FORMAL DESIGN OF SMIL DOCUMENTS

Abdelkader Belkhir and Samia Bouyakoub-Smail

Department of Computer Science, Faculty of Electronics and Computer Science, USTHB University, BP 32 El-Alia ,

Algiers, ALGERIA

Keywords: Multimedia presentations, modelling and verification, formal techniques, Petri nets, SMIL.

Abstract: This article introduces a new formal technique based on extended timed Petri nets, called SMIL-Net for the

modelling and the verification of SMIL presentations. We also propose a technique for the temporal and

hyper-temporal consistency checking of the document based on the SMIL-Net model.

1 INTRODUCTION

Synchronized Multimedia Integration Language

(SMIL) (Hoschka,1998) (Ayars,2001) was

developed by the World Wide Web Consortium

(W3C) to address the lack of HTML for multimedia

over the Web. It provides an easy way to compose

multimedia presentations. This paper thus focuses on

the SMIL-based presentations.

The complexity of SMIL documents can lead

authors, in certain cases, to specify synchronization

relations which could not be satisfied during the

presentation of the document, thus characterizing the

occurrence of inconsistencies, making it necessary to

use formal techniques for their specification and for

verifying their logical and temporal properties

(Sampaio,2003) (Huiqun,2002) (Yang,2000). This

article introduces a new formal technique based on

extended timed Petri nets, called SMIL-Net (which

means SMIL - Petri Net) for the modelling and the

verification of SMIL presentations (Smail,2004).

The major innovation of the SMIL-Net model is the

modelling of the hyper-temporal dimension of SMIL

documents, and the verification of temporal and

hyper-temporal consistency within the same model.

The paper is organized as follows: First of all, we

present the SMIL-Net model in section 2. The

conversion techniques from SMIL to SMIL-Net are

presented in section 3. The methods for detecting

temporal and hyper-temporal inconsistencies are

explained in section 4. Finally, section 5 concludes

this paper.

2 SMIL-NET

The elements in SMIL-Net include places, tokens,

transitions and arcs.

we define three kinds of places to model the

different time-dependant elements of SMIL: The

regular place represents a media element and its

associated duration.

The link place represents a

linking element (a and area elements of SMIL)

and its activation interval. Finally, the virtual place

represents a synchronization attribute (begin, end,

dur).

The model defines two types of tokens: State tokens

that define the state of the place, and Control tokens

that act on the state of the place. These tokens are

transported by different types of arcs: The simple arc

is used to transport state tokens. The virtual arc is

used to transport control tokens. The dynamic arc is

used to model hyperlink firing. The master arc

controls the firing of master transitions.

To model the different semantics of SMIL, three

types of transitions are defined (subset of the

transition semantics defined in TSPN model

(Senac,1995) (Senac,1996-a) (Senac,1996-b)): The

simple transition fires if all its input places are active

and have unlocked tokens. The Master transition

fires if the place having a master arc is active and

has an unlocked token. The First transition fires if

one of its input places is active and has an unlocked

token.

The next section will illustrate the use of these

different elements in the modelling of SMIL

elements.

396

Belkhir A. and Bouyakoub-Smail S. (2007).

FORMAL DESIGN OF SMIL DOCUMENTS.

In Proceedings of the Third International Conference on Web Information Systems and Technologies - Web Interfaces and Applications, pages 396-399

DOI: 10.5220/0001271403960399

 SciTePress

3 CONVERTING SMIL

DOCUMENT TO SMIL-NET

In this section, the synchronization and hypermedia

elements of SMIL are examined, and the

mechanisms that convert these elements to SMIL-

Net are presented.

We assume that the syntax of the input SMIL

script has been checked before starting the

transformation.

3.1 Converting a Media Object

Figure 1: Converting a media object.

A media object is represented by a regular place and

a pair of ordinary transitions (Ts, Te).

However, the attributes associated with the

element require some more conversion. Since the

“begin” attribute specifies the time for the explicit

begin of an element, one virtual place representing

the begin time with the specified duration is added in

front of the element. The “end” attribute specifies

the explicit end of an element, so one virtual place

with the end value is added between the original

start transition and the end transition. A master

transition is added at the end to force the object to

end with the value specified in the end attribute. The

“dur” attribute specifies the explicit duration of an

element, thus a virtual place between the actual start

transition and the end transition is added. The end

transition is also a master transition .

3.2 Converting the <seq> Element

The <seq> element defines a sequence of elements

in which elements play one after the other. Since the

children of a <seq> element form a temporal

sequence, we concatenate each child of <seq> one

by one in SMIL-Net,

Figure 2: Converting a <seq> element.

3.3 Converting the <par> Element

The <par> element defines a simple parallel time

grouping in which multiple elements can play back

at the same time. Thus, all children of <par> should

be within the same pair of transition (Ts, Te).

There are three variations for <par> according to

the value of the "endsync" attribute. The “endsync”

attribute controls the end of the <par> element, as a

function of children end time. Legal values for the

attribute are “last”, “first”, and “id-ref’.

Figure 3: Converting the <par> element.

The value of “last” requires <par> to end with

the last end of all the child elements, so Te is

modelled by a simple transition. The value of “id-

ref’ requires <par> to end with the specified child.

So we change the end transition Te to a Master

transition, and the arc between the specified child

and transition to a master arc. The value of “first”

requires <par> to end with the earliest end of all the

child elements. Therefore, we should change the end

transition Te to a First transition so that the child

that ends first will fire the transition. Other

synchronization attributes, such as “begin”, “end”,

and “dur”, could also be associated with <seq> and

<par> elements, but the conversion is similar to that

in the media object elements.

