CVPR/ICCV 09 Paper ReadingCVPR/ICCV 09 Paper Reading

Dan WangNov. 6, 2009

Papers

• CVPR:– Learning Color and Locality Cues for Moving

Object Detection and Segmentation

• ICCV: – Texel-based Texture Segmentation

Learning Color and Locality Cues for Moving Object Detection and Segmentation

Feng Liu and Michael Gleicher

• Authors• What does the paper do?• Problem of previous methods?• How to do?

• Authors• What does the paper do?• Problem of previous methods?• How to do?

Author 1 Feng Liu

• Computer Sciences Department at the University of Wisconsin, Madison

• Graduate student• Publications

– Feng Liu, Michael Gleicher, Hailin Jin and Aseem Agarwala. Content-Preserving Warps for 3D Video Stabilization. ACM SIGGRAPH 2009

– Feng Liu, Yu-hen Hu and Michael Gleicher. Discovering Panoramas in Web Videos. ACM Multimedia 2008

– Feng Liu and Michael Gleicher. Texture-Consistent Shadow Removal. ECCV 2008

– Feng Liu and Michael Gleicher. Video Retargeting: Automating Pan and Scan. ACM Multimedia 2006

Author 2 Michael Gleicher

• Computer Sciences Department at the University of Wisconsin, Madison

• Professor• Positions – 2009 – present – Professor– 2004 – 2009 – Associate Professor– 1998 - 2004 – Assistant Professor

• http://pages.cs.wisc.edu/~gleicher/CV.pdf

• Authors• What does the paper do?• Problem of previous methods?• How to do?

Problem of Previous Methods

• Most previous automatic methods rely on methods rely on object or camera motionobject or camera motion to detect the moving object.

• Small motion Small motion of object or camera do not provide sufficient information for these methods.

Abstract• The paper presents an algorithm for automatically

detecting and segmenting a moving object from a monocular video.

• Existing method: Rely on motion to detect moving object. When motion is sparse and insufficient, it fails…

• What does this paper do ?– Unsupervised algorithm to learn object color and locality

cues from the sparse motion information.• How to do?– Detect key frames, sub-objects– Learning from the sub-objects: color and locality cues– Combining cues in MRF framework

• What is “Moving ObjectMoving Object”?– Some compact regions with different apparent

motion from background.

• How to detect moving object– Estimate the global motion in a video– Calculate the discrepancy at each between the

object motion and global motion

Detect Moving Object

• Model: – Use Homography to model the global motion

between two consecutive frames.

• Feature:– Use a SIFT feature-based method to estimate the

homography

projective transformationDetails in [19]

Detect Moving Object

• With the homography, we calculate the motion cue(mc) at pixel (x,y) as :

Key Frame Extraction

• Definition– The frame where a moving object or its part can

be reliably inferred from motion cues.– Motion cues are likely reliable when they are

strong and compact.

Segment Moving Sub-objects from Key Frames

• Problem– Not all pixels of the moving object have significant

motion cues. Motion cues are Sparse!

Segment Moving Sub-objects from Key Frames

• Solution– Neighboring pixels are likely to have the same

label.– Neighboring pixels with similar colors are more

likely to have the same label.

• Solved by Graph cut algorithm

Segment Moving Sub-objects from Key Frames

• MRF priors on labels to model the interaction

Pixel label

Number of pixels Neighbor

of pixel i

Segment Moving Sub-objects from Key Frames

• The likelihood of image I given a labeling can be modeled as follows:

Learning Color and Locality Cues

• Assumption– The moving sub-objects from all the key frames

form a complete sampling of the moving objects.

• Procedure– Lab color space– Build GMM

Learning Color and Locality Cues

• The spatial affinity of pixel I to the moving object:

• Location likelihood:

F: sub-objectparameter

position

Experiments

• Insignificant camera and object motion

• Significant camera and uneven object

• Significant camera and object motion

Discussion• Extracting moving object from a video with less object

and camera motion is not easier than more objects and camera motion

• Contribution– Unsupervised?

• Currently– Off-line– Motion estimation is time consuming

• Future– Parameters– Background modeling

Texel-based Texture Segmentation

Sinisa Todorovic, Narendra AhujaReporter: Wang Dan

Authors

• Sinisa Todorovic– Assistant Professor– School of EECS, Oregon State University – Publication: CVPR 2009, ICCV 2009, TPAMI 2008...

• Narendra Ahuja– Donald Biggar Willet Professor– Beckman Institute ， UIUC– Publication: ICCV 2009, IJCV 2008, TPAMI…

Problem

• Given an arbitrary image, segment all texture subimages– Texture = Spatial repetition of texture elements, i.e.,texels– Texels are not identical, but only statistically similar to one

another.– Texel placement along the texture surface is not periodic, but

only statistically uniform.– Texels in the image are not homogeneous, but regions that

may contain subregions.

