Re: discussion about temporal scenario modelisation

Warner ten Kate wrote:
> 
> Thanks for your interesting paper. Sorry for taking so much
> time before sending this reaction/question.
> 
> In your paper you distinguish between a hierarchical and
> a constraint-based approach to specify temporal structure.
> 
> If I understand you, you categorize SMIL and CMIF as members
> of the first type, and Madeus (among others) as representative
> of the other type. I have to confess that I do not know much
> about CMIF and Madeus.
> 
> What I get from the figures in your document, in Madeus you
> specify constraints between media-objects; sets of media-objects
> with their constraints can be taken together into logical groups.
> 
> Is this correct?
> Is there any additional semantics to these logical groups
> eg. are there (structured) dependicies between these groups?
> 
> As an example of a constraint-based specification you give
> 
>   A meets B
>   B meets C
>   I starts A
> 
> Your suggestion is that this approach is more author friendly.
> 
> I would say that this type of authoring is also supported by SMIL:
> 
>   <B begin="id(A)(end)"/>
>   <C begin="id(B)(end)"/>
>   <A begin="id(I)(begin)"/>
> 

we do not understand how this group can be incorporated in a SMIL
specification ?
You are obliged to encapsulate these three objects by using a seq or par
node, aren't you ?
 
If so, how the synchronization by qualified events is merged with the
seq or par semantics ?

The aim of the two following remarks/questions is to show that the
temporal "events synchronization" proposed in SMIL is not equivalent to
temporal constraints as used in Madeus (or ISIS, or CHIMP, TIEMPO, ...)

1) Suppose i want to add to your example a new objects E in sequence
before A.
The temporal constraints to addd to the previous set 
  A meets B 
  B meets C  
  I starts A

is : E meets A 

What about now with the events synchronization :
 The previous set was :

 <B begin="id(A)(end)"/>
 <C begin="id(B)(end)"/>
 <A begin="id(I)(begin)"/>

I have to add a new assertion <A begin="id(E)(end)> which is in conflict
with <A begin="id(I)(begin)"/>.

Conflicts in smil are often solved by taking something like a "first
arrived" policy (minimum between two possible durations).
If this is the case here, it means that one of the two synchronisations
will be lost. 

2)
When I express in Madeus that two objects are linked by an equal
constraint
A equal B. 
One can think that it is equivalent to 
par
  <A begin ="id(B) (begin)", end = "id(B) (end)>
  <B>
/par

Suppose now that A and B are two moving objects which have to reach some
different fixed points on the screen. In addition the author can adjust
their speed as he wants.

The semantic expressed by A equal B is: A and B starts and finishes at
the same time, and none of them has been interrupted (i.e they reached
their final position). 
With a constraint-based approach the temporal formatter chooses a
well-suited speed for each objects. 

In SMIL, (from our understanding) the author is obliged to fix the
duration (and so the speed) of each objects, then the three following
cases are possible :
1) he give the same duration of the two objects => everything is ok.
2) A is shorter than B =>  the end point syncronization is not satisfied
3) A is longer than B => A is interrupted when B ends, it means that A
do not reached the position wished by the author. 

 On this example, it is very simple to give the same duration of the two
objects, but if I take a more complex one (Two sequences (A,B,C) and
(E,D) synchronized at their beginning and end point). 

A meets B
B mets C
D meets E 
A starts D
E finishes C

this task is not so trivial for the author. Moreover, if he inserts a
new object in one
sequence, he has to compute again these values.
Here, we point out one major advantage of using constraints.

Sincerely yours,
Cecile Roisin, Muriel jourdan
 
ps : you can find at the following address a survey on authoring
techniques for temporal scenarios of multimedia documents to be
published in "The handbook of multimedia" at :
http://opera.inrialpes.fr/OPERA/BibOpera.html

Received on Tuesday, 10 February 1998 05:35:32 UTC