Lecture’7:’’...

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  • Lecture 7 - !!!

    Fei-Fei Li!

    Lecture 7: Finding Features (part 2/2)

    Professor Fei-Fei Li Stanford Vision Lab

    2-Oct-14 1

  • Lecture 7 - !!!

    Fei-Fei Li!

    What we will learn today?

    Local invariant features MoHvaHon Requirements, invariances

    Keypoint localizaHon Harris corner detector

    Scale invariant region selecHon AutomaHc scale selecHon Difference-of-Gaussian (DoG) detector

    SIFT: an image region descriptor

    2-Oct-14 2

    Previous lecture (#6)

    Some background reading: David Lowe, IJCV 2004

  • Lecture 7 - !!!

    Fei-Fei Li!

    A quick review

    Local invariant features MoHvaHon Requirements, invariances

    Keypoint localizaHon Harris corner detector

    Scale invariant region selecHon AutomaHc scale selecHon Difference-of-Gaussian (DoG) detector

    SIFT: an image region descriptor

    2-Oct-14 3

  • Lecture 7 - !!!

    Fei-Fei Li!

    Quick review: Harris Corner Detector

    2-Oct-14 4

    edge: no change along the edge direction

    corner: significant change in all directions

    flat region: no change in all directions

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  • Lecture 7 - !!!

    Fei-Fei Li!

    Quick review: Harris Corner Detector

    2-Oct-14 5

    Fast approximaHon Avoid compuHng the eigenvalues

    : constant (0.04 to 0.06)

    = det(M ) trace(M )2 = 12 (1 +2 )2

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    Corner > 0

    Edge < 0

    Edge < 0

    Flat region

    1

  • Lecture 7 - !!!

    Fei-Fei Li!

    Quick review: Harris Corner Detector

    2-Oct-14 6

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  • Lecture 7 - !!!

    Fei-Fei Li!

    Quick review: Harris Corner Detector

    2-Oct-14 7

    TranslaHon invariance RotaHon invariance Scale invariance?

    Not invariant to image scale!

    All points will be classified as edges!

    Corner

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  • Lecture 7 - !!!

    Fei-Fei Li!

    What we will learn today?

    Local invariant features MoHvaHon Requirements, invariances

    Keypoint localizaHon Harris corner detector

    Scale invariant region selecHon AutomaHc scale selecHon Difference-of-Gaussian (DoG) detector

    SIFT: an image region descriptor

    2-Oct-14 8

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon Consider regions (e.g. circles) of different sizes around a point

    Regions of corresponding sizes will look the same in both images

    2-Oct-14 9

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon

    The problem: how do we choose corresponding circles independently in each image?

    2-Oct-14 10

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon SoluHon:

    Design a funcHon on the region (circle), which is scale invariant (the same for corresponding regions, even if they are at different scales)

    Example: average intensity. For corresponding regions (even of different sizes) it will be the same.

    scale = 1/2

    For a point in one image, we can consider it as a funcHon of region size (circle radius)

    f

    region size

    Image 1 f

    region size

    Image 2

    2-Oct-14 11

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon Common approach:

    scale = 1/2

    f

    region size

    Image 1 f

    region size

    Image 2

    Take a local maximum of this funcHon

    ObservaHon: region size, for which the maximum is achieved, should be invariant to image scale.

    s1 s2

    Important: this scale invariant region size is found in each image independently!

    2-Oct-14 12

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon

    A good funcHon for scale detecHon: has one stable sharp peak

    f

    region size

    bad

    f

    region size

    bad

    f

    region size

    Good !

    For usual images: a good funcHon would be a one which responds to contrast (sharp local intensity change)

    2-Oct-14 13

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon FuncHons for determining scale

    2 2

    21 22

    ( , , )x y

    G x y e

    +

    =

    ( )2 ( , , ) ( , , )xx yyL G x y G x y = +

    ( , , ) ( , , )DoG G x y k G x y =

    Kernel Imagef = Kernels:

    where Gaussian

    Note: both kernels are invariant to scale and rotation

    (Laplacian)

    (Difference of Gaussians)

    2-Oct-14 14

  • Lecture 7 - !!!

    Fei-Fei Li!

