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Distinctive Image Features from Scale-Invariant Keypoints

Note 이 ppt 에 있는 자료들은 Lowe 의 논문들이나 google에서 찾은 feature 관련 paper, ppt 파일들을 참조해서 개인용으로 만든 것입니다.

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이 점을 감안해서 혹시 그림이나 사진 등을 참고할 생각이시라면 인터넷을 통해 해당하는 paper, ppt, web-page 등을 찾으셔서 레퍼런스로 활용해주시기 바랍니 다.

( 레퍼런스 자료들의 제작자 분들께 매우 죄송하다는 말을 전합니다 ㅠㅠ )

Contents

    Introduction SIFT Overview • Scale-space extrema detection • Keypoint localization • Orientation assignment • Keypoint descriptor Test and Result Conclusion

Introduction

 Ideal Interest Points / Regions

Introduction

 Harris Corner Detector • • Rotation invariance Partial invariance to affine change

Introduction

 Harris Corner Detector edge corner • Non-invariance to scale change

Introduction

 SIFT • The Scale Invariant Feature Transform • • • • Choosing features that are invariant to image scaling and rotation Partially invariant to changes in illumination and 3D camera viewpoint • Well localized in both spatial and frequency domain - Resistant to noise, clutter, and occlusion Features are highly distinctive, matched with high probability against large number of features See……  Application • Object recognition • • • • • Automatic mosaic Tracking Robot localization 3D scene modeling Panoramas

Introduction

Introduction

SIFT overview

1.

2.

Detector Scale-space extrema detection Keypoint localization and filtering 3.

4.

Descriptor Orientation assignment Keypoint descriptor

SIFT overview

1.

2.

Detector Scale-space extrema detection Keypoint localization and filtering 3.

4.

Descriptor Orientation assignment Keypoint descriptor

1. Scale-space extrema detection  A “good” function for scale detection has one stable sharp peak f Good bad region size bad  L or DOG(Difference of Gaussians) kernel is a matching filter !

1. Scale-space extrema detection  DOG(Difference of Gaussians) • Construct scale-space • Take differences Downsample Convolve with Gaussian

1. Scale-space extrema detection  For example • Construct scale-space

1. Scale-space extrema detection  For example • Take differences

1. Scale-space extrema detection  Scale-space extrema • Compare a pixel with its 26 neighbors in 3*3 regions at the current and adjacent scales

1. Scale-space extrema detection  For example • Scale-space extrema

SIFT overview

1.

2.

Detector Scale-space extrema detection Keypoint localization and filtering 3.

4.

Descriptor Orientation assignment Keypoint descriptor

2. Keypoint localization and filtering  Reject points with bad contrast • DOG smaller than 0.03 (image values in [0, 1])

2. Keypoint localization and filtering  Reject points with strong edge response in one direction only • To check if ratio of principal curvature is below some threshold, r

SIFT overview

1.

2.

Detector Scale-space extrema detection Keypoint localization and filtering 3.

4.

Descriptor Orientation assignment Keypoint descriptor

3. Orientation assignment  Descriptor computed relative to keypoint’s orientation achieves rotation invariance  Let, for a keypointm L is the image with the closest scale • Compute the orientation histogram - within a region around the keypoint (16 Ⅹ 16) • Compute gradient magnitude and orientation using finite differences

GradientVector

        1)  

3. Orientation assignment

3. Orientation assignment

3. Orientation assignment

3. Orientation assignment

3. Orientation assignment

SIFT overview

1.

2.

Detector Scale-space extrema detection Keypoint localization and filtering 3.

4.

Descriptor Orientation assignment Keypoint descriptor

Keypoint descriptor  The computation of the keypoint descriptor • A set of keypoints are obtained from each reference image • Each such keypoint has a graphical descriptor – which is a 128 component vector (4Ⅹ4Ⅹ8) ← keypoint descriptor’s complexity

 Storing Keypoint descriptor

Keypoint descriptor  Matching • Test image gives a new set of (keypoint, vector) pair • Find the nearest (top 2) descriptors in database  Acceptance of a match • Storage using k-d trees - Use the Best-Bin-First(BBF) algorithm • Ratio of distance to first nearest descriptor to that of second < threshold (0.8)

Test and Result <3D object recognition>

Test and Result

Test and Result

 SIFT didn’t work • Large illumination change Test and Result

 SIFT didn’t work • Non-rigid deformation Test and Result

Conclusion  SIFT • A novel method for detecting interest points • • Invariant to - image scaling - translation - rotation Robust matching across substantial range of - distortion - change in 3D view point - addition of noise - change in illumination  SIFT extensions • PCA-SIFT • • • SURF Approx SIFT GPU implementation