Optical Flow, Motion Segmentation, Feature Tracking

Transcription

1 BIL Computer Vision May 21, 2014 Optical Flow, Motion Segmentation, Feature Tracking Aykut Erdem Dept. of Computer Engineering Hacettepe University

2 Motion Optical Flow Motion Segmentation Feature Tracking Today

3 Motion Optical Flow Motion Segmentation Feature Tracking Today

4 Slide credit: Fei-Fei Li From images to videos A video is a sequence of frames captured over time Now our image data is a function of space (x, y) and time (t)

5 Slide credit: J. Hays Motion and perceptual organization Sometimes, motion is the only cue

6 Slide credit: K. Grauman Motion and perceptual organization Sometimes, motion is foremost cue

7 G. Johansson, Visual Perception of Biological Motion and a Model For Its Analysis, Perception and Psychophysics 14, , Slide credit: J. Hays Motion and perceptual organization Even impoverished motion data can evoke a strong percept

8 G. Johansson, Visual Perception of Biological Motion and a Model For Its Analysis, Perception and Psychophysics 14, , Slide credit: J. Hays Motion and perceptual organization Even impoverished motion data can evoke a strong percept

9 Slide credit: J. Hays Uses of motion Estimating 3D structure Segmenting objects based on motion cues Learning and tracking dynamical models Recognizing events and activities

10 Motion estimation techniques Optical flow Recover image motion at each pixel from spatio-temporal image brightness variations (optical flow) Feature-tracking Extract visual features (corners, textured areas) and track them over multiple frames Slide credit: Fei-Fei Li, S. Savarese

11 Motion Optical Flow Motion Segmentation Feature Tracking Today

12 Slide credit: Fei-Fei Li Optical flow Vector field function of the spatiotemporal image brightness variations Picture courtesy of Selim Temizer - Learning and Intelligent Systems (LIS) Group, MIT

13 Slide credit: J. Hays Motion field The motion field is the projection of the 3D scene motion into the image

14 Motion field and parallax X(t) is a moving 3D point Velocity of scene point: V = dx/dt x(t) = (x(t),y(t)) is the projection of X in the image Apparent velocity v in the image: given by components v x = dx/dt and v y = dy/dt These components are known as the motion field of the image X(t) V x(t) v X(t+dt) x(t+dt) Slide credit: A. Efros

15 Motion field and parallax To find image velocity v, differentiate x=(x,y) with respect to t (using quotient rule): X(t) V X(t+dt) x = f X Z v x ZVx V f Z f Vx Vzx Z = 2 = z X v x(t+dt) y = f Y Z v y = f V y V Z z y x(t) Image motion is a function of both the 3D motion (V) and the depth of the 3D point (Z) Slide credit: A. Efros

16 Slide credit: A. Efros Motion field and parallax Pure translation: V is constant everywhere f Vx V vx = Z f Vy V vy = Z z z x v y 1 = ( v 0 Vzx), ( f V ) x, f V y Z v = 0

17 Slide credit: A. Efros Motion field and parallax Pure translation: V is constant everywhere 1 v ( v 0 Vzx), v = 0 f V x, f V y = ( ) Z The length of the motion vectors is inversely proportional to the depth Z V z is nonzero: Every motion vector points toward (or away from) the vanishing point of the translation direction

18 Slide credit: A. Efros Motion field and parallax Pure translation: V is constant everywhere 1 v ( v ), 0 Vzx v 0 = f V x, f V y = ( ) Z The length of the motion vectors is inversely proportional to the depth Z V z is nonzero: Every motion vector points toward (or away from) the vanishing point of the translation direction V z is zero: Motion is parallel to the image plane, all the motion vectors are parallel

19 Optical flow Definition: optical flow is the apparent motion of brightness patterns in the image Ideally, optical flow would be the same as the motion field Have to be careful: apparent motion can be caused by lighting changes without any actual motion Think of a uniform rotating sphere under fixed lighting vs. a stationary sphere under moving illumination figure from Jahne et al, 1999 Slide credit: J. Hays

