recognizing specific objects & facesthe viola/jones face detector • a seminal approach to...
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Recognizing Specific Objects & Faces
Lecture 8
Many slides from: S. Savarese, S. Lazebnik, D. Lowe, P. Viola
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Overview
• Specific Object Recognition– Lowe ‘04– Rothganger et al. ‘03
• Face Dectection– Sub-space methods– Neural Network-based approaches– Viola & Jones
• Face Recognition
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Specific Object Recognition
Slides from S. Savarese, D. Lowe and F. Rothganger
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Slide: S. Savarese
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Challenges:
• Variability due to:
– View point
– Illumination
– Occlusions
– But not intra-class variation
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Overview
• Specific Object Recognition– Lowe ‘04– Rothganger et al. ‘03
• Face Dectection– Sub-space methods– Neural Network-based approaches– Viola & Jones
• Face Recognition
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Face Detection & Recognition
Slides from P. Viola, S. Lazebnik, A. Torralba
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Face detection
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Face detection and recognition
Detection Recognition “Sally”
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Consumer application: iPhoto 2009
http://www.apple.com/ilife/iphoto/
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Consumer application: iPhoto 2009
• Can be trained to recognize pets!
http://www.maclife.com/article/news/iphotos_faces_recognizes_cats
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Challenges:
• Variability due to:
– Intra-class variation (but fairly small)
– Illumination
– Occlusions (eyeglasses, facial hair, hair)
– Fairly limited viewpoint (frontal, profile)
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Practical Challenges of face detection• Sliding window detector must evaluate tens of
thousands of location/scale combinations
• Faces are rare: 0–10 per image– For computational efficiency, we should try to
spend as little time as possible on the non-face windows
– A megapixel image has ~106 pixels and a comparable number of candidate face locations
– To avoid having a false positive in every image image, our false positive rate has to be less than 10-6
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Overview
• Specific Object Recognition– Lowe ‘04– Rothganger et al. ‘03
• Face Dectection– Sub-space methods– Neural Network-based approaches– Viola & Jones
• Face Recognition
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Subspace Methods
• PCA (“Eigenfaces”, Turk and Pentland)
• PCA (Bayesian, Moghaddam and Pentland)
• LDA/FLD (“Fisherfaces”, Belhumeur &
Kreigman)
• ICA
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Principal Component Analysis
• Given: N data points x1, … ,xN in Rd
• We want to find a new set of features that are
linear combinations of original ones:
u(xi) = uT(xi – µ)
(µ: mean of data points)
• What unit vector u in Rd captures the most
variance of the data?
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Principal Component Analysis
• Direction that maximizes the variance of the projected data:
Projection of data point
Covariance matrix of data
The direction that maximizes the variance is the eigenvector
associated with the largest eigenvalue of Σ
N
N
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Eigenfaces: Key idea
• Assume that most face images lie on
a low-dimensional subspace determined by
the first k (k<d) directions of maximum
variance
• Use PCA to determine the vectors u1,…uk
that span that subspace:
x ≈ μ + w1u1 + w2u2 + … + wkuk
• Represent each face using its “face space”
coordinates (w1,…wk)
• Perform nearest-neighbor recognition in “face
space”
M. Turk and A. Pentland, Face Recognition using Eigenfaces, CVPR 1991
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Eigenfaces example
Training
images
x1,…,xN
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Eigenfaces example
Top eigenvectors:
u1,…uk
Mean: μ
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Eigenfaces example
• Face x in “face space” coordinates:
=
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Eigenfaces example
• Face x in “face space” coordinates:
• Reconstruction:
= +
µ + w1u1 + w2u2 + w3u3 + w4u4 + …
=
^x =
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Summary: Recognition with eigenfaces
Process labeled training images:
• Find mean µ and covariance matrix Σ
• Find k principal components (eigenvectors of Σ) u1,…uk
• Project each training image xi onto subspace spanned by principal components:(wi1,…,wik) = (u1
T(xi – µ), … , ukT(xi – µ))
Given novel image x:
• Project onto subspace:(w1,…,wk) = (u1
T(x – µ), … , ukT(x – µ))
• Optional: check reconstruction error x – x to determine whether image is really a face
• Classify as closest training face in k-dimensional subspace
^
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Limitations
• Global appearance method: not robust to
misalignment, background variation
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Limitations
• PCA assumes that the data has a Gaussian
distribution (mean µ, covariance matrix Σ)
The shape of this dataset is not well described by its principal components
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Limitations
• The direction of maximum variance is not
always good for classification
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Distribution-Based Face Detector• Learn face and nonface models from examples [Sung and Poggio 95]
• Cluster and project the examples to a lower dimensional space using Gaussian distributions and PCA
• Detect faces using distance metric to face and nonface clusters
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Distribution-Based Face Detector• Learn face and nonface models from examples [Sung and Poggio 95]
Training Database
1000+ Real, 3000+ VIRTUAL
50,0000+ Non-Face Pattern
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Overview
• Specific Object Recognition– Lowe ‘04– Rothganger et al. ‘03
• Face Dectection– Sub-space methods– Neural Network-based approaches– Viola & Jones
• Face Recognition
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Neural Network-Based Face Detector
• Train a set of multilayer perceptrons and arbitrate a decision among all outputs [Rowley et al. 98]
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From: http://www.ius.cs.cmu.edu/IUS/har2/har/www/CMU-CS-95-158R/
The steps in preprocessing a window. First, a linear function is fit to the intensity values in the window,
and then subtracted out, correcting for some extreme lighting conditions. Then, histogram equalization is applied, to correct for different camera gains and to improve contrast. For each of these steps, the mapping is computed based on pixels inside the oval mask, while the mapping is applied to the entire window.
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Example CMU face detector results
input
All images from: http://www.ius.cs.cmu.edu/demos/facedemo.html
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From: http://www.ius.cs.cmu.edu/IUS/har2/har/www/CMU-CS-95-158R/
Example face images, randomly mirrored, rotated, translated, and scaled by small amounts (photos are of the three authors).
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From: http://www.ius.cs.cmu.edu/IUS/har2/har/www/CMU-CS-95-158R/
During training, the partially-trained system is applied to images of scenery which do not contain faces (like the one on the left). Any regions in the image detected as faces (which are expanded and shown on the right) are errors, which can be added into the set of negative training examples.
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From: http://www.ius.cs.cmu.edu/IUS/har2/har/www/CMU-CS-95-158R/
Images with all the above threshold detections indicated by boxes.
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From: http://www.ius.cs.cmu.edu/IUS/har2/har/www/CMU-CS-95-158R/
The framework used for merging multiple detections from a single network: A) The detections are recorded in an image pyramid. B) The detections are ``spread out'' and a threshold is applied. C) The centroids in scale and position are computed, and the regions contributing to each centroid are collapsed to single points. In the example shown, this leaves only two detections in the output pyramid. D) The final step is to check the proposed face locations for overlaps, and E) to remove overlapping detections if they exist. In this example, removing the overlapping detection eliminates what would otherwise be a false positive.
