@c -*-texinfo-*-
@node Introduction, Building, Top, Top
@chapter Introduction
@cindex Introduction

@menu
* What Is CEL:: What Is Crystal Entity Layer?
* Concepts::
* How To Read:: How To Read This Manual
* Current Status::
* Using CEL For Your Project:: Using Crystal Entity Layer For Your Project
* Guide:: A Guide To This Documentation
* Features:: Feature List
* Where To Get CEL:: Where to Get Crystal Entity Layer
* Mailing Lists::
* Licence:: GNU Library General Public License
@end menu

@node What Is CEL, Concepts, Introduction, Introduction
@section What Is Crystal Entity Layer?

Crystal Entity Layer is a plugin developed to be used in conjunction with 
Crystal Space.  It is designed to make it easier for game developers to 
create games based on Crystal Space by providing such concepts as entities,
properties and behaviours.

Crystal Space is a portable modular 3D SDK, including many components for 
building various applications and games.  For more information on Crystal 
Space please see the
@uref{http://www.crystalspace3d.org/,Crystal Space web page}.

It is assumed here that you know the basic ideas behind Crystal Space. More
specifically you should have a notion about @sc{scf} and @sc{vfs} as those
are used a lot in @sc{cel} too.

Crystal Entity Layer is still in development.  It's development efforts are
currently hosted by SourceForge.  Anybody wishing to contribute should visit
the @uref{http://cel.crystalspace3d.org/,Crystal Entity Layer} project page and
@uref{http://www.sourceforge.net/,SourceForge}.

Crystal Entity Layer is free and should remain free. Crystal Entity Layer 
falls under the GNU copyleft license for libraries (LGPL). A copy of of the 
LGPL is included in this manual for your perusal. In short, the LGPL allows 
you to use CrystalSpace as a library in your products, even commercial 
products, but modifications to the library or derivative works incorporating 
parts of the library must be made freely available to everyone, under the 
LGPL's terms. 

@node Concepts, How To Read, What Is CEL, Introduction
@section Concepts
Every object within the game world is represented by an entity.  This includes
the world itself and all the objects within it, such as players, non-player
characters, items and cameras.

The base Crystal Entity Layer Entities are managed within the Physical Layer.
These basic Entities cannot do very much at all as they are.  In order to add
functionality one, or usually more, Physical Layer Property Classes are added
to the Entity.

Property Classes are used to provide specific attributes, functionality and
constraints to Entities and also provide the interface with the Crystal Space
library.  This may be a simple attribute, such as allowing an entity to store
it's weight, or it may provide more complex functionality such as receiving
input from the keyboard or drawing a 3D object such as a mesh.  The set of 
properties provides by all the property classes together defines what the
entity really is, and what functionality is provided.

Property Classes are added to Entities at run-time.  What this means for the
developer is that if required Property Classes can be added, or removed, from
Entities on-the-fly.

Property Classes are generated by Property Class Factories.  These are usually
implemented as a Crystal Space plugin.  This means that you load the plugin, 
and then invoke the plugin as required to create property classes.

The Physical Layer is also responsible for providing the Persistence Layer.  
This is a system that is responsible for being able to save and load the state 
of each and every Entity.  This may be a file on a disk, or to some form of 
database.  Single-player games would use this to provide save-game 
functionality for example.

The Behaviour Layer handles things such as game logic.  It is this layer that 
dictates how an Entity will behave within the constraints placed upon it by 
the Physical Layer.  

The Physical Layer is able to generate events, which are passed to the 
Behaviour Layer in order to be acted upon.  Examples of such events are when 
an Entity is clicked on by the mouse pointer, when a particular key is hit or 
when an item is added to an inventory.  Such events are usually generated from 
within the Property Classes when a predetermined set of conditions is 
achieved.

The majority of the Behaviour Layer may be implemented in a scripting language 
such as Python if this is suitable for the project.  This allows the behavior 
of Entities to be easily scripted without having to recompile the entire 
project, and also for new behaviours to be added in the same manner.

In summary, Crystal Entity Layer is made up itself of two layers, these being 
the Physical Layer and the Behaviour Layer.

The Physical Layer is responsible for all managing base Entities, providing 
the Property Class system to manage attributes, functionality and constraints 
to Entities, to prove a system for the Property Classes to communicate with 
the Behaviour Layer and to provide a Persistence Layer in order to be able to 
load and save Entities to some form of storage.

The Behaviour Layer is responsible for managing how Entities behave within the 
restrictions placed up on by the Physical Layer.  This may be implemented in 
C++, or it may be implemented in some form of scripting language.

@node How To Read, Current Status, Concepts, Introduction
@section How To Read This Manual

This manual was written and is currently maintained by Daniel Thoroughgood.
Constructive feedback, suggestions and contributions are always welcome.

If you find something in this manual which you believe is not correct please
notify the mailing list in order to have this investigated and corrected.

This manual is intended to also serve the purpose of a reference and as such
the order in which it is read is not important for the experienced developer.
People that are new to Crystal Entity Layer however should read the manual
front to back.

@node Current Status, Using CEL For Your Project, How To Read, Introduction
@section Current Status

Crystal Entity Layer is still in Beta status, and is likely to be for the
foreseeable future.  Crystal Space, on which this work is based, is still
under very active development and from time to time changes may be made to 
the Crystal Space sources which will cause Crystal Entity Layer not to work
as expected, or not to work at all.