20 Estimating optical flow I(x,y,t-1) I(x,y,t) Given two subsequent frames, estimate the apparent motion field u(x,y), v(x,y) between them Key assumptions Brightness constancy: projection of the same point looks the same in every frame Small motion: points do not move very far Spatial coherence: points move like their neighbors Slide credit: S. Savarese

21 The brightness constancy constraint I(x,y, t-1) I(x,y, t) Brightness Constancy Equation: I x, y, t) I( x + u, y + v ( =, t + 1 Linearizing the right side using Taylor expansion: Hence, I (x+ u, y+ v,t) I (x, y,t 1)+ I x u+ I y v+ I t I (x+ u, y+ v,t) I (x, y,t 1) = +I x u+ I y v+ I t I x u + I y v + I t 0 Image derivative along x Difference over frames I ) [ ] T u v + I = 0 t Slide credit: S. Savarese

22 Slide credit: J. Hays How does this make sense? I [ ] T u v + I = 0 What do the static image gradients have to do with motion estimation? t

23 The brightness constancy constraint Can we use this equation to recover image motion (u,v) at each pixel? I [ ] T u v + I = 0 How many equations and unknowns per pixel? One equation (this is a scalar equation), two unknowns (u,v) The component of the motion perpendicular to the gradient (i.e., parallel to the edge) cannot be measured t If (u, v) satisfies the equation, so does (u+u, v+v ) if I [ u' v' ] T = 0 gradient (u,v) (u+u,v+v ) (u,v ) edge Slide credit: S. Savarese

24 Slide credit: J. Hays The aperture problem Actual motion

25 Slide credit: J. Hays The aperture problem Perceived motion

26 The barber pole illusion Slide credit: J. Hays

27 The barber pole illusion Slide credit: J. Hays

28 Slide credit: S. Savarese B. Lucas and T. Kanade. An iterative image registration technique with an application to stereo vision. In IJCAI, pp , Solving the ambiguity How to get more equations for a pixel? Spatial coherence constraint Assume the pixel s neighbors have the same (u,v) If we use a 5x5 window, that gives us 25 equations per pixel

29 Slide credit: S. Savarese Lucas-Kanade flow Overconstrained linear system:

30 Slide credit: S. Savarese Conditions for solvability When is this system solvable? What if the window contains just a single straight edge?

31 Slide credit: S. Savarese Lucas-Kanade flow Overconstrained linear system Least squares solution for d given by The summations are over all pixels in the K x K window

32 Conditions for solvability Optimal (u, v) satisfies Lucas-Kanade equation When is this solvable? i.e., what are good points to track? A T A should be invertible A T A should not be too small due to noise eigenvalues λ 1 and λ 2 of A T A should not be too small A T A should be well-conditioned λ 1 / λ 2 should not be too large (λ 1 = larger eigenvalue) Does this remind you of anything? Criteria for Harris corner detector Slide credit: S. Saverese

33 Slide credit: S. Savarese M = A T A is the second moment matrix (Harris corner detector ) Eigenvectors and eigenvalues of A T A relate to edge direction and magnitude The eigenvector associated with the larger eigenvalue points in the direction of fastest intensity change The other eigenvector is orthogonal to it

34 Slide credit: S. Savarese, Darya Frolova, Denis Simakov Interpreting the eigenvalues Classification of image points using eigenvalues of M: λ 2 Edge λ 2 >> λ 1 Corner λ 1 and λ 2 are large, λ 1 ~ λ 2 ; E increases in all directions λ 1 and λ 2 are small; E is almost constant in all directions Flat region Edge λ 1 >> λ 2 λ 1

35 Slide credit: S. Savarese Low-texture region gradients have small magnitude small l 1, small l 2

36 Slide credit: S. Savarese Edge gradients very large or very small large l 1, small l 2

37 Slide credit: S. Savarese High-texture region gradients are different, large magnitudes large l 1, large l 2

38 Slide credit: S. Savarese What are good features to track? Can measure quality of features from just a single image Hence: tracking Harris corners (or equivalent) guarantees small error sensitivity

