Re: [Nel] NeL Status Update?

[Cyril Corvazier]

Hi,

> E> Is there any way I can get an ETA on that 3DS plugin, and detailed
documentation of how to structure the data for my world, so NeL will work
with it?

What you said about the NeL file format makes a lot of sense,  and it match
what we planned to do in a near future.

Actually, i'm going to describe the current data management.

Today, our export plug-ins generate NeL binary files using the NeL
serialisation system.
The serialisation system is described in the document inserted at the end of
the mail. This document
will be available on the web site in the Doxygen Related Pages during the
next week.

Here is the URL of another document that describes how to build NeL 3d data
from your 3d editor and export them in NeL binary format:

http://www.nevrax.org/docs/doxygen/nel/3d_data_howto.html

---

NeL Files and Serialisation

* Introduction

This is really quite a difficult subject to write about - so this file is an
introduction which describes the basic features and principles of our
system.


* How our files work in NeL

The NeL files are NOT designed to be man-readable.  Interpretation and
generation of file contents is performed by the objects that are to be read
and written using a standardised mechanism. This mechanism was inspired by
the system provided by Java.

We use the term 'serialisable' to describe a class  that can be read from/
written to a NeL data file.
Counter-intuitive as it may, at first, appear, each 'serialisable' class
supplies a single method that is used for both reading and writing.

Note that the files are encoded in little-endian and that the NeL library
code deals with conversion of endian-ness for big-endian platforms


* Serialisation beyond files

The serialisation system can be used for generating binary data buffers in
memory (without writing the result to a file) or for packing and unpacking
data for transfer over a LAN.


* How it works

Technically, we define a 'serialisable' class as a class that can be passed
to IStream::serial().
In order for a class to be serialisable it is sufficient for it to include
the following method:
 void serial(IStream&).
The fact that we use a template method definition means that a serialisable
class does not have to be derived from any other class.
All standard types are serialisable due to a non-template prototypes shown
below.
STL containers of serialisable types are serialisadble
Pointers to non-polymorphic serialisable types are serialisable.

The IStream class definition looks something like this:

 class IStream
 {
 ...
  void serial (int&);
  void serial (float&);
 ...
  template <class T> void serial (T&t)
  {
   t.serial (*this);
  }
 };


Example:
To make the following class serialisable:

 class myFirstClass
 {
  int a,b;
 };

you would need to extend the class as follows:

 class myFirstClass
 {
  int a,b;
  void serial (IStream&istream)
  {
   istream.serial(a);
   istream.serial(b);
  }
 };

The following example shows how to serialise a more complicated data
structure

 class myFirstClass
 {
  void serial (IStream&);
 };

 class myclass
 {
  int     BaseType;
  myFirstClass    SerialisableClass
  std::vector< myFirstClass>  STLContainerOfSerialisableClass;
  myFirstClass    *PointerToSerialisableClass;
  std::vector< myFirstClass*>  STLContainerOfPointersToSerialisableClass;

  void serial (IStream&istream)
  {
   istream.serial(BaseType);
   istream.serial(SerialisableClass);
   istream.serialCont(STLContainerOfSerialisableClass);
   istream.serialPtr(PointerToSerialisableClass);
   istream.serialContPtr(STLContainerOfPointersToSerialisableClass);
  }
 };



* Dealing with cross referenced or hierarchical data

If an object contains a pointer to another object in memory then the
serialPtr() method is used to read/ write the referenced object.
The NeL library code writes a value corresponding to the pointer to the
serialised data, followed by the data that the pointer points to (In the
case of a NULL pointer the value 0 is written without any following data)
The NeL library code automatically deals with the cases where two or more
objects reference the same object or there is a circular reference.  Each
time a pointer is de-referenced, for writing, NeL checks against a table of
previous pointers;  if the pointer value already exists in the table then no
data is written.  At read time the data structures are faithfully
reconstructed.


* Dealing with polymorphism within cross referenced data

In a nut shell, in order to un-serialise a data record that one only has an
interface type for, one needs to store an additional identifier with the
data record that identifies it's real type.  The mechanism for doing this is
best shown with an example:

 class IBaseClass : public IStreamable
 {
  // This class is an interface. It is polymorphic.
  virtual void foo ()=0;

  // It must declare it's name
  NLMISC_DECLARE_CLASS (MyClass);
 };

 class CClassToSerialise
 {
  IBaseClass  *PointerToAPolymorphicClass;

  void serial (IStream& s)
  {
   s.serialPolyPtr (PointerToAPolymorphicClass);
  }
 };

 void main ()
 {
 ...
  // The polymorphic class must be registered in the registry
  NLMISC_REGISTER_CLASS (MyClass);
 ...
 }


* Dealing with file format evolution

 void serial (IStream& s)
 {
  // At the begining of the serial process, read/ write the version number
of the class implementation

  // In the following example - at read time 'version' contains the version
read from the stream. At
  // write time version code '3' is written to the stream and to the
variable 'version'.
  int version=s.serialVersion (3);

  // Now switch the version
  switch (version)
  {
  case 3:
   // The last field added in the class
   s.serial (LastField);

   // do some different stuff at read time and write time
   if (s.isReading())
   {
    // at read time
    ...
   }
   else
   {
    // at write time
    ...
   }

  case 2:
   // note that the code provided as of here allows for the reading of old
versions of the class

   s.serial (Toto);

   // in the case where the evolution from my version 1 implementation to my
version 2
   // is not simply an extension of version 1 we need to break execution
here
   break;

  case 1:
   s.serial (Foo);
  case 0:
   s.serial (Truc);
  }

 }


* NeL File Headers

The objective of NeL file headers is to verify that a file is in the right
format before attempting to interpret the contents.

 // The NeL team use the following advise serialise a file this way:
 void CFileRootClass::serial (IStream& s)
 {
  // First write / read-check the header
  s.serialCheck ((uint32)'_LEN');
  s.serialCheck ((uint32)'HSEM');

  // This code write / read-check the header 'NEL_MESH' at the beginning of
the file.
  // If the check fails, serialCheck throws the EInvalidDataStream
exception.
 }

* Good examples to look at:

 include/nel/misc/stream.h   // Stream base classes
 class CTileBank in src/3d/tile_bank.cpp // Good example of file format
evolution
 class CAnimation in src/3d/animation.cpp // Good example of polymorphism


---
Cyril Corvazier
Lead 3d programmer
Nevrax France