\chapter{Hits and Digitization}

\section{Design Philosophy}
In {\sc Geant4} a {\it hit} is a snapshot of a physical interaction or an 
accumulation of interactions of a track or tracks in a ``sensitive'' detector 
component.  A digitization, or {\it digit}, represents a detector output, such 
as an ADC/TDC count or a trigger signal.  A {\it digit} is created from one or 
more hits and/or other {\it digits}.  Given the wide variety of {\sc Geant4} 
applications, ways of describing detector sensitivity and the quantities to 
be stored in the {\it hits} and {\it digits} vary greatly.  This category 
therefore provides only abstract classes for both detector sensitivity and 
{\it hits}/{\it digits}.  It also provides tools for organizing the 
{\it hits}/{\it digits} into collections.

\section{Class Design}

\begin{itemize}

\item {\bf G4SensitiveDetectorManager} -
   a list of G4SensitiveDetectors.
 
\item {\bf G4HitsStructure} -
   a tree-like structure of G4Hit collections. Each branch represents the 
   hits in given sub-detector.  For example, the first level of branches 
   may consist of a tracker, ECAL, and HCAL, while the second level, in HCAL,
   consists of the barrel and endcaps. Finally the barrel may have phi-slices, 
   Z-slices, etc.
 
\item {\bf G4VSensitiveDetector} -
   an abstract class of all of sensitive volumes.
 
\item {\bf G4HitsCollection} -
   a collection of hits. Instantiates an RWCollection class.
 
\item {\bf G4VHit} -
   this class has all the information about a particular hit caused by a 
   single step.
 
\item {\bf G4VDigitizer} -
   the class of objects which transform the hits deposited by particles into 
   digitizations.
 
\item {\bf G4DigitizerManager} -
   the (single) object dispatching common messages to individual digitizers.
 
\item {\bf G4VDigi} -
   an abstract (base) class for all G4 digitizations.  This could be data as 
   simple as a singe byte, or as complex as an Ntuple.
 
\item {\bf G4DigiStructure} -
   digitizations are organized as a structure, which could be anything
   between a single value and an Ntuple.

\end{itemize} 


The object-oriented design of the 'hit' related classes is shown in the 
following class diagrams. The diagrams are described in the Booch notation.  
Fig. \ref{figure:hit-1} shows the general management of hit classes.
Fig. \ref{figure:hit-2} shows the OO design of user-related hit classes.
Fig. \ref{figure:hit-3} shows the OO design of the readout geometry.

\begin{figure}
\begin{center}
\includegraphics[angle=0,scale=0.55]{OOAnalysisDesign/Hit/classDgmHitMain.eps}
\vspace{10pt}
\caption{Overview of hit classes management}
\label{figure:hit-1}
\end{center}
\end{figure}

\begin{figure}
\begin{center}
\includegraphics[angle=0,scale=0.5]{OOAnalysisDesign/Hit/classDgmHitUser.eps}
\vspace{10pt}
\caption{User hit classes}
\label{figure:hit-2}
\end{center}
\end{figure}

\begin{figure}
\begin{center}
\includegraphics[angle=0,scale=0.6]{OOAnalysisDesign/Hit/classDgmReadoutGeom.eps}
\vspace{10pt}
\caption{Readout geometry}
\label{figure:hit-3}
\end{center}
\end{figure}

\section{Status of this chapter}

27.06.05 section on design philosophy added (from Geant4 general paper) by
         D.H. Wright \\