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149 lines
3.4 KiB
Plaintext
149 lines
3.4 KiB
Plaintext
GPAC Scene Graph generator documentation - v0.4.0
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Last Modified: July 2005
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0 - Foreword
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The sets (MPEG4, X3D and SVG) of nodes handled by the GPAC are genrated through "Scene Generators" applications:
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applications/generators/MPEG4 for MPEG4 along with the official MPEG-4 template files needed to generate nodes and their encoding tables.
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applications/generators/X3D for X3D along with the X3D template file needed to generate nodes.
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applications/generators/SVG for SVG along with the SVG template.
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MPEG-4 Template files are numbered by versions of amendments to the MPEG-4 systems standard.
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X3D and SVG templates do not have versionning.
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1 - Regenerating the scene graph
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First recompile the appropriated scene generator application. You don't need to recompile GPAC to recompile it.
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The generators DIRECTLY overwrites source code files in the GPAC distribution, you MUST NOT try to run it from a different
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location than gpac/applications/generators/*
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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
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first version of the standard (1998) described in templates1.txt file, just type:
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MPEG4Gen templates1.txt
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Template files MUST be fed in order, and versions cannot be skipped: you SHALL NOT try to generate version1 and version3 without version2.
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To generate a scene graph handling v1 to v3 of the BIFS system, type:
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MPEG4Gen templates1.txt templates2.txt templates3.txt
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For X3D, simply run X3DGen, it will automatically load the "templates_X3D.txt" file
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For SVG, simply run SVGGen completesvgt12rng.xml
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2 - Customizing the scene graph
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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
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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.
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2.1 - Customizing the MPEG-4 scene graph
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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.
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However MPEG4Gen allows you to specify which nodes should be supported or not in the scene graph. This is currently specified
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with a simple text file where unwanted nodes are listed one by line ('#' acting as a line comment). The file is specified by
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the "-p " switch
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For example, generating a scene graph for BIFS V1 without support for the BIFS audio nodes will be:
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MPEG4Gen -p skip_audio.txt templates1.txt
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and the content of skip_audio.txt file will be
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AudioBuffer
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AudioClip
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AudioDelay
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AudioFX
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AudioMix
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AudioSwitch
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AudioSource
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2.2 - Customizing the X3D scene graph
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X3DGen uses the same mechanism as MPEG4Gen for node skinpping, eg: X3DGen skipfile
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2.3 - Customizing the SVG scene graph
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This is undocumented and probably not supported
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3 - Advanced Manupulations
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It is possible to develop custom templates. The resulting encoding/decoding will not be compliant with MPEG-4 BIFS but it can
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be interesting to see how a single-version mechanism with only the desired nodes reduces applications and bitstreams sizes
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To write your own templates, you must:
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follow the syntax of regular templates
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make sure the SFWorldNode type is defined and used by all nodes (needed for BIFS updates)
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make sure at least one node will be of type SFTopNode (needed for BIFS Scene Replace command)
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