A Robust Real Time Face Detection

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A Robust Real Time Face Detection
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Outline

• AdaBoost Learning Algorithm
• Face Detection in real life
• Using AdaBoost for Face Detection
• Improvements
• Demonstration

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AdaBoost

• A short Introduction to Boosting (Freund
  Schapire, 1999)
• Logistic Regression, AdaBoost and Bregman
  Distances (Collins, Schapire, Singer, 2002)

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Boosting

• The Horse-Racing Gambler Problem
• Rules of thumb for a set of races
• How should we choose the set of races in order to
  get the best rules of thumb?
• How should the rules be combined into a single
  highly accurate prediction rule?
• Boosting !

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• Boosting

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AdaBoost - the idea

• AdaBoost agglomerates many weak classifiers into
  one strong classifier.

• Initialize sample weights
• For each cycle
• Find a classifier that performs well on the
  weighted sample
• Increase weights of misclassified examples
• Return a weighted list of classifiers

IQ
Shoe size
Shoe size
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AdaBoost - algorithm
distribution
example
decision
step
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AdaBoost training error

• Freund and Schapire (1997) proved that
• AdaBoost adapts to the error rates of the
  individual weak hypotheses.
• Therefore it is called ADABoost.

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AdaBoost generalization error

• Freund and Schapire (1997) showed that

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AdaBoost generalization error

• The analysis implies that boosting will overfit
  if the algorithm is run for too many rounds
• However, it was observed empirically that
  AdaBoost does not overfit,
• even when run thousands of rounds.
• Moreover, it was observed that the generalization
  error continues to drive down long after training
  error reached zero

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AdaBoost generalization error

• An alternative analysis was presented by Schapire
  et al. (1998), that suits the empirical findings

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AdaBoost different point of view

• We try to solve the problem of approximating the
  ys using a linear combination of weak hypotheses
• In other words, we are interested in the problem
  of finding a vector of parameters a such that
  is a good approximation of yi
• For classification problems we try to match the
  sign of f(xi) to yi

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AdaBoost different point of view

• Sometimes it is advantageous to minimize some
  other (non-negative) loss function instead of the
  number of classification errors
• For AdaBoost the loss function is
• This point of view was used by Collins, Schapire
  and Singer (2002) to demonstrate that AdaBoost
  converges to optimality

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Face Detection (not face recognition)
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Face Detection in Monkeys

• There are cells that detect faces

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Face Detection in Human

• There are processes of face detection

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Faces Are Special

• We humans analyze faces in a different way

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Faces Are Special

• We humans analyze faces in a different way

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Faces Are Special
We analyze faces in a different way
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Face Recognition in Human

• We analyze faces in a specific location

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Robust Real-Time Face Detection

• Viola and Jones, 2003

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Features

• Picture analysis, Integral Image

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Features

• The system classifies images based on the value
  of simple features

Two-rectangle
Value ? (pixels in white area) - ? (pixels in
black area)
Three-rectangle
Four-rectangle
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Contrast Features
Source
Result
Notice that each feature is related to a special
location in the sub-window
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Features

• Notice that each feature is related to a special
  location in the sub-window
• Why features and not pixels?
• Encode domain knowledge
• Feature based system operates faster
• Inspiration from human vision

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Features

• Later we will see that there are other features
  that can be used to implement an efficient face
  detector
• The original system of Viola and Jones used only
  rectangle features

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Computing Features

• Given a detection resolution of 24x24, and size
  of 200x200, the set of rectangle features is
  160,000 !
• We need to find a way to rapidly compute the
  features

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Integral Image

• Intermediate representation of the image
• Computed in one pass over the original image

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Integral Image
Using the integral image representation one can
compute the value of any rectangular sum in
constant time. For example the integral sum
inside rectangle D we can compute as ii(4)
ii(1) ii(2) ii(3)
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Integral Image
Integral Image
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Building a Detector

• Cascading, training a cascade

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Main Ideas

• The Features will be used as weak classifiers
• We will concatenate several detectors serially
  into a cascade
• We will boost (using a version of AdaBoost) a
  number of features to get good enough detectors

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Weak Classifiers

• Weak Classifier A feature which best separates
  the examples
• Given a sub-window (x), a feature (f), a
  threshold (T), and a polarity (p) indicating the
  direction of the inequality

