Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 8

SURF descriptors

n  SURF – Speeded Up Robust Features [Bay et al. 2006]!

n  Compute horizontal and vertical pixel differences, dx, dy (in local coordinate system for rotation and scale invariance, window size 20σ x 20σ, where σ2 is feature scale)

n  Sum dx, dy, and |dx|,|dy| over 4x4 subregions (SURF-64) or 3x3 subregions (SURF-36)"n  Normalize vector for gain invariance, but distinguish bright blobs and dark blobs based on sign of

Laplacian (trace of Hessian matrix)"

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 9

DxxDyy-(0.9Dxy)2

Filters"

Dxx

Dxy

Dyy

Blob Response"

Gray"

Computing feature descriptors

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 10

DxxDyy-(0.9Dxy)2

Filters"

Dxx

Dxy

Dyy

Blob Response"

Gray"

Computing feature descriptors

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 11

DxxDyy-(0.9Dxy)2

Filters"

Dxx

Dxy

Dyy

Blob Response"

Gray"

Computing feature descriptors

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 12

Computing feature descriptors

DxxDyy-(0.9Dxy)2

Filters"

Dxx

Dxy

Dyy

Blob Response"

Gray"

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 13

Computing feature descriptors

Maxima"

x"

y"

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 14

Computing feature descriptors

Gradient Field for Oriented Patch"

Orient Along "Dominant Gradient"

Digital Image Processing: Bernd Girod, © 2013 Stanford University -- Image Matching 15

Computing feature descriptors SU

RF

Des

crip

tor"

Σ dx Σ dy Σ|dx| Σ|dy|

Σ dx Σ dy Σ|dx| Σ|dy|

Σ dx Σ dy Σ|dx| Σ|dy|

SIFT

Des

crip

tor"

Σ Σ Σ Σ Σ Σ Σ Σ

dx

dy

…"

…"