    1 2

    1 2

    dettrace

    MM

    =

    = +

    blob detecHon; Marr 1982; Voorhees and Poggio 1987; Blostein and Ahuja 1989;

    trace det

    scale scale

    From Lindeberg 1998

    2-Oct-14 15

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant Detectors Harris-Laplacian1

    Find local maximum of: Harris corner detector in space (image coordinates)

    Laplacian in scale

    1 K.Mikolajczyk, C.Schmid. Indexing Based on Scale Invariant Interest Points. ICCV 2001 2 D.Lowe. DisHncHve Image Features from Scale-Invariant Keypoints. IJCV 2004

    scale

    x

    y

    Harris

    L

    apla

    cian

    SIFT (Lowe)2 Find local maximum of: Difference of Gaussians in space

    and scale

    scale

    x

    y

    DoG

    D

    oG

    2-Oct-14 16

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant Detectors

    K.Mikolajczyk, C.Schmid. Indexing Based on Scale Invariant Interest Points. ICCV 2001

    Experimental evaluaHon of detectors w.r.t. scale change

    Repeatability rate: # correspondences # possible correspondences

    2-Oct-14 17

  • Lecture 7 - !!!

    Fei-Fei Li!

    Scale Invariant DetecHon: Summary

    Given: two images of the same scene with a large scale difference between them

    Goal: find the same interest points independently in each image

    SoluHon: search for maxima of suitable funcHons in scale and in space (over the image)

    Methods:

    1. Harris-Laplacian [Mikolajczyk, Schmid]: maximize Laplacian over scale, Harris measure of corner response over the image

    2. SIFT [Lowe]: maximize Difference of Gaussians over scale and space

    2-Oct-14 18

  • Lecture 7 - !!!

    Fei-Fei Li!

    What we will learn today?

    Local invariant features MoHvaHon Requirements, invariances

    Keypoint localizaHon Harris corner detector

    Scale invariant region selecHon AutomaHc scale selecHon Difference-of-Gaussian (DoG) detector

    SIFT: an image region descriptor

    2-Oct-14 19

  • Lecture 7 - !!!

    Fei-Fei Li!

    Local Descriptors We know how to detect points Next quesHon:

    How to describe them for matching?

    ? Point descriptor should be:

    1. Invariant 2. Distinctive Sli

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    2-Oct-14 20

  • Lecture 7 - !!!

    Fei-Fei Li!

    CVPR 2003 Tutorial

    Recogni5on and Matching Based on Local Invariant Features

    David Lowe Computer Science Department University of BriHsh Columbia

    2-Oct-14 21

  • Lecture 7 - !!!

    Invariant Local Features

    Image content is transformed into local feature coordinates that are invariant to translaHon, rotaHon, scale, and other imaging parameters

    2-Oct-14 22

  • Lecture 7 - !!!

    Fei-Fei Li!

    Advantages of invariant local features Locality: features are local, so robust to occlusion and cluoer (no prior segmentaHon)

    Dis5nc5veness: individual features can be matched to a large database of objects

    Quan5ty: many features can be generated for even small objects

    Efficiency: close to real-Hme performance Extensibility: can easily be extended to wide range of differing feature types, with each adding robustness

    2-Oct-14 23

  • Lecture 7 - !!!

    Scale invariance Requires a method to repeatably select points in loca5on

    and scale: The only reasonable scale-space kernel is a Gaussian

    (Koenderink, 1984; Lindeberg, 1994) An efficient choice is to detect peaks in the difference of

    Gaussian pyramid (Burt & Adelson, 1983; Crowley & Parker, 1984 but examining more scales)

    Difference-of-Gaussian with constant raHo of scales is a close approximaHon to Lindebergs scale-normalized Laplacian (can be shown from the heat diffusion equaHon)

    Blur

    Resample

    Subtract

    Blur

    Resample

    Subtract

    2-Oct-14

  • Lecture 7 - !!!

    Fei-Fei Li!

    Becoming rotaHon invariant

    We are given a keypoint and its scale from DoG

    We will select a characterisHc orientaHon for the keypoint (based on the most prominent gradient there; discussed next slide)

    We will describe all features rela5ve to this orientaHon

    Causes features to be rotaHon invariant! If the keypoint appears rotated in

    another image, the features will be the same, because theyre rela5ve to the characterisHc orientaHon

    0 2

    2-Oct-14 25

  • Lecture 7 - !!!

    Fei-Fei Li!

    Becoming rotaHon invariant Choosing characterisHc orientaHon: Use the blurred image associated with

    the keypoints scale. Look at pixels in a