face detection - matematikcentrum...face detection skin color • example of skin pixel classifier...

FACE DETECTIONMikael Nilsson

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FACE DETECTION

Outline

• Introduction

• Skin Color

• Appearance Based Learning

– Timeline, ~1994 ->

• Non Maximum Suppression

• Bicycle Counting Project

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FACE DETECTION

Introduction

• Goal of Face Detection

– Given an arbitrary image, the goal of face detection is to determine whether or not there are any faces in the image and, if present, return the image location and extent of each face.

[Yang et al, Detecting Faces in Images: a Survey, Pami 2002]

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FACE DETECTION

Introduction

• Goal of Face Detection version 2

– Given an image, tell whether there is any human face, if there is, where is it (or where they are).

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FACE DETECTION

Introduction

• Goal of Face Detection version 3

– Identify and locate human faces in an image regardless of their • position

• scale

• in-plane rotation

• orientation

• pose (out-of-plane rotation)

• and illumination

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FACE DETECTION

Introduction

• Why is it hard?– Pose Variation

– Beards, Glasses

– Facial Expression

– Occlusion

– Image Orientation

– Lighting, Noise

– Resolution

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FACE DETECTION

Introduction

• Related problems– Facial Feature Extraction (like eyes, nose, mouth localization)

– Face Recognition (who is the person)

– Facial Expression Analysis (example: only “smile-photos”)

– Head Pose Estimation

– Gender/Age/Race Classification

– Face Tracking

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FACE DETECTION

Introduction

• Two methods discussed

– Skin Color

– Appearance Based Learning

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FACE DETECTION

Skin Color

• Skin Color

– General Idea• Find skin color pixels in image

• Use found pixels and do blob analysis

• Skin color methods has been proposed as preprocessing before other face detection methods (all ”orange” objects detected)

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FACE DETECTION

Skin Color

• A skin pixel classifier using likelihood ratio approach

)|(

)|(skinrgbP

skinrgbP

rgb

P(rgb)

skin non-skin

)|( skinrgbP

)|( skinrgbP

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FACE DETECTION

Skin Color

• Example of skin pixel classifier

– Model andusing Gaussian Mixture Model (GMM)

– Train two GMMs, one with skin pixels and another with nonskin pixels

– Find likelihood ratio using GMMs and set threshold

)|( skinrgbP )|( skinrgbP

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FACE DETECTION

Demo

• Skin Color– Michael J. Jones and James M. Rehg, ”Statistical Color Models with Application to Skin Detection”, International Journal of Computer Vision, 1999, pp.274—280.

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FACE DETECTION

Appearance Based Learning

• Apperance Based Learning

– General Idea• Collect a large set of resized (e.g. 19x19, 20x20, 22x22 or 32x32) face and nonface images and learn a classifier to discriminate them.

• Given a test image, detect faces by applying the classifier at each position and scale of the image. 22x22

nonface

face

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FACE DETECTION


• Handling position and scale

– Search all positions and relevant scales

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FACE DETECTION


• Timeline

– Kah Kay Sung and Tomaso Poggio, ”Example-based Learning for View-based Human Face Detection”, MIT A.I. Memo No. 1521, 1994, C.B.C.L. Paper No. 112

– H. Rowley, S. Baluja, and T. Kanade, “Neural network-based face detection”, CVPR, 1996, pp. 203–208.

– H. Schneiderman, T. Kanade. "Probabilistic Modeling of Local Appearance and Spatial Relationships for Object Recognition”, CVPR,

1998, pp. 45-51.

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FACE DETECTION


• Timeline– D. Roth, M. Yang, and N. Ahuja, “A snow-based face detector”, in NIPS 12, 2000, pp. 855–861.

– P. Viola and M. Jones, “Rapid object detection using a boosted cascade of simple features”, CVPR, 2001, vol. 1, pp. 511–518.

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FACE DETECTION


• Sung & Poggio 1994– They collected 1067 face patterns and for each one of these training images they did:

• Resizing to 19x19 pixels

• Masking to avoid background pixels

• Lighting correction / Histogram Equalization

19x19

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FACE DETECTION


• Sung & Poggio 1994– In addition, for each one of the 1067face patterns, new virtual samples were generated by rotation and mirror operations, totalizing 4150 face patterns.

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FACE DETECTION


• Sung & Poggio 1994– Each 19x19 pattern can be seen as a point in a high dimensional space.

– The K-means algorithm is used to cluster face and non-face samples into 6 (six) clusters.

– Each cluster is described by a multi-dimensional Gaussian with a centroid and covariance matrix.

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FACE DETECTION


• Sung & Poggio 1994

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FACE DETECTION


• Sung & Poggio 1994– Compute the distance of a sample to all face and non-face clusters

– Final classifier use a Multi-Layer Perceptron (MLP) trained using backpropagation on the distance vector

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FACE DETECTION



– Bootstrapping• 1. Start with a small set of

nonface examples in the training set

• 2. Train a MLP classifier with the current training set

• 3. Run the learned face detector on a sequence of random images (not containing any faces).

• 4. Collect all the nonface patterns that the current system wrongly classifies as faces (i.e., false positives)

• 5. Add these nonface patterns to the training set

• 6. Got to Step 2 or stop if satisfied

Test Image

False Detections

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FACE DETECTION



– Result• Able to detect upright frontal faces in complex scenes.