3.4 Converting the <a> Element

The functionality of the <a> element is very similar

to the functionality of the <a> element in HTML, it

defines a link which can be activated by one of its

child elements without influencing their

synchronization.

The link <a> is active during all the interval of

activation of the source element, thus its interval of

activation is [0,*,source-duration].

The <a> element is modelled by a link place and

a dynamic arc to fire the link. When the active

duration of the source element begins, the link place

receives a pause token and waits for user interaction.

If an interaction happens during the activation

interval of the link (which is, in this case the same as

the active interval of the source element), the

Elt2

Elt 1

elt 1

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elt 1

elt 2

FORMAL DESIGN OF SMIL DOCUMENTS

397

dynamic arc is transformed to a simple arc and the

following transition (the link) fires.

There are three variations for <a> element

according to the "show" attribute. The “show”

attribute specifies how to handle the current state of

the presentation at the time in which the link is

activated. Legal values for the attribute are “new”,

“pause”, and “replace”. The “pause” value

requires the source to be paused while playing the

destination element, so we use virtual arcs with

pause/resume tokens to pause and resume the source

element. The “replace” value require the destination

element to replace the source element and to play in

the same context, so the firing of the transition

retrieves a token to the source place in order to

disable it. The “new” value requires the destination

element to play in a new context without affecting

the source element.

3.5 Converting the <area> Element

The functionality of the <a> element is restricted in

that it only allows associating a link with a complete

media object. The <area> element allows associating

a link with spatial and/or temporal portions of an

object.

When the <area> element is associated with a

spatial portion of an object, it has the same temporal

behavior as the <a> element since it stills active

during all the duration of the source object.

When the <area> element is associated with a

temporal portion of an object using attributes such as

begin and end attributes, the interval of activation

associated to the link place is restricted to

[begin,*,end].

The <area> element also accepts the ‘show’

attribute, the SMIL-Net associated with the <area>

element with the different values of the ‘show’

attribute is the same as in the <a> element with a

different activation interval for the link place.

4 TEMPORAL AND

HYPER-TEMPORAL

VERIFICATION

The complexity of SMIL documents can lead

authors, in certain cases, to specify synchronization

relations which could not be satisfied during the

presentation of the document, thus characterizing the

occurrence of temporal inconsistencies. For this

reason, we associate verification techniques to

SMIL-Net to detect temporal inconsistencies.

The hyper-temporal inconsistencies resulting of

the definition of inconsistent temporal links are also

an important aspect of the verification process, and

are detected by SMIL-Net.

4.1 Temporal Verification

The time conflict is defined in (Yang,2000) as the

case of conflicting values of attributes in the SMIL

script.

There are two types of time conflicts for SMIL

presentations, the intra-element time conflict and the

inter-elements time conflict.

4.1.1 The Intra-element Time Conflict

The intra-element time conflict is the case of

conflicting attributes associated with a single

element. Therefore, to detect the intra-element time

conflict, we only need to examine the values of

attributes associated with a single element.

The intra-element time conflict is detected in

SMIL-Net when a master transition has two master

arcs coming from its input places.

4.1.2 The Inter-elements Time Conflict

The inter-elements time conflict is the case of

conflicting attributes among different elements.

In SMIL-Net, the firing time of transition should

be earlier than that of the following transition. It

implies that if the values of attributes set by the

author make the firing time of a transition later than

the firing time of some following transition, it results

in the inter-elements time conflict.

The inter-element time conflict could be detected

by comparing the computed firing times of

transitions.

Thus, it is necessary to calculate the firing time

for all the transitions in the SMIL-Net.

To compute the firing time for each transition,

we have to reduce the SMIL-Net by removing all

non-temporal elements.

Then, the firing time for each transition is

computed by traversing the reduced SMIL-Net

transition by transition from the initial state. The

computation rule for calculating the firing time of a

transition depends on its type: For simple transitions,

the firing time of the transition is the maximum

value of the “firing time of the preceding transition”

plus “the nominal duration of the following place of

the preceding transition”. For First transitions, the

firing time of the transition is the minimum value of

the “firing time of the preceding transition” plus “the

nominal duration of the following place of the

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preceding transition”. For Master transitions, the

firing time of the transition is the value of the “firing

time of the preceding transition” plus “the nominal

duration of the place associated with a master arc”.

When the firing time of each transition is computed,

we traverse the SMIL-Net again from the initial

place and locate the pair of transitions at which the

conflicting firing times occur. The corresponding

pair of elements with the conflicting transitions is

therefore the source of the inter-elements time

conflict.

4.2 The Hyper-Temporal Verification

The hyper-temporal inconsistency is concerned with

the definition of temporal links pointing on a non-

active object. This case of inconsistency is detected

when the activation interval of the link place is not

included in the duration interval of the source place.

In the case of local links, we should ensure that

the link firing may not lead to temporal

inconsistency. Thus, we assume that the link fires,

we replace the dynamic arc by a simple arc, and we

apply the temporal verification techniques seen

above on the obtained SMIL-Net.

5 CONCLUSION

This paper introduces a new forma model, based on

the temporal extensions of Petri nets for modelling

and verifying temporal and hyper-temporal

consistency of SMIL documents, called SMIL-Net.

The main contributions of this work are:

 The definition of a formal model covering both

of the temporal and the hyper-temporal

aspects of SMIL documents.

 The definition of an approach to automatically

translate SMIL documents into SMIL-Net

specification.

 The temporal and hyper-temporal consistency

checking based on the same model.

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