**Conditional Models for Contextual Human Motion Recognition.** Cristian Smimchisescu, Atul Kanaujia, Zhiguo Li and Dimitris Metaxus. In Proceedings of the International Conference on Computer Vision, (ICCV 2005), Beijing, China, 2005.

We present algorithms for recognizing human motion in monocular video sequences, based on discriminative Conditional Random Field (CRF) and Maximum Entropy Markov Models (MEMM). Existing approaches to this problem typically use generative (joint) structures like the Hidden Markov Model (HMM). Therefore they have to make simplifying, often unrealistic assumptions on the conditional independence of observations given the motion class labels and cannot accommodate overlapping features or long term contextual dependencies in the observation sequence. In contrast, conditional models like the CRFs seamlessly represent contextual dependencies, support efficient, exact inference using dynamic programming, and their parameters can be trained using convex optimization. We introduce conditional graphical models as complementary tools for human motion recognition and present an extensive set of experiments that show how these typically outperform HMMs in classifying not only diverse human activities like walking, jumping, running, picking or dancing, but also for discriminating among subtle motion styles like normal walk and wander walk.

**A Dynamic Conditional Random Field Model for Object Segmentation in Image Sequences**. Yang Wang and Qiang Ji. In IEEE Computer Society Conference on Computer Vision and Pattern Recognition (CVPR 2005), Volume 1, 2005.

This paper presents a dynamic conditional random field (DCRF) model to integrate contextual constraints for object segmentation in image sequences. Spatial and temporal dependencies within the segmentation process are unified by a dynamic probabilistic framework based on the conditional random field (CRF). An efficient approximate filtering algorithm is derived for the DCRF model to recursively estimate the segmentation field from the history of video frames. The segmentation method employs both intensity and motion cues, and it combines dynamic information and spatial interaction of the observed data. Experimental results show that the proposed approach effectively fuses contextual constraints in video sequences and improves the accuracy of object segmentation.

**Gene Prediction with Conditional Random Fields**. Aron Culotta, David Kulp and Andrew McCallum. Technical Report UM-CS-2005-028. University of Massachusetts, Amherst, 2005.

Given a sequence of DNA nucleotide bases, the task of gene prediction is to find subsequences of bases that encode proteins. Reasonable performance on this task has been achieved using generatively trained sequence models, such as hidden Markov models. We propose instead the use of a discrimini- tively trained sequence model, the conditional random field (CRF). CRFs can naturally incorporate arbitrary, non-independent features of the input without making conditional independence assumptions among the features. This can be particularly important for gene finding, where including evidence from protein databases, EST data, or tiling arrays may improve accuracy. We evaluate our model on human genomic data, and show that CRFs perform better than HMM-based models at incorporating homology evidence from protein databases, achieving a 10% reduction in base-level errors.

**CRF package**. Sunita Sarawagi.

The CRF package is a Java implementation of conditional random fields for sequential labeling.

**Bayesian Conditional Random Fields.** Yuan Qi, Martin Szummer and Thomas P. Minka. To appear in Proceedings of the Tenth International Workshop on Artificial Intelligence and Statistics (AISTATS 2005), 2005.

We propose Bayesian Conditional Random Fields (BCRFs) for classifying interdependent and structured data, such as sequences, images or webs. BCRFs are a Bayesian approach to training and inference with conditional random fields, which were previously trained by maximizing likelihood (ML) (Lafferty et al., 2001). Our framework eliminates the problem of overfitting, and offers the full advantages of a Bayesian treatment. Unlike the ML approach, we estimate the posterior distribution of the model parameters during training, and average over this posterior during inference. We apply an extension of EP method, the power EP method, to incorporate the partition function. For algorithmic stability and accuracy, we flatten the approximation structures to avoid two-level approximations. We demonstrate the superior prediction accuracy of BCRFs over conditional random fields trained with ML or MAP on synthetic and real datasets.

**Semi-Markov Conditional Random Fields for Information Extraction.** Sunita Sarawagi and William W. Cohen. In Advances in Neural Information Processing Systems 17 (NIPS 2004), 2005.

We describe semi-Markov conditional random fields (semi-CRFs), a conditionally trained version of semi-Markov chains. Intuitively, a semi-CRF on an input sequence x outputs a "segmentation" of x, in which labels are assigned to segments (i.e., subsequences) of x rather than to individual elements x_i of x. Importantly, features for semi-CRFs can measure properties of segments, and transitions within a segment can be non-Markovian. In spite of this additional power, exact learning and inference algorithms for semi-CRFs are polynomial-time---often only a small constant factor slower than conventional CRFs. In experiments on five named entity recognition problems, semi-CRFs generally outperform conventional CRFs.

**Contextual models for object detection using boosted random fields.** Antonio Torralba, Kevin P. Murphy, William T. Freeman. In Advances in Neural Information Processing Systems 17 (NIPS 2004), 2005.

We seek to both detect and segment objects in images. To exploit both local image data as well as contextual information, we introduce Boosted Random Fields (BRFs), which uses Boosting to learn the graph structure and local evidence of a conditional random field (CRF). The graph structure is learned by assembling graph fragments in an additive model. The connections between individual pixels are not very informative, but by using dense graphs, we can pool information from large regions of the image; dense models also support efficient inference. We show how contextual information from other objects can improve detection performance, both in terms of accuracy and speed, by using a computational cascade. We apply our system to detect stuff and things in office and street scenes.

**Markov Networks for Detecting Overlapping Elements in Sequence Data.** Jospeh Bockhorst and Mark Craven. In Advances in Neural Information Processing Systems 17 (NIPS 2004), 2005.

Many sequential prediction tasks involve locating instances of pat- terns in sequences. Generative probabilistic language models, such as hidden Markov models (HMMs), have been successfully applied to many of these tasks. A limitation of these models however, is that they cannot naturally handle cases in which pattern instances overlap in arbitrary ways. We present an alternative approach, based on conditional Markov networks, that can naturally represent arbitrarily overlapping elements. We show how to efficiently train and perform inference with these models. Experimental results from a genomics domain show that our models are more accurate at locating instances of overlapping patterns than are baseline models based on HMMs.

**Conditional Random Fields for Object Recognition.** Ariadna Quattoni, Michael Collins and Trevor Darrel. In Advances in Neural Information Processing Systems 17 (NIPS 2004), 2005.

We present a discriminative part-based approach for the recognition of object classes from unsegmented cluttered scenes. Objects are modeled as flexible constellations of parts conditioned on local observations found by an interest operator. For each object class the probability of a given assignment of parts to local features is modeled by a Conditional Random Field (CRF). We propose an extension of the CRF framework that incorporates hidden variables and combines class conditional CRFs into a unified framework for part-based object recognition. The parameters of the CRF are estimated in a maximum likelihood framework and recognition proceeds by finding the most likely class under our model. The main advantage of the proposed CRF framework is that it allows us to relax the assumption of conditional independence of the observed data (i.e. local features) often used in generative approaches, an assumption that might be too restrictive for a considerable number of object classes. We illustrate the potential of the model in the task of recognizing cars from rear and side views.