• Computationally

expensive

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FACE DETECTION


• Rowley et al. 1996– Similar to Sung and Poggio

– 20x20 instead of 19x19, no mask used

– Same techniques for bootstrap, preprocessing, etc.

– Neural Network applied directly into the image.

– Different heuristics

– Faster than Sung & Poggio (but still far from real-time)

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FACE DETECTION


• Rowley et al. 1996

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FACE DETECTION



– Rowley extended his initial work in 1997 by adressing rotations also

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FACE DETECTION



– Router network, input and output

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FACE DETECTION



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FACE DETECTION


• Schneiderman et al. 1998– Used parts of image

– Three different classifiers are created: frontal face detector, left profile, and right profile. The right profile classifier is the same as the left profile, but take as input mirrored images.

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FACE DETECTION


• Schneiderman et al. 1998

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FACE DETECTION


• Schneiderman et al. 1998

– Left-, right profile and frontal face

– Also for cars

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FACE DETECTION


• Yang et al.

– Created sparse boolean primitive input space (intensity x position) from 20x20 patches

– Also evaluated to extend input space with (position x intensity mean x intensity variance) with some improvements

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FACE DETECTION


• Yang et al.

– Example sparse input, 2 bits per value and 5 values => (2^2)*5=20 boolean vector

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FACE DETECTION


• Utilized Winnow update rule

• Winnow

– Promotion: if

– Demotion: if

• Perceptron

– Promotion: if

– Demotion: if

thresholdactivation

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FACE DETECTION


• Sparse Input and Winnow update rule

– Sparse Network of Winnows (SNoW)

+

= SNoW

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FACE DETECTION


• Main conclusion of…

– Sung & Poggio

– Rowley et al.

– Schneiderman et al.

– Yang et al.

• …is that they works quite well in detecting faces but are too slowfor real-time operation (at leastseveral seconds per image)

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FACE DETECTION


• Viola and Jones 2001

– Breakthourgh regarding detection speed ~15 FPS on 384x288 size image

– Rectangle features (“Haar-like features”)

– Adaboost learning

– Classifier cascade

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FACE DETECTION


• Rectangle features– Uses the integral image

– The integral image (ii) computes a value at each pixel (x,y) that is the sum of the pixel values above and to the left of (x,y) including the point.

(x,y)

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FACE DETECTION

• Integral image


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FACE DETECTION


• Integral image

ii(x, y-1)

s(x-1, y)

i(x, y)

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FACE DETECTION


• 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)

• Integral image in MATLAB – ii = cumsum(cumsum(double(i)), 2);

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FACE DETECTION


• Let a,b,c,d be corner coordinates

• Then the sum of original image values within the rectangle can be computed as:

– D=ii(d)+ii(a)–ii(b)–ii(c)

• Only 3 additions (2-, 1+) and 4 table lookups are required to find sum over area for any size of rectangle!

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FACE DETECTION


• Rectangle features

• Value=sum(pixels in white area)-sum(pixels in black area)

• sums done with integral image

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FACE DETECTION


• Rectangle features

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FACE DETECTION


• Which rectangle features to use

• Design of classifier

• Adaboost

Strong classifier

Weak classifier

WeightFeaturesvector

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FACE DETECTION

• Given a nested set of classifier hypothesis classes

• Computational Risk Minimization


FACEIMAGESUB-WINDOW

Classifier 1

F

T

NON-FACE

Classifier 3T

F

NON-FACE

F

T

NON-FACE

Classifier 2T

F

NON-FACE

vs false neg determined by

% False Pos

% D

etec

tion

0 50

0

100

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FACE DETECTION


• Classifier cascade

FACEIMAGESUB-WINDOW

Classifier 1

F

T

NON-FACE

Classifier 3T

F

NON-FACE

F

T

NON-FACE

Classifier 2T

F

NON-FACE

Classifier 1 Classifier 2 Classifier 3

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FACE DETECTION

Non Maximum Supression

• Non Maximum Supression

– Note that there will typically be several detections on a face• This is true for all appearance based methods discussed

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FACE DETECTION


• Merge overlapping detection boxes using non maximum supression

– Set a threshold on overlap ratio (e.g. 0.2)

– Use the rectangle with the highest (max) score from the classifier

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FACE DETECTION


• Non Maximum Supression (NMS)

NMS

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FACE DETECTION

Demo

• Face Detection– M. Nilsson, J. Nordberg and I. Claesson, ”Face Detection using Local SMQT Features and Split Up SNoW Classifier”, ICASSP, 2007, pp. II-589 -II-592.

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FACE DETECTION

Bicycle Counting Project

• Bicycle Counting Project

– Note that other objects than faces can be used• For example Schneiderman tested on cars also

– In the Bicycle Counting project bicycles are considered• in low resolution to keep sensor cost low

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FACE DETECTION


• Design system that can classify bicycles…

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FACE DETECTION


• …in videos like this

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FACE DETECTION


• Start making positive samples

– Example of workflow

1

2

3

Zoom

Mark three points

Warp to patch

32x32

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FACE DETECTION


• Negatvie samples

– Start with some random images

– Utilize Bootstrapping (see Sung & Poggio) once classifier method is in place and build up database with more samples

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FACE DETECTION


• Design method to find bicycles

– Suggested method• Histogram of Oriented Gradients (HOG)

• Support Vector Machine (SVM)

– Aim:• Calculate bicycles going in both directions

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FACE DETECTION


• Example bicycle couting final result

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face detection - matematikcentrum...face detection skin color • example of skin pixel classifier...

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