shape context
DESCRIPTION
Scene segmentation & interpretation Shape context, a descriptor for object recognition in computer vision.TRANSCRIPT
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Shape Context
Rocío Cabrera u1908272
Vanya Valindria u1908259
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Introduction
Can you guess what number it is?
Objectives
“Have descriptors that can be computed in one image and used to find corresponding points, if visible, in another image.”
“Given a query model image, to develop an algorithm capable of retrieving similar-shaped images from an extensive database”
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Process Stages
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Solve the correspondence problem between
the two shapes
Use the correspondences to estimate an
aligning transform
Compute the distance between
the two shapes
Evaluate the distance and classify the
shape
. .... ??
SHAPE CONTEXT“A novel approach to measuring similarities between shapes and exploit it for object classification/recognition”
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Shape Context Computation
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Step 1. Obtain from ShapeP and ShapeQ n-samples uniformly spaced taken from their edge elements
Shape Context Computation
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Step 2. Compute the Euclidean distance (r) and the angle (θ) from each point in the set to all the other n-1 points. Normalize r by the median distance (λ) and measure the angle relative to the positive x-axis.
Shape Context Computation
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Step 3. Compute the log of the r vector.Discretize the distance and angle measurements
Shape Context Computation
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Step 4. For each origin point, capture number of points that lie a given θ,R bin.
Each shape context is a log-polar histogram of the coordinates of the n-1 points measured from the origin reference point.
Shape Context Computation
Shape context of the sample points in ShapeP and ShapeQ.
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Matching Shape Contexts
How can we assign the sample points of ShapeP to correspond to those of ShapeQ?
Determining shape correspondences such that:
1. Corresponding points have very similar descriptors
2. The correspondences are unique
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Matching Shape Contexts
Define matching cost function
Shape context Distance between the two normalized histograms
Local appearance Dissimilarity of the tangent angles
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Matching Shape Contexts
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Modeling Transformation
Given a set of correspondences, estimate a transformation that maps the model into the target
Euclidean transformation
Affine model
Thin Plate Spline (TPS)
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Classification/Recognition
This enables a measure of shape similarity The dissimilarity between two shapes can be computed
as the sum of matching errors between corresponding points, together with a term measuring the magnitude of the aligning transform
Given a dissimilarity measure, a k-NN technique can be used for object classification/recognition
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Method Evaluation
Advantages Incorporates invariance to:
Translation
Scale
Rotation
Occlusions
Drawbacks Sensitive local distortion or
blurred edges
Problems in cluttered background
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Applications
Digit recognition
Silhouette similarity-based retrieval
3 D object recognition
Trademark retrieval
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Database for Digit Recognition
MNIST datasets of handwritten digits:
60,000 training and 10,000 test digits
Links:
http://yann.lecun.com/exdb/mnist/
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Database for Silhouette
MPEG-7 shape silhouette database (Core Experiment CE-Shape-1 part B)
1400 images: 70 shapes categories and 20 images per category
Links:
http://mpeg.chiariglione.org/standards/mpeg-7/mpeg-7.htm
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Database for 3-D object recognition
COIL-20 database
20 common household objects; turned every 5˚ for a total of 72 views per object
Links:
http://www1.cs.columbia.edu/CAVE/software/softlib/coil-20.php
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Database for Trademark retrieval
300 different real-world trademark
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MATLAB DEMO
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Conclusions
The shape context method is simple to implement yet it is a rich shape descriptor
The methodology makes it invariant to translation, scale and rotation
Useful tool for shape matching and recognition
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