 0, 0, 360,
new Scalar(255, 0, 255));
Mat faceROI = frameGray.submat(face);
// -- In each face, detect eyes
MatOfRect eyes = new MatOfRect();
eyesCascade.detectMultiScale(faceROI, eyes);
List<Rect> listOfEyes = eyes.toList();
for (Rect eye : listOfEyes) {
Point eyeCenter = new Point(face.x + eye.x + eye.width / 2, face.y + eye.y + eye.height / 2);
int radius = (int) Math.round((eye.width + eye.height) * 0.25);
Imgproc.circle(frame, eyeCenter, radius, new Scalar(255, 0, 0), 4);
}
}
//-- Show what you got
HighGui.imshow("Capture - Face detection", frame );
}
public void run(String[] args) {
String filenameFaceCascade = args.length > 2 ? args[0] : "../../data/haarcascades/haarcascade_frontalface_alt.xml";
String filenameEyesCascade = args.length > 2 ? args[1] : "../../data/haarcascades/haarcascade_eye_tree_eyeglasses.xml";
int cameraDevice = args.length > 2 ? Integer.parseInt(args[2]) : 0;
CascadeClassifier faceCascade = new CascadeClassifier();
CascadeClassifier eyesCascade = new CascadeClassifier();
if (!faceCascade.load(filenameFaceCascade)) {
System.err.println("--(!)Error loading face cascade: " + filenameFaceCascade);
System.exit(0);
}
if (!eyesCascade.load(filenameEyesCascade)) {
System.err.println("--(!)Error loading eyes cascade: " + filenameEyesCascade);
System.exit(0);
}
VideoCapture capture = new VideoCapture(cameraDevice);
if (!capture.isOpened()) {
System.err.println("--(!)Error opening video capture");
System.exit(0);
}
Mat frame = new Mat();
while (capture.read(frame)) {
if (frame.empty()) {
System.err.println("--(!) No captured frame -- Break!");
break;
}
//-- 3. Apply the classifier to the frame
detectAndDisplay(frame, faceCascade, eyesCascade);
if (HighGui.waitKey(10) == 27) {
break;// escape
}
}
System.exit(0);
}
}
public class ObjectDetectionDemo {
public static void main(String[] args) {
// Load the native OpenCV library
System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
new ObjectDetection().run(args);
}
}
Python

This tutorial code's is shown lines below. You can also download it from here

from __future__ import print_function
import cv2 as cv
import argparse
def detectAndDisplay(frame):
frame_gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
frame_gray = cv.equalizeHist(frame_gray)
#-- Detect faces
faces = face_cascade.detectMultiScale(frame_gray)
for (x,y,w,h) in faces:
center = (x + w//2, y + h//2)
frame = cv.ellipse(frame, center, (w//2, h//2), 0, 0, 360, (255, 0, 255), 4)
faceROI = frame_gray[y:y+h,x:x+w]
#-- In each face, detect eyes
eyes = eyes_cascade.detectMultiScale(faceROI)
for (x2,y2,w2,h2) in eyes:
eye_center = (x + x2 + w2//2, y + y2 + h2//2)
radius = int(round((w2 + h2)*0.25))
frame = cv.circle(frame, eye_center, radius, (255, 0, 0 ), 4)
cv.imshow('Capture - Face detection', frame)
parser = argparse.ArgumentParser(description='Code for Cascade Classifier tutorial.')
parser.add_argument('--face_cascade', help='Path to face cascade.', default='data/haarcascades/haarcascade_frontalface_alt.xml')
parser.add_argument('--eyes_cascade', help='Path to eyes cascade.', default='data/haarcascades/haarcascade_eye_tree_eyeglasses.xml')
parser.add_argument('--camera', help='Camera divide number.', type=int, default=0)
args = parser.parse_args()
face_cascade_name = args.face_cascade
eyes_cascade_name = args.eyes_cascade
face_cascade = cv.CascadeClassifier()
eyes_cascade = cv.CascadeClassifier()
#-- 1. Load the cascades
if not face_cascade.load(cv.samples.findFile(face_cascade_name)):
print('--(!)Error loading face cascade')
exit(0)
if not eyes_cascade.load(cv.samples.findFile(eyes_cascade_name)):
print('--(!)Error loading eyes cascade')
exit(0)
camera_device = args.camera
#-- 2. Read the video stream
cap = cv.VideoCapture(camera_device)
if not cap.isOpened:
print('--(!)Error opening video capture')
exit(0)
while True:
ret, frame = cap.read()
if frame is None:
print('--(!) No captured frame -- Break!')
break
detectAndDisplay(frame)
if cv.waitKey(10) == 27:
break
cv::samples::findFile
cv::String findFile(const cv::String &relative_path, bool required=true, bool silentMode=false)
Try to find requested data file.
cv::circle
void circle(InputOutputArray img, Point center, int radius, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws a circle.

# Result

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

Be sure the program will find the path of files haarcascade_frontalface_alt.xml and haarcascade_eye_tree_eyeglasses.xml. They are located in opencv_contrib/modules/xobjdetect/data/haarcascades

1. 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.

# Additional Resources

1. Paul Viola and Michael J. Jones. Robust real-time face detection. International Journal of Computer Vision, 57(2):137–154, 2004. [287]
2. Rainer Lienhart and Jochen Maydt. An extended set of haar-like features for rapid object detection. In Image Processing. 2002. Proceedings. 2002 International Conference on, volume 1, pages I–900. IEEE, 2002. [168]
3. Video Lecture on Face Detection and Tracking
4. An interesting interview regarding Face Detection by Adam Harvey
5. OpenCV Face Detection: Visualized on Vimeo by Adam Harvey