Rational

• Texels occupy image regions– If the image contains texture, many regions will

have similar properties• color, shape, layout of subregions• orientation, relative displacements

– The pdf of region properties will have modes

Texture detection and segmentation

Detection of modes of pdf of region properties

Method Overview

Contributions

• No assumptions about the pdf of texel properties

• Both appearance and placement of the texels are allowed to be stochastic and correlated

• New hierarchical, adaptive-bandwidth kernel to capture texel structural properties

Method Description

• Define a feature space of region propertiesregion properties• Descriptor of each region = Data point in the

feature space• Partition the feature space Partition the feature space into bins by Voronoi

tessellation• Run the meanshift with the new, hierarchical new, hierarchical

kernelkernel• Regions under a pdf mode comprise the texture

subimage

The feature space of region properties

• Image hierarchical structure

• Use multiscale segmentation algorithm[1], [4]

The feature space of region properties

• A descriptor vector of prperties xi of image region i1. Average contrast across i’s boundary2. Area3. Standard deviation of i’s children area4. Displacement vector between the centroids of i and its

parent region5. Perimeter6. Aspect ratio of intercepts of i’s principal axes with the

i’s boundary7. Orientation: the angle between principal axes and x-axis8. Centroid of I

PCA 95% Not scale and rotation invariance

Voronoi Diagram

AA

BB

CC

Voronoi Diagram

• Definition:– Let P = {p1,p2, ... , pn} be a set of points in the plane (or in

any dimensional space), which we call sites. – Define V(pi), the Voronoi cell for pi, to be the set of points

q in the plane that are closer to pi than to any other site. That is, the Voronoi cell for pi is defined to be:

• V(pi ) = {q | dist(pi,q) < dist(pj, q), for j != i}:

• Anyway, it can partition the feature space…Anyway, it can partition the feature space…• http://www.dma.fi.upm.es/mabellanas/tfcs/fvd/voro

noi.html

Voronoi-based Binned Meanshift

• New variable-bandwidth matrix

Gaussian kernel

Hierarchical kernel

• Motivation– Texels, in general, are not homogenous-intensity

regions, but may contain hierarchically embedded subregions.

– Since region descriptors represent image regions, we can define hierarchical relationships between the descriptors based on the embedding of corresponding smaller regions within larger regions in the image.

• Voronoi partitioning of the feature space– Suppose x belongs to Bi, and x’ belongs to Bj

•Arbitrary points

•Region descriptors

x x’

xi xj

• x belongs to Bi . Compute by finding the maximum subtree isomorphism between two trees rooted at at xi and xj as:

Experimental Evaluation

• Qualitative Evaluation• Quantitative evaluation– G: the area of true texture– D: the area of a subimage that our approach

segments– Segmentation error per texture:

• (1) 100 collages of randomly mosaicked, 111 distinct Brodatz textures, where each texture occupies at least 1/6 of the collage

• (2) 180 collages of randomly mosaicked, 140 distinct Prague textures from 10 thematic classes (e.g., flowers, plants, rocks, textile, wood, etc.), where each texture occupies at least 1/6 of the collage .

• (3) 100 Aerial-Produce images, where 50 aerial images show housing developments, agricultural fields, and landscapes, and 50 images show produce aisles in supermarkets .

• (4) Berkeley segmentation dataset

Quantitative evaluation(Brodaz)

93.3%+3.7

structural properties vs ignoring them 77.9%+4.1 Simple Gaussian kernel, Variable bandwidth

Variable bandwidth in this paper 62.3%+7.8 Simple Gaussian kernel, Variable bandwidth [7]

• Quantitative evaluation– G: the area of true texture– D: the area of a subimage that our approach

segments– Segmentation error per texture:

Quantitative evaluation(Prague)– G: the area of true texture– D: the area of a subimage that our approach segments– CS: Correct segmentation– OS: Over segmentation means G that is split into smaller regions D– US: DG– ME counts every G that does not belong to CS– NE counts every D that does not belong to US

“The state-of-the-art unsrpervised texture

segmentation”

Conclusion• We have presented a texel-based approach to

segmenting image parts occupied by distinct textures. This is done by capturing intrinsic and intrinsic and placement placement properties of distinct groups of texels.

• Experimental evaluation on texture mosaics and real-world images suggests that capturing structural properties structural properties of texels is very important for texture segmentation.

• To account for texel substructure, we have derived and used a hierarchical, variable a hierarchical, variable bandwidth kernel bandwidth kernel in the meanshift.

Slices are partly from Sinisa Todorovic