share/doc/SceneGenerators

		GPAC Scene Graph generator documentation - v0.4.0

			Last Modified: July 2005



0 - Foreword



The sets (MPEG4, X3D and SVG) of nodes handled by the GPAC are genrated through "Scene Generators" applications:

applications/generators/MPEG4 for MPEG4 along with the official MPEG-4 template files needed to generate nodes and their encoding tables.

applications/generators/X3D for X3D along with the X3D template file needed to generate nodes.

applications/generators/SVG for SVG along with the SVG template.



MPEG-4 Template files are numbered by versions of amendments to the MPEG-4 systems standard. 

X3D and SVG templates do not have versionning.



1 - Regenerating the scene graph



First recompile the appropriated scene generator application. You don't need to recompile GPAC to recompile it.

The generators DIRECTLY overwrites source code files in the GPAC distribution, you MUST NOT try to run it from a different 

location than gpac/applications/generators/*



For MPEG-4 you must provide MPEG4Gen with the set of template files. For example, if you're planning to use only nodes defined in the 

first version of the standard (1998) described in templates1.txt file, just type:



MPEG4Gen templates1.txt



Template files MUST be fed in order, and versions cannot be skipped: you SHALL NOT try to generate version1 and version3 without version2. 

To generate a scene graph handling v1 to v3 of the BIFS system, type:

MPEG4Gen templates1.txt templates2.txt templates3.txt



For X3D, simply run X3DGen, it will automatically load the "templates_X3D.txt" file

For SVG, simply run SVGGen completesvgt12rng.xml





2 - Customizing the scene graph



As of 0.2.2, all nodes in current gpac version are supported by renderers and cannot be removed. You will therefore have to REMOVE some code in the renderers

and the scene graph in order to recompile GPAC. You should therefore not try to customize the scene graph unless you know what you're doing.



	2.1 - Customizing the MPEG-4 scene graph



As said above, it is not possible to skip a BIFS version when regenerating the scene graph since this will break binary encoding of the nodes.

However MPEG4Gen allows you to specify which nodes should be supported or not in the scene graph. This is currently specified

with a simple text file where unwanted nodes are listed one by line ('#' acting as a line comment). The file is specified by

the "-p " switch



For example, generating a scene graph for BIFS V1 without support for the BIFS audio nodes will be:

MPEG4Gen -p skip_audio.txt templates1.txt



and the content of skip_audio.txt file will be



AudioBuffer

AudioClip

AudioDelay

AudioFX

AudioMix

AudioSwitch

AudioSource



	2.2 - Customizing the X3D scene graph

	

X3DGen uses the same mechanism as MPEG4Gen for node skinpping, eg: X3DGen skipfile



	2.3 - Customizing the SVG scene graph



This is undocumented and probably not supported



3 - Advanced Manupulations



	It is possible to develop custom templates. The resulting encoding/decoding will not be compliant with MPEG-4 BIFS but it can

be interesting to see how a single-version mechanism with only the desired nodes reduces applications and bitstreams sizes

To write your own templates, you must:

	follow the syntax of regular templates

	make sure the SFWorldNode type is defined and used by all nodes (needed for BIFS updates)

	make sure at least one node will be of type SFTopNode (needed for BIFS Scene Replace command)