MPEG-4 Systems Binary Format For Scene (BIFS) 1. What is BIFS? 1. What is BIFS?BIFS is an abbreviation for "BInary Format for Scenes". BIFS provides a complete framework for the presentation engine of MPEG-4 terminals. BIFS enables to mix various MPEG-4 media together with 2D and 3D graphics, handle interactivity, and deal with the local or remote changes of the scene over time. BIFS has been designed as an extension of the VRML 2.0 specification in a binary form. BIFS is actually composed of 4 elements:
2. Why BIFS ?A central concept in the MPEG-4 design is transmission and
interaction with audio-visual objects, of synthetic or natural nature. The
Audio, Visual part of the standard provide the encoding algorithms for
individual audio-visual objects. In order to combine these media together into
complete presentations, a scene description capability is needed.
3. What is the MPEG-4 model of an audio-visual object?In the MPEG-4 model, audio-visual objects have both a spatial and a temporal extent. Temporally, all AV objects have a single dimension. Each AV object has a local coordinate system in which the object has a fixed spatio-temporal location and scale. AV objects are positioned in a scene by specifying one or more coordinate transformations from the object's local coordinate system into a common, global coordinate system, or scene coordinate system. An audio-visual object in a BIFS scene is usually represented by one BIFS node or a sub-tree of the BIFS scene graph. 4. Why is scene description information separate from audio-visual objects?Scene description information is a property of the scene's structure rather than of particular AV objects. Consequently, it is transmitted as a separate stream. This is an important feature for bitstream editing and one of the essential content based functionalities in MPEG-4. For bitstream editing, one can change the composition of AV objects without having to decode their bitstreams and change their content. If the position of the object were part of the object's bitstream, this would become very difficult. 5. Can the scene description be changed?The scene description can be dynamically changed at any time. An initial scene description is provided at the beginning of an MPEG-4 stream. It can be as simple as a single node, or as complex as one wants (within limits that are established for ensuring conformance). BIFS-Commands are used to modify a set of properties of the scene at a given time. It is possible to insert, delete and replace nodes, fields and ROUTEs as well as to replace the entire scene. For continuous changes of the parameters of the scene, BIFS-Anim can be used; it specifically addresses the continuous update of the fields of a particular node. BIFS-Anim is used to integrate different kinds of animation, including the ability to animate face models as well as meshes, 2D and 3D positions, rotations, scale factors, and color attributes. The BIFS-Anim information is conveyed in its own elementary stream. 6. What is the difference between BIFS and VRML?BIFS has been designed as an extension to the VRML 2.0 specification. In Version 2 of MPEG-4 Systems, all VRML nodes are supported. BIFS extended the base VRML specification in various aspects:
7. The Scene Description looks similar to VRML. Is it?The scene description has several similarities to VRML, as the set of nodes defined by VRML was used as an initial set of composition nodes for MPEG-4. The environment that MPEG-4 addresses, however, is quite different from VRML because a key requirement is support for high quality real-time audio-visual content. In addition, rather than using a static scene description, MPEG-4 defines a dynamic one in which objects can be added, changed, or removed from the scene description at any point in time. The MPEG group collaborates closely with VRML in order to ensure alignment and maximize the synergy of the work of both international bodies. 8. How is interactivity handled in MPEG-4?Interactivity in MPEG-4 Systems is separated into two major categories: client side and server side. The former is available locally at an MPEG-4 terminal while the latter requires communication between the terminal and the sender. Client side interactivity can be further divided in simple object manipulation (repositioning, hiding, changing attributes, etc.) that does not require normative support from the standard, and more general types of events (hyper linking, triggers, etc.) that do require normative support. Note that server side interactivity also requires normative support. Client-side interactivity is handled via VRML's ROUTE mechanism, that links event source fields to event sink fields in the BIFS node tree. Server-based interactivity is provided via a Version 2 BIFS node, called ServerCommand. Additional interactivity can be provided by an application, by translating application events into local scene description updates. Sophisticated interactive applications can be created using the programmatic features of MPEG-4 Systems (ECMAScript as well as Java). 9. So, the scene description is streamed?Yes. There are elementary streams (just as any visual or audio stream) with BIFS commands and elementary streams conveying BIFS animation data. This allows to attach time stamps to such information, same as time stamps are attached, e.g., to an audio frame. 10. Can there be multiple scene description streams in an MPEG-4 presentation?Yes. For example, the BIFS scene may be composed from multiple sub-scenes that are Inlined to a main scene. In that case each sub-scene would have its own scene description stream. |
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