\chapter{Effective codebooks for human action categorization}\label{chap:stip}
{

\begin{quote}
\textit{In this chapter we propose a new method for human action categorization, combining novel gradient and optic flow descriptors, and creating an effective bag-of-words model. Recent approaches have represented videos using bag of spatio-temporal visual words, following the successful results achieved in object and scene classification. In such cases codebooks are usually obtained by k-means clustering and hard assignment of visual features to the more representative codewords. Our main contribution is two-fold. First, we define a novel 3D gradient descriptor that combined with optic flow outperforms the state-of-the-art, without requiring fine parameter tuning. Second, we show that for spatio-temporal features the popular k-means algorithm is insufficient, because cluster centers are attracted by the denser regions of the sample distribution, providing a non-uniform description of the feature space and thus failing to code other informative regions. We obtain a more effective codebook by applying a radius-based clustering method and a soft assignment that considers the information of two or more relevant codeword candidates.
\footnote{A preliminary version of the work presented in this chapter has been published as \dquote{Effective Codebooks for Human Action Categorization} in \textit{Proc.~of ICCV International Workshop on Video-oriented Object and Event Classification (VOEC), 2009}~\cite{ballan-voec09}.}
}
\end{quote}


%-------------------------------------------------------------------------
\section{Introduction and previous work}


%-------------------------------------------------------------------------
\section{Detector and descriptors}\label{sec:detector_descriptor}

\subsection{Detector}

\subsection{Descriptors}


%-------------------------------------------------------------------------
\section{Action representation and categorization}\label{sec:representation7}

\subsection{Codebook formation}\label{sec:codebook}

\subsection{Codeword assignment}\label{sec:word_assignment}


%-------------------------------------------------------------------------
\section{Experimental results}\label{sec:results7}

\subsection{Evaluation of our descriptor}\label{sec:exp_descriptor}

\subsection{Performances obtained by effective codebooks}\label{sec:final_results}

\subsection{Comparison to state-of-the-art}\label{sec:exp_comparison}


%-------------------------------------------------------------------------
\section{Conclusion}\label{sec:conclusion7}

}