Likewise Crystal Entity Layer is still in development and you may find that
some features do not work as expected, or may not work at all.  Some features
may be only partially implemented and there may be both known and unknown
bugs.

Please do not let this discourage you from using Crystal Entity Layer.  Most
of the core functions are implemented and working correctly.  Any bugs which
are reported will be investigated as soon as possible and contributions are 
always welcome.

@node Using CEL For Your Project, Guide, Current Status, Introduction
@section Using Crystal Entity Layer For Your Project

If you wish to use Crystal Entity Layer for your project you should obtain
and build sources of both Crystal Space and Crystal Entity Layer.

Read the Crystal Space documentation and take a look through the samples and
examples to become familiar with how it works.

You should also join the appropriate Crystal Space and Crystal Entity Layer
mailing lists in order to watch how things develop.  

You also need to agree to the LGPL (GNU Library General Public Licence).

If you are using Crystal Entity Layer for a project please let us know as we
are very interested in hearing what people are doing with both Crystal Space
and Crystal Entity Layer.

@node Guide, Features, Using CEL For Your Project, Introduction
@section A Guide To This Documentation
TODO: Complete this section when the layout of the manual is solid and cross
references are all in place.

@node Features, Where To Get CEL, Guide, Introduction
@section Feature List

@node Where To Get CEL, Mailing Lists, Features, Introduction
@section Where to Get Crystal Entity Layer
The Crystal Entity Layer homepage contains information on obtaining both the 
official releases of Crystal Entity Layer and the current development version.

Please keep in mind that both types of releases are still Beta.  The main
difference being that release versions should be well tested and generally 
stable.

With the development version your milage may vary.  Sometimes it may not
compile easily on all platforms, or may not compile at all.  There may be
features that are only partially implemented and these probably will not
be very well documented.

However, the development version will always have all the most recent features
and will probably contain features that are not available in the release
version.

You will also need Crystal Space in order to use Crystal Entity Layer.  Like
Crystal Entity Layer this is also available in both official release and
development status.  For more information on the two of these please see
the Crystal Space documentation.

Crystal Entity Layer is available at:@*
@uref{http://cel.crystalspace3d.org/}

Crystal Space is available at:@*
@uref{http://www.crystalspace3d.org/}

Generally a release version of Crystal Entity Layer should be built against
the matching release of Crystal Space.  Likewise development versions should
be build against a development version of Crystal Space from the same date.

Mixing and matching of versions is not guaranteed to work, and in fact quite
likely will not.

@subheading SVN
As Crystal Entity Layer is being actively developed changes are quite often
made to the sources on a daily basis.

The latest sources to Crystal Entity Layer may be downloaded from 
SourceForge's @sc{svn} Servers.

To checkout @sc{svn} trunk in a @file{cel} subdirectory, use:
@example
svn co https://svn.sourceforge.net/svnroot/cel/cel/trunk cel
@end example

@node Mailing Lists, Licence, Where To Get CEL, Introduction
@section Mailing Lists
There are currently two Crystal Entity Layer mailing lists that may be of 
interest to to users and developers.

The main list is for general discussion, questions and bug reports.  You may
subscribe to this list by visiting:@*
@uref{http://lists.sourceforge.net/mailman/listinfo/cel-main}

You may also view an archive of prior posts to this list at:@*
@uref{http://sourceforge.net/mailarchive/forum.php?forum_id=1108}

The @sc{svn} update list is for notification of changes to the @sc{svn} source
repository.  All users subscribed to this list will receive an e-mail every
time a developer updates the source.   You may subscribe to this list at:@*
@uref{http://lists.sourceforge.net/mailman/listinfo/cel-cvs-update}

You may also view an archive of prior posts to this list at:@*
@uref{http://sourceforge.net/mailarchive/forum.php?forum_id=33225}

@node Licence,  , Mailing Lists, Introduction
@section GNU Library General Public License

@noindent
Version 2, June 1991

Copyright @copyright{} 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, @sc{ma} 02139, @sc{usa}
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@subheading Gnu Library General Public License Terms And Conditions For Copying, Distribution And Modification

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@subsubheading No Warranty

@enumerate j
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@subsubheading End Of Terms And Conditions

@subheading Appendix: How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest
possible use to the public, we recommend making it free software that
everyone can redistribute and change.  You can do so by permitting
redistribution under these terms (or, alternatively, under the terms of the
ordinary General Public License).

To apply these terms, attach the following notices to the library.  It is
safest to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least the
``copyright'' line and a pointer to where the full notice is found.

<one line to give the library's name and a brief idea of what it does.>
Copyright @copyright{} <year> <name of author>

This library is free software; you can redistribute it and/or
modify it under the terms of the @sc{gnu} Library General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but @sc{without any warranty}; without even the implied warranty of
@sc{merchantability} or @sc{fitness for a particular purpose}.  See the @sc{gnu}
Library General Public License for more details.

You should have received a copy of the @sc{gnu} Library General Public
License along with this library; if not, write to the Free
Software Foundation, Inc., 675 Mass Ave, Cambridge, @sc{ma} 02139, @sc{usa}.

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the library, if
necessary.  Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in the
library `Frob' (a library for tweaking knobs) written by James Random Hacker.

<signature of Ty Coon>, 1 April 1990
Ty Coon, President of Vice

That's all there is to it!

