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2.1 Design and Implementation of an MHEG-5 Engine
This section describes the design and implementation work which is being carried out by the authors. The
process or set of processes that interpret MHEG-5 objects and deliver them to an application for
presentation is referred to in the standard as an MHEG-5 engine. A link engine processes such events and
associated data, and fires the links that have their link condition satisfied. The model adopted by the
authors is shown in Figure 1.
The Object Processor (OP) is the module responsible for the effective interpretation of MHEG-5 objects
and execution of actions. When Link objects have to be activated, the OP registers them in the Link
Engine (LE), where a table mapping Link-Conditions into Link-Effect is kept. The Object Store (OS)
supplies prepared objects to the OP, and the Object Decoder (OD) deals with decoding and preparing
MHEG-5 objects.
When the MHEG-5 engine 1 is first activated, it runs a cold start process to retrieve the first Application
object to be processed. From this first object users can choice their desired Application object.
Once the OP gets the identification of the object to be processed, it requests the prepared object from the
OS module. The OS keeps a cache with Scene objects prepared in advance, with the aim of minimize
access time, and objects that had been active before and have high probability to become active again in a
short time. If the OS does not have the required object, it requests the OD to decode the object and then
prepares it.
Based on the type of the object, the OP knows what attributes to look for in the data structure resulting
from the object preparation. If it is processing an Application or Scene object then it starts the execution
of the Action object specified in their On-Start-Up attribute. All elementary actions are queued and
processed accordingly. Link objects found by OP in Scene or Application objects are activated and
registered at the Link Table.
Some events may occur as a consequence of the OP processing activities (synchronous events) or
generated by processes that are asynchronous to the MHEG-5 engine, e.g. user interactions, firing of the
timer, etc. In the later case, the occurrence of events are passed on to the OP through the primitive
External Input(). These external events are translated to proper events recognized by the standard and,
same as happen with the synchronous events, they are reported to the LE module.
Present()
Presentation and User Interface
Action Queue
Event Table
Object Processor
Is Avaiable
Object
Store
Cache
External Input()
ReportEvent()
Register
Link()
: :
Request Object()
Prepare Object()
MHEG Object
Object Decoder
Prepared Object
Process
Action()
Link Table
Link Engine : :
Load Object()
Figure 1: MHEG-5 engine model
[1] An MHEG-5 engine can be seen as an application that takes MHEG-5 objects as inputs and provides
their presentations
Network
ASN.1