39 Slide credit: S. Savarese Recap Key assumptions (Errors in Lucas-Kanade) Small motion: points do not move very far Brightness constancy: projection of the same point looks the same in every frame Spatial coherence: points move like their neighbors

40 Revisiting the small motion assumption Is this motion small enough? Probably not it s much larger than one pixel (2nd order terms dominate) How might we solve this problem? * From Khurram Hassan-Shafique CAP5415 Computer Vision 2003 Slide credit: Fei-Fei Li

41 Reduce the resolution * From Khurram Hassan-Shafique CAP5415 Computer Vision 2003 Slide credit: J. Hays

42 Multi-resolution Lucas Kanade Algorithm Slide credit: S. Savarese

43 Slide credit: S. Savarese Coarse-to-fine optical flow estimation u=1.25 pixels u=2.5 pixels u=5 pixels image 1H u=10 pixels image 2I Gaussian pyramid of image 1 Gaussian pyramid of image 2

44 Iterative Refinement Iterative Lukas-Kanade Algorithm 1. Estimate displacement at each pixel by solving Lucas-Kanade equations 2. Warp I(t) towards I(t+1) using the estimated flow field - Basically, just interpolation 3. Repeat until convergence * From Khurram Hassan-Shafique CAP5415 Computer Vision 2003 Slide credit: Fei-Fei Li

45 Slide credit: J. Hays Coarse-to-fine optical flow estimation run iterative L-K run iterative L-K warp & upsample... image 1J image 2I Gaussian pyramid of image 1 (t) Gaussian pyramid of image 2 (t+1)

46 Optical Flow Results * From Khurram Hassan-Shafique CAP5415 Computer Vision 2003 Slide credit: J. Hays

47 Optical Flow Results * From Khurram Hassan-Shafique CAP5415 Computer Vision 2003 Slide credit: J. Hays

48 Slide credit: S. Savarese Recap Key assumptions (Errors in Lucas-Kanade) Small motion: points do not move very far Brightness constancy: projection of the same point looks the same in every frame Spatial coherence: points move like their neighbors

49 Motion Optical Flow Motion Segmentation Feature Tracking Today

50 Slide credit: Fei-Fei Li Motion segmentation How do we represent the motion in this scene?

51 Motion segmentation Break image sequence into layers each of which has a coherent (affine) motion J. Wang and E. Adelson. Layered Representation for Motion Analysis. CVPR Slide credit: S. Savarese

52 What are layers? Each layer is defined by an alpha mask and an affine motion model J. Wang and E. Adelson. Layered Representation for Motion Analysis. CVPR Slide credit: S. Savarese

53 Affine motion Slide credit: S. Savarese Substituting into the brightness constancy equation: Each pixel provides 1 linear constraint in 6 unknowns Least squares minimization: y a x a a y x v y a x a a y x u ), ( ), ( 0 ) ( ) ( t y x I y a x a a I y a x a a I 2 t y x I y a x a a I y a x a a I a Err ) ( ) ( ) (

54 Slide credit: S. Savarese How do we estimate the layers? 1. Obtain a set of initial affine motion hypotheses Divide the image into blocks and estimate affine motion parameters in each block by least squares Eliminate hypotheses with high residual error Map into motion parameter space Perform k-means clustering on affine motion parameters Merge clusters that are close and retain the largest clusters to obtain a smaller set of Merge clusters that are close and retain the largest clusters to obtain a smaller set of hypotheses to describe all the motions in the scene to in scene

55 Slide credit: S. Savarese How do we estimate the layers? 1. Obtain a set of initial affine motion hypotheses Divide the image into blocks and estimate affine motion parameters in each block by least squares Eliminate hypotheses with high residual error Map into motion parameter space Perform k-means clustering on affine motion parameters Merge clusters that are close and retain the largest clusters to obtain a smaller set of hypotheses to describe all the motions in the scene 2. Iterate until convergence: Assign each pixel to best hypothesis Pixels with high residual error remain unassigned

56 Slide credit: S. Savarese How do we estimate the layers? 1. Obtain a set of initial affine motion hypotheses Divide the image into blocks and estimate affine motion parameters in each block by least squares Eliminate hypotheses with high residual error Map into motion parameter space Perform k-means clustering on affine motion parameters Merge clusters that are close and retain the largest clusters to obtain a smaller set of hypotheses to describe all the motions in the scene