Probability for this threshold
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Weak Classifiers

• A weak classifier is a combination of a feature
  and a threshold
• We have K features
• We have N thresholds where N is the number of
  examples
• Thus there are KN weak classifiers

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Weak Classifier Selection

• For each feature sort the examples based on
  feature value
• For each element evaluate the total sum of
  positive/negative example weights (T/T-) and the
  sum of positive/negative weights below the
  current example (S/S-)
• The error for a threshold which splits the range
  between the current and previous example in the
  sorted list is

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An example
positive/negative example weights
the sum of positive/negative weights below the
current example
For e calculation
weight
Feature value

e B A S- S T- T W f y x
2/5 3/5 2/5 0 0 2/5 3/5 1/5 2 -1 X1
1/5 4/5 1/5 1/5 0 2/5 3/5 1/5 3 -1 X2
0 5/5 0 2/5 0 2/5 3/5 1/5 5 1 X3
1/5 4/5 1/5 2/5 1/5 2/5 3/5 1/5 7 1 X4
2/5 3/5 2/5 2/5 2/5 2/5 3/5 1/5 8 1 X5
Error min(A,B)
examples
decision
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Main Ideas Cascading

• The Features will be used as weak classifiers
• We will concatenate several detectors serially
  into a cascade
• We will boost (using a version of AdaBoost) a
  number of features to get good enough detectors

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Cascading

• We start with simple classifiers which reject
  many of the negative sub-windows while detecting
  almost all positive sub-windows
• Positive results from the first classifier
  triggers the evaluation of a second (more
  complex) classifier, and so on
• A negative outcome at any point leads to the
  immediate rejection of the sub-window

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Cascading
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Main Ideas Boosting

• The Features will be used as weak classifiers
• We will concatenate several detectors serially
  into a cascade
• We will boost (using a version of AdaBoost) a
  number of features to get good enough detectors

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Training a cascade

• User selects values for
• Maximum acceptable false positive rate per layer
• Minimum acceptable detection rate per layer
• Target overall false positive rate
• User gives a set of positive and negative examples

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Training a cascade (cont.)

• While the overall false positive rate is not met
• While the false positive rate of current layer is
  less than the maximum per layer
• Train a classifier with n features using AdaBoost
  on a set of positive and negative examples
• Decrease threshold when the current classifier
  detection rate of the layer is more than the
  minimum
• Evaluate current cascade classifier on validation
  set
• Evaluate current cascade detector on a set of non
  faces images and put any false detections into
  the negative training set

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Results
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Training Data Set

• 4916 hand labeled faces
• Aligned to base resolution (24x24)
• Non faces for first layer were collected from
  9500 non faces images
• Non faces for subsequent layers were obtained by
  scanning the partial cascade across non faces and
  collecting false positives (max 6000 for each
  layer)

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Structure of the Detector

• 38 layer cascade
• 6060 features

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Speed of final Detector

• On a 700Mhz Pentium III processor, the face
  detector can process a 384 by 288 pixel image in
  about .067 seconds

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Improvements

• Learning Object Detection from a Small Number of
  Examples the Importance of Good Features (Levy
  Weiss, 2004)

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Improvements

• Performance depends crucially on the features
  that are used to represent the objects (Levy
  Weiss, 2004)
• Good Features imply
• Good results from small training databases
• Better generalization abilities
• Shorter (faster) classifiers

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Edge Orientation Histogram

• Invariant to global illumination changes
• Captures geometric properties of faces
• Domain knowledge represented
• Inner part of the face includes more horizontal
  edges then vertical
• The ration between vertical and horizontal edges
  is bounded
• The area of the eyes includes mainly horizontal
  edges
• The chin has more or less the same number of
  oblique edges on both sides

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Edge Orientation Histogram

• Called EOH
• The EOH can be calculated using some kind of
  Integral Image
• We find the gradients at the point (x,y) using
  Sobel masks
• We calculate the orientation of the edge (x,y)
• We divide the edges into K bins
• The result is stored in K matrices
• We use the same idea of Integral Image for the
  matrices

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EOH Features

• The ratio between two orientations
• The dominance of a given orientation
• Symmetry Features

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Results

• Already with only 250 positive examples we can
  see above 90 detection rate
• Faster classifier
• Better performance in profile faces

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DemoImplementing Viola Jones systemFrank
Fritze, 2004