DataDriven Markov Chain Monte Carlo Presented by Tomasz

Data-Driven Markov Chain Monte Carlo Presented by Tomasz Malisiewicz for Advanced Perception 3/1/2006

Overview of Talk • What is Image Segmentation? Image segmentation in a Bayesian statistical framework • How to find a good segmentation? Markov Chain Monte Carlo for exploring the space of all segmentations • DDMCMC results Data-Driven methods for exploiting image data and speeding up MCMC

DDMCMC Motivation • Iterative approach: consider many different segmentations and keep the good ones • Few tunable parameters, ex) # of segments encoded into prior • DDMCMC vs Ncuts

Berkeley Segmentation Database Image 326038 Berkeley Ncuts K=30 DDMCMC

Why a rigorous formulation? • Allows us to define what we want the segmentation algorithm to return • Assigning a Score to a segmentation

Formulation #1 (and you thought you knew what image segmentation was) • Image Lattice: • Image: • For any point either An image partition or into disjoint regions is not An image segmentation! Regions Contents Are Key! • Lattice partition into K disjoint regions: • Region is discrete label map: • Region Boundary is Continuous:

Formulation #2 (and you thought you knew what image segmentation was) • Each Image Region is a realization from a probabilistic model • are parameters of model indexed by • A segmentation is denoted by a vector of hidden Space of all variables W; K is number of regions segmentations • Bayesian Framework: Posterior Likelihood Prior

Prior over segmentations (do you like exponentials? ) # of model params Want less regions Want round-ish regions ~ uniform Want less complex models Want small regions •

Likelihood for Images • Visual Patterns are independent stochastic processes Grayscale • • • is model-type index is model parameter vector is image appearance in i-th region Color

Four Gray-level Models Uniform Gaussian Clutter Intensity Histogram • Gray-level model space: Texture FB Response Histogram Shading B-Spline

Three Color Models (L*, u*, v*) • Gaussian • Mixture of 2 Gaussians • Bezier Spline • Color model space:

Calibration • Likelihoods are calibrated using empirical study • Calibration required to make likelihoods for different models comparable (necessary for model competition) Principled? or Hack?

What did we just do? Def. of Segmentation: Score (probability) of Segmentation: Likelihood of Image = product of region likelihoods Regions defined by k-partition:

What do we do with scores? Search

Search through what? Anatomy of Solution Space • Space of all k-partitions • General partition space Scene Space • Space of all segmentations or Partition space K Model spaces

Searching through segmentations Exhaustive Enumeration of all segmentations Takes too long! Greedy Search (Gradient Ascent) Local minima! Stochastic Search Takes too long MCMC based exploration Described in the rest of this talk!

Why MCMC • What is it? -A clever way of searching through a high-dimensional space -A general purpose technique of generating samples from a probability • What does it do? -Iteratively searches through space of all segmentations by constructing a Markov Chain which converges to stationary distribution

Designing Markov Chains • Three Markov Chain requirements • Ergodic: from an initial segmentation W 0, any other state W can be visited in finite time (no greedy algorithms); ensured by jump-diffusion dynamics • Aperiodic: ensured by random dynamics • Detailed Balance: every move is reversible

5 Dynamics 1. ) Boundary Diffusion 2. ) Model Adaptation 3. ) Split Region 4. ) Merge Region 5. ) Switch Region Model At each iteration, we choose a dynamic with probability q(1), q(2), q(3), q(4), q(5)

Dynamics 1: Boundary Diffusion • Diffusion* within Boundary Between Regions i and j *Movement within partition space Temperature Decreases over Time Brownian Motion Along Curve Normal

Dynamics 2: Model Adaptation • Fit the parameters* of a region by steepest ascent *Movement within cue space

Dynamics 3 -4: Split and Merge • Split one region into two Remaining Variables Are unchanged Probability of Proposed Split Conditional Probability of how likely chain proposes to move to W’ from W Data-Driven Speedup

Dynamics 3 -4: Split and Merge • Merge two Regions Remaining Variables Are unchanged Data-Driven Speedup Probability of Proposed Merge

Dynamics 5: Model Switching • Change models • Proposal Probabilities Data-Driven Speedup

Motivation of DD • Region Splitting: How to decide where to split a region? vs • Model Switching: Once we switch to a new model, what parameters do we jump to? Model Adaptation Required some initial parameter vector

Data Driven Methods • Focus on boundaries and model parameters derived from data: compute these before MCMC starts • Cue Particles: Clustering in Model Space • K-partition Particles: Edge Detection • Particles Encode Probabilities Parzen Window Style

Cue Particles In Action Clustering in Color Space

K-partition Particles in Action • Edge detection gives us a good idea of where we expect a boundary to be located

Particles or Parzen Window* Locations? • What is this particle business about? • A particle is just the position of a parzenwindow which is used for density estimation *Parzen Windowing also known as: Kernel Density Estimation, Non-parametric density estimation 1 D particles

Are you awake: What did we just do? • Scores (Probability of Segmentation) Search So what type of answer does the • 5 MCMC dynamics Markov Chain return? What can we do with this answer? How many answers to to wemaking want? • Data-Driven Speedup (key MCMC work in finite time)

Multiple Solutions • MAP gives us one solution • Output of MCMC sampling How do we get multiple solutions? Parzen Windows: Again Scene Particles

Why multiple solutions? • Segmentation is often not the final stage of computation • A higher level task such as recognition can utilize a segmentation • We don’t want to make any hard decision before recognition • multiple segmentations = good idea

K-adventurers • We want to keep a fixed number K of segmentations but we don’t want to keep trivially different segmentations • Goal: Keep the K segmentations that best preserve the posterior probability in KL-sense • Greedy Algorithm: - Add new particle, remove worst particle

Results (Multiple Solutions)

Results

Results (Color Images) http: //www. stat. ucla. edu/~ztu/DDMCMC/benchmark_color. htm

Conclusions • DDMCMC: Combines Generative (topdown) and Discriminative (bottom-up) approaches • Traverse the space of all segmentations via Markov Chains • Does your head hurt? • Questions?

References • DDMCMC Paper: http: //www. cs. cmu. edu/~efros/courses/AP 06/Pa pers/tu-pami-02. pdf • DDMCMC Website: http: //www. stat. ucla. edu/%7 Eztu/DDMCMC/DD MCMC_segmentation. htm • MCMC Tutorial by Authors: http: //civs. stat. ucla. edu/MCMC_tutorial. ht m