57 Slide credit: S. Savarese How do we estimate the layers? 1. Obtain a set of initial affine motion hypotheses Divide the image into blocks and estimate affine motion parameters in each block by least squares Eliminate hypotheses with high residual error Map into motion parameter space Perform k-means clustering on affine motion parameters Merge clusters that are close and retain the largest clusters to obtain a smaller set of hypotheses to describe all the motions in the scene 2. Iterate until convergence: Assign each pixel to best hypothesis Pixels with high residual error remain unassigned Perform region filtering to enforce spatial constraints Re-estimate affine motions in each region

58 Example result J. Wang and E. Adelson. Layered Representation for Motion Analysis. CVPR Slide credit: S. Savarese

59 Motion Optical Flow Motion Segmentation Feature Tracking Today

60 Tracking Slide credit: Fei-Fei Li, K. Grauman, D. Ramanan

61 Slide credit: D. Parikh Why is tracking challenging? Small, few visual features Erratic movements, moving very quickly Occlusions, leaving and coming back Surrounding similar-looking objects

62 Slide credit: D. Parikh Strategies for tracking Tracking by repeated detection - Works well if object is easily detectable (e.g., face or colored glove) and there is only one - Need some way to link up detections - Best you can do, if you can t predict motion

63 Slide credit: D. Parikh Tracking with dynamics Key idea: Based on a model of expected motion, predict where objects will occur in next frame, before even seeing the image - Restrict search for the object - measurement noise is reduced by trajectory smoothness - Robustness to missing or weak observations

64 Optical flow for tracking? If we have more than just a pair of frames, we could compute flow from one to the next: But flow only reliable for small motions, and we may have occlusions, textureless regions that yield bad estimates anyway Slide credit: K. Grauman

65 Motion estimation techniques Optical flow Directly recover image motion at each pixel from spatiotemporal image brightness variations Dense motion fields, but sensitive to appearance variations Suitable for video and when image motion is small Feature-based methods Extract visual features (corners, textured areas) and track them over multiple frames Sparse motion fields, but more robust tracking Suitable when image motion is large (10s of pixels) Slide credit: K. Grauman

66 Slide credit: K. Grauman Feature-based matching for motion Interesting point Best matching neighborhood Search window Time t Time t+1 Search window is centered at the point where we last saw the feature, in image I1. Best match = position where we have the highest normalized cross-correlation value.

67 Feature-based matching for motion For a discrete matching search, what are the tradeoffs of the chosen search window size? Which patches to track? Select interest points e.g. corners Where should the search window be placed? Near match at previous frame More generally, taking into account the expected dynamics of the object Slide credit: K. Grauman

68 Example: A Camera Mouse Video interface: use feature tracking as mouse replacement User clicks on the feature to be tracked Take the 15x15 pixel square of the feature In the next image do a search to find the 15x15 region with the highest correlation Move the mouse pointer accordingly Repeat in the background every 1/30th of a second James Gips and Margrit Betke Slide credit: K. Grauman

69 Slide credit: K. Grauman Feature-based matching for motion For a discrete matching search, what are the tradeoffs of the chosen search window size? Which patches to track? Select interest points e.g. corners Where should the search window be placed? Near match at previous frame More generally, taking into account the expected dynamics of the object

70 Slide credit: K. Grauman Detection vs. tracking t=1 t=2 t=20 t=21

71 Slide credit: K. Grauman Detection vs. tracking Detection: We detect the object independently in each frame and can record its position over time, e.g., based on blob s centroid or detection window coordinates

72 Slide credit: K. Grauman Detection vs. tracking Tracking with dynamics: We use image measurements to estimate position of object, but also incorporate position predicted by dynamics, i.e., our expectation of object s motion pattern.

73 Slide credit: K. Grauman Detection vs. tracking Tracking with dynamics: We use image measurements to estimate position of object, but also incorporate position predicted by dynamics, i.e., our expectation of object s motion pattern.