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• Application of Convolutional Neural Networks to Face Detection
Osadchy, Miller, LeCun. Face Detection and Pose Estimation, 2004
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• Non-linear dimensionality reduction
Osadchy, Miller, LeCun. Face Detection and Pose Estimation, 2004
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Osadchy, Miller, LeCun. Face Detection and Pose Estimation, 2004
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Overview
• Specific Object Recognition– Lowe ‘04– Rothganger et al. ‘03
• Face Dectection– Sub-space methods– Neural Network-based approaches– Viola & Jones
• Face Recognition
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Paul Viola Michael J. JonesMitsubishi Electric Research Laboratories (MERL)
Cambridge, MA
Most of this work was done at Compaq CRL before the authors moved to MERL
Rapid Object Detection Using a Boosted Cascade of Simple Features
http://citeseer.ist.psu.edu/cache/papers/cs/23183/http:zSzzSzwww.ai.mit.eduzSzpeoplezSzviolazSzresearchzSzpublicationszSzICCV01-Viola-Jones.pdf/viola01robust.pdf
Manuscript available on web:
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The Viola/Jones Face Detector
• A seminal approach to real-time object
detection
• Training is slow, but detection is very fast
• Key ideas• Integral images for fast feature evaluation
• Boosting for feature selection
• Attentional cascade for fast rejection of non-face windows
P. Viola and M. Jones. Rapid object detection using a boosted cascade of
simple features. CVPR 2001.
P. Viola and M. Jones. Robust real-time face detection. IJCV 57(2), 2004.
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Image Features
“Rectangle filters”
Value =
∑ (pixels in white area) –
∑ (pixels in black area)
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Example
Source
Result
Slide: S. Lazebnik
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Fast computation with integral images
• The integral image
computes a value at each
pixel (x,y) that is the sum
of the pixel values above
and to the left of (x,y),
inclusive
• This can quickly be
computed in one pass
through the image
(x,y)
Slide: S. Lazebnik
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Computing the integral image
Slide: S. Lazebnik
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Computing the integral image
Cumulative row sum: s(x, y) = s(x–1, y) + i(x, y)
Integral image: ii(x, y) = ii(x, y−1) + s(x, y)
ii(x, y-1)
s(x-1, y)
i(x, y)
MATLAB: ii = cumsum(cumsum(double(i)), 2);Slide: S. Lazebnik
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Computing sum within a rectangle
• Let A,B,C,D be the values of the integral image at the corners of a rectangle
• Then the sum of original image values within the rectangle can be computed as:
sum = A – B – C + D
• Only 3 additions are required for any size of rectangle!
D B
C A
Slide: S. Lazebnik
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Example
-1 +1
+2
-1
-2
+1
Integral
Image
Slide: S. Lazebnik
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Feature selection
• For a 24x24 detection region, the number of
possible rectangle features is ~160,000!
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Feature selection
• For a 24x24 detection region, the number of
possible rectangle features is ~160,000!
• At test time, it is impractical to evaluate the
entire feature set
• Can we create a good classifier using just a
small subset of all possible features?
• How to select such a subset?
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Boosting
• Boosting is a classification scheme that works
by combining weak learners into a more
accurate ensemble classifier
• Training consists of multiple boosting rounds• During each boosting round, we select a weak learner that
does well on examples that were hard for the previous weak
learners
• “Hardness” is captured by weights attached to training
examples
Y. Freund and R. Schapire, A short introduction to boosting, Journal of
Japanese Society for Artificial Intelligence, 14(5):771-780, September, 1999.
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Training procedure
• Initially, weight each training example equally
• In each boosting round:• Find the weak learner that achieves the lowest weighted
training error
• Raise the weights of training examples misclassified by current
weak learner
• Compute final classifier as linear combination
of all weak learners (weight of each learner is
directly proportional to its accuracy)
• Exact formulas for re-weighting and combining
weak learners depend on the particular
boosting scheme (e.g., AdaBoost)
Y. Freund and R. Schapire, A short introduction to boosting, Journal of
Japanese Society for Artificial Intelligence, 14(5):771-780, September, 1999.
