source: trunk/documents/UserDoc/DocBookUsersGuides/ForApplicationDeveloper/xml/Fundamentals/classCategory.xml @ 921

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<!--    Converted to DocBook: Katsuya Amako, Aug-2006         -->
<!--    Changed by: Katsuya Amako,  4-Aug-1998                -->
<!--    Changed by: Michel MAIRE,   3-Aug-1998                -->
<!--    Changed by: Katsuya Amako, 30-Nov-1998                -->
<!--    Changed by: Dennis Wright, 20-Nov-2001                -->
<!--    Proof read by: Joe Chuma,  28-Jun-1999                -->
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<!-- ******************* Section (Level#1) ****************** -->
<sect1 id="sect.ClassCate">
<title>
Class Categories and Domains
</title>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.ClassCate.WhatIsClassCate">
<title>
What is a class category?
</title>

<para>
In the design of a large software system such as Geant4, it is
essential to partition it into smaller logical units. This makes
the design well organized and easier to develop. Once the logical
units are defined independent to each other as much as possible,
they can be developed in parallel without serious interference.
</para>

<para>
In object-oriented analysis and design methodology by Grady Booch 
<citation>
<xref linkend="biblio.booch1994" endterm="biblio.booch1994.abbrev" />
</citation>
, class categories are used to
create logical units. They are defined as "clusters of classes that
are themselves cohesive, but are loosely coupled relative to other
clusters." This means that a class category contains classes which
have a close relationship (for example, the "has-a" relation).
However, relationships between classes which belong to different
class categories are weak, i.e., only limitted classes of these
have "uses" relations. The class categories and their relations are
presented by a class category diagram. The class category diagram
designed for Geant4 is shown in the figure below. Each box in the
figure represents a class category, and a "uses" relation by a
straight line. The circle at an end of a straight line means the
class category which has this circle uses the other category.

<figure id="fig.G4ClassCategory">
<title>
Geant4 class categories
</title>

<mediaobject>
  <imageobject role="fo">
    <imagedata fileref="./AllResources/Fundamentals/classCategory.jpg"
               format="JPG" contentwidth="7.0cm" align="center" />
  </imageobject>
  <imageobject role="html">
    <imagedata fileref="./AllResources/Fundamentals/classCategory.jpg"
               format="JPG" align="center" />
  </imageobject>
</mediaobject>
</figure>
</para>

<para>
The file organization of the Geant4 codes follows basically the
structure of this class cateogory. This <emphasis>User's Manual</emphasis> 
is also organized according to class categories.
</para>

<para>
In the development and maintenance of Geant4, one software team
will be assigned to a class category. This team will have a
responsibility to develop and maintain all classes belonging to the
class category.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.ClassCate.ClassCateInGeant4">
<title>
Class categories in Geant4
</title>

<para>
The following is a brief summary of the role of each class category
in Geant4.

<orderedlist spacing="compact">
  <listitem><para><emphasis role="bold">Run and Event</emphasis>
            <para>These are categories related to the generation of events,
                  interfaces to event generators, and any secondary particles
                  produced. Their roles are principally to provide particles to be
                  tracked to the Tracking Management.
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Tracking and Track</emphasis>
            <para>These are categories related to propagating a particle by
                  analyzing the factors limiting the step and applying the relevant
                  physics processes. The important aspect of the design was that a
                  generalized Geant4 physics process (or interaction) could perform
                  actions, along a tracking step, either localized in space, or in
                  time, or distributed in space and time (and all the possible
                  combinations that could be built from these cases).
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Geometry and Magnetic Field</emphasis>
            <para>These categories manage the geometrical definition of a detector
                  (solid modeling) and the computation of distances to solids (also
                  in a magnetic field). The Geant4 geometry solid modeler is based on
                  the ISO STEP standard and it is fully compliant with it, in order
                  to allow in future the exchange of geometrical information with CAD
                  systems. A key feature of the Geant4 geometry is that the volume
                  definitions are independent of the solid representation. By this
                  abstract interface for the G4 solids, the tracking component works
                  identically for various representations. The treatment of the
                  propagation in the presence of fields has been provided within
                  specified accuracy. An OO design allows us to exchange different
                  numerical algorithms and/or different fields (not only B-field),
                  without affecting any other component of the toolkit.
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Particle Definition and Matter</emphasis>
            <para>These two categories manage the the definition of materials and
                  particles.
            </para>
            </para></listitem>
            <listitem><para><emphasis role="bold">Physics</emphasis>
            <para>This category manages all physics processes participating in the
                  interactions of particles in matter. The abstract interface of
                  physics processes allows multiple implementations of physics models
                  per interaction or per channel. Models can be selected by energy
                  range, particle type, material, etc. Data encapsulation and
                  polymorphism make it possible to give transparent access to the
                  cross sections (independently of the choice of reading from an
                  ascii file, or of interpolating from a tabulated set, or of
                  computing analytically from a formula). Electromagnetic and
                  hadronic physics were handled in a uniform way in such a design,
                  opening up the physics to the users.
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Hits and Digitization</emphasis>
            <para>These two categories manage the creation of hits and their use
                  for the digitization phase. The basic design and implementation of
                  the Hits and Digi had been realized, and also several prototypes,
                  test cases and scenarios had been developed before the
                  alpha-release. Volumes (not necessarily the ones used by the
                  tracking) are aggregated in sensitive detectors, while hits
                  collections represent the logical read out of the detector.
                  Different ways of creating and managing hits collections had been
                  delivered and tested, notably for both single hits and calorimetry
                  hits types. In all cases, hits collections had been successfully
                  stored into and retrieved from an Object Data Base Management
                  System.
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Visualization</emphasis>
            <para>This manages the visualization of solids, trajectories and hits,
                  and interacts with underlying graphical libraries (the
                  Visualization class category). The basic and most frequently used
                  graphics functionality had been implemented already by the
                  alpha-release. The OO design of the visualization component allowed
                  us to develop several drivers independently, such as for OpenGL, Qt and
                  OpenInventor (for X11 and Windows), DAWN, Postscript (via DAWN) and
                  VRML.
            </para>
  </para></listitem>
  <listitem><para><emphasis role="bold">Interfaces</emphasis>
            <para>This category handles the production of the graphical user
                  interface (GUI) and the interactions with external software
                  (OODBMS, reconstruction etc.).
            </para>
  </para></listitem>
</orderedlist>
</para>


</sect2>
</sect1>