74 Slide credit: K. Grauman Tracking with dynamics Use model of expected motion to predict where objects will occur in next frame, even before seeing the image. Intent: Do less work looking for the object, restrict the search. Get improved estimates since measurement noise is tempered by smoothness, dynamics priors. Assumption: continuous motion patterns: Camera is not moving instantly to new viewpoint Objects do not disappear and reappear in different places in the scene Gradual change in pose between camera and scene

75 Slide credit: K. Grauman Tracking as inference The hidden state consists of the true parameters we care about, denoted X. The measurement is our noisy observation that results from the underlying state, denoted Y. At each time step, state changes (from X t-1 to X t ) and we get a new observation Y t.

76 Slide credit: K. Grauman State vs. observation Hidden state : parameters of interest Measurement : what we get to directly observe

77 Slide credit: K. Grauman Tracking as inference The hidden state consists of the true parameters we care about, denoted X. The measurement is our noisy observation that results from the underlying state, denoted Y. At each time step, state changes (from X t-1 to X t ) and we get a new observation Y t. Our goal: recover most likely state X t given All observations seen so far. Knowledge about dynamics of state transitions.

78 Slide credit: K. Grauman Tracking as inference: intuition Time t Time t+1 Belief Measurement Corrected prediction

79 Slide credit: K. Grauman Tracking as inference: intuition measurement Belief: prediction Belief: prediction Corrected prediction measurement old belief Time t Time t+1

80 Slide credit: K. Grauman Independence assumptions Only immediate past state influences current state P ( X X X ) P( X X ) t 0,, t 1 = t t 1 dynamics model Measurement at time t depends on current state ( Y X Y, X, Y X ) P( Y X ) P = t 0, 0 t 1 t 1, t t observation model t

81 Prediction: Given the measurements we have seen up to this point, what state should we predict? Correction: Tracking as inference P ( ) X y y t t,, 0 1 Now given the current measurement, what state should we predict? ( ) X P,, y y t t 0 Slide credit: K. Grauman

82 Slide credit: K. Grauman Questions How to represent the known dynamics that govern the changes in the states? How to represent relationship between state and measurements, plus our uncertainty in the measurements? How to compute each cycle of updates? Representation: We ll consider the class of linear dynamic models, with associated Gaussian pdfs. Updates: via the Kalman filter.

83 Slide credit: K. Grauman Notation reminder x ~ N( µ, Σ) Random variable with Gaussian probability distribution that has the mean vector μ and covariance matrix Σ. x and μ are d-dimensional, Σ is d x d. d=2 d=1 If x is 1-d, we just have one Σ parameter -à the variance: σ 2

84 Slide credit: K. Grauman Linear dynamic model Describe the a priori knowledge about System dynamics model: represents evolution of state over time. x ~ N( Dx ; Σ t t 1 d n x 1 n x n n x 1 Measurement model: at every time step we get a noisy measurement of the state. ) y ~ N( Mx ; Σ t t m ) m x 1 m x n n x 1

85 Slide credit: K. Grauman Example: randomly drifting points xt ~ N( Dxt 1; Σd ) Consider a stationary object, with state as position Position is constant, only motion due to random noise term. State evolution is described by identity matrix D=I

86 Example: Constant velocity (1D points) Slide credit: K. Grauman 1 d position 1 d position measurements states time

87 State vector: position p and velocity v Measurement is position only Example: Constant velocity (1D points) ξ ε + = + Δ + = ) ( t t t t t v v v t p p " # $ % & = t t t v p x noise v p t noise D x x t t t t t + " # $ % & " # $ % & Δ = + = [ ] noise v p noise Mx y t t t t + " # $ % & = + = 0 1 ) ; ( ~ 1 d t t N Σ Dx x ) ; ( ~ m t t N Σ Mx y Slide credit: K. Grauman

88 Slide credit: K. Grauman Questions How to represent the known dynamics that govern the changes in the states? How to represent relationship between state and measurements, plus our uncertainty in the measurements? How to compute each cycle of updates? Representation: We ll consider the class of linear dynamic models, with associated Gaussian pdfs. Updates: via the Kalman filter.