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Boosting
Weak
Classifier 1
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Boosting
Weights
Increased
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Boosting
Weak
Classifier 2
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Boosting
Weights
Increased
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Boosting
Weak
Classifier 3
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Boosting
Final classifier is
linear combination of
weak classifiers
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Boosting vs. SVM
• Advantages of boosting• Integrates classifier training with feature selection
• Complexity of training is linear instead of quadratic in the
number of training examples
• Flexibility in the choice of weak learners, boosting scheme
• Testing is fast
• Easy to implement
• Disadvantages• Needs many training examples
• Training is slow
• Often doesn’t work as well as SVM (especially for many-
class problems)
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Boosting for face detection
• Define weak learners based on rectangle
features
otherwise 0
)( if 1)(
tttt
t
pxfpxh
window
value of rectangle feature
parity threshold
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• Define weak learners based on rectangle
features
• For each round of boosting:• Evaluate each rectangle filter on each example
• Select best filter/threshold combination based on weighted
training error
• Reweight examples
Boosting for face detection
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Boosting for face detection
• First two features selected by boosting:
This feature combination can yield 100%
detection rate and 50% false positive rate
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Boosting for face detection
• A 200-feature classifier can yield 95% detection
rate and a false positive rate of 1 in 14084
Not good enough!
Receiver operating characteristic (ROC) curve
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Attentional cascade
• We start with simple classifiers which reject
many of the negative sub-windows while
detecting almost all positive sub-windows
• Positive response 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
FACEIMAGE
SUB-WINDOWClassifier 1
TClassifier 3
T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
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Attentional cascade
• Chain classifiers that are
progressively more complex
and have lower false positive
rates:vs false neg determined by
% False Pos
% D
etec
tion
0 50
0 100
FACEIMAGE
SUB-WINDOWClassifier 1
TClassifier 3
T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
Receiver operating
characteristic
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Attentional cascade
• The detection rate and the false positive rate of
the cascade are found by multiplying the
respective rates of the individual stages
• A detection rate of 0.9 and a false positive rate
on the order of 10-6 can be achieved by a
10-stage cascade if each stage has a detection
rate of 0.99 (0.9910 ≈ 0.9) and a false positive
rate of about 0.30 (0.310 ≈ 6×10-6)
FACEIMAGE
SUB-WINDOWClassifier 1
TClassifier 3
T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
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Training the cascade
• Set target detection and false positive rates for
each stage
• Keep adding features to the current stage until
its target rates have been met • Need to lower AdaBoost threshold to maximize detection (as
opposed to minimizing total classification error)
• Test on a validation set
• If the overall false positive rate is not low
enough, then add another stage
• Use false positives from current stage as the
negative training examples for the next stage
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The implemented system
• Training Data• 5000 faces
– All frontal, rescaled to
24x24 pixels
• 300 million non-faces
– 9500 non-face images
• Faces are normalized
– Scale, translation
• Many variations• Across individuals
• Illumination
• Pose
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System performance
• Training time: “weeks” on 466 MHz Sun
workstation
• 38 layers, total of 6061 features
• Average of 10 features evaluated per window
on test set
• “On a 700 Mhz Pentium III processor, the
face detector can process a 384 by 288 pixel
image in about .067 seconds” • 15 Hz
• 15 times faster than previous detector of comparable
accuracy (Rowley et al., 1998)
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Output of Face Detector on Test Images
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Funny Nikon ads
"The Nikon S60 detects up to 12 faces."
Slide: S. Lazebnik
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Other detection tasks
Facial Feature Localization
Male vs.
female
Profile Detection
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Profile Detection
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Profile Features
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Summary: Viola/Jones detector
• Rectangle features
• Integral images for fast computation
• Boosting for feature selection
• Attentional cascade for fast rejection of
negative windows
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Face Recognition
N. Kumar, A. C. Berg, P. N. Belhumeur, and S. K.
Nayar, "Attribute and Simile Classifiers for Face
Verification," ICCV 2009.
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Face Recognition
N. Kumar, A. C. Berg, P. N. Belhumeur, and S. K.
Nayar, "Attribute and Simile Classifiers for Face
Verification," ICCV 2009.
Attributes for training Similes for training
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Face Recognition
N. Kumar, A. C. Berg, P. N. Belhumeur, and S. K.
Nayar, "Attribute and Simile Classifiers for Face
Verification," ICCV 2009.
Results on Labeled Faces in the Wild Dataset