89 Slide credit: K. Grauman The Kalman filter Method for tracking linear dynamical models in Gaussian noise The predicted/corrected state distributions are Gaussian Only need to maintain the mean and covariance The calculations are easy

90 Slide credit: K. Grauman Kalman filter Know corrected state from previous time step, and all measurements up to the current one à Predict distribution over next state. Time update ( Predict ) Receive measurement Know prediction of state, and next measurement à Update distribution over current state. Measurement update ( Correct ) P ( X y ) t 0,, y t 1 Mean and std. dev. of predicted state: σ t t µ, Time advances: t++ ( X y, ) P, t 0 y t Mean and std. dev. of corrected state: + σ + t t µ,

91 Slide credit: L. Lazebnik 1D Kalman filter: Prediction Have linear dynamic model defining predicted state evolution, with noise Want to estimate predicted distribution for next state Update the mean: P Update the variance: ( ) ( 2 ) X y,, y N µ,( σ t X ( σ 2 ) ~, t N dx t 1 d = ) 0 t 1 t t = + 1 µ t dµ t ( σ t ) = σ d + ( dσ t 1 ) 2

92 1D Kalman filter: Correction Have linear model defining the mapping of state to measurements: Want to estimate corrected distribution given latest meas.: Update the mean: Update the variance: ( ) 2, ~ m t mx t N Y σ ( ) ( ) 2 0 ),(,, + + = t t t t N y y X P σ µ ) ( ) ( = t m t t m t t m my σ σ σ σ µ µ ) ( ) ( ) ( + + = t m t m t m σ σ σ σ σ Slide credit: L. Lazebnik

93 µ Prediction vs. correction σ + my ( σ t m t t + 2 m t µ t = ( σ t ) = σ m + m ( σ t ) σ m + m ( σ t ) What if there is no prediction uncertainty + = t t µ σ + t 2 = µ ( ) 0 What if there is no measurement uncertainty ) The measurement is ignored + = t y t 2 = µ ( σ ) 0 m t The prediction is ignored + σ ( σ ( σ t = 0)? ) ( σ m = 0)? Slide credit: L. Lazebnik

94 Constant velocity model Kalman filter processing o state x measurement * predicted mean estimate position + corrected mean estimate bars: variance estimates before and after measurements Slide credit: K. Grauman time Time t Time t+1

95 Constant velocity model Kalman filter processing o state x measurement * predicted mean estimate position + corrected mean estimate bars: variance estimates before and after measurements Slide credit: K. Grauman time Time t Time t+1

96 Constant velocity model Kalman filter processing o state x measurement * predicted mean estimate position + corrected mean estimate bars: variance estimates before and after measurements Slide credit: K. Grauman time Time t Time t+1

97 Constant velocity model Kalman filter processing o state x measurement * predicted mean estimate position + corrected mean estimate bars: variance estimates before and after measurements Slide credit: K. Grauman time Time t Time t+1

98 Example: Kalman Filter Ground Truth Observation Correction Slide credit: D. Parikh Prediction Next Frame Update Location, Velocity, etc. 98

99 Slide credit: D. Parikh Comparison Ground Truth Observation Correction

100 Slide credit: K. Grauman

101 A bat census Slide credit: K. Grauman

102 Slide credit: K. Grauman Tracking: issues Initialization Often done manually Background subtraction, detection can also be used Data association, multiple tracked objects Occlusions, clutter Which measurements go with which tracks?

103 Slide credit: K. Grauman Tracking: issues Initialization Often done manually Background subtraction, detection can also be used Data association, multiple tracked objects Occlusions, clutter Deformable and articulated objects Constructing accurate models of dynamics E.g., Fitting parameters for a linear dynamics model Drift Accumulation of errors over time

104 Drift D. Ramanan, D. Forsyth, and A. Zisserman. Tracking People by Learning their Appearance. PAMI

105 Slide credit: K. Grauman Tracking Summary Tracking as inference Goal: estimate posterior of object position given measurement Linear models of dynamics Represent state evolution and measurement models Kalman filters Recursive prediction/correction updates to refine measurement General tracking challenges