[Insight-users] Cycles in SpatialObject possible

[Stephen R. Aylward]

Hi,

Julien will be able to give more info, but basically, a scene cannot be 
stored as a DAG.  SpatialObjects allow only one parent per object since 
each child object has a transform that positions it in its parent 
object's space...  If we allowed multiple parents, it would be 
problematic to define an object-to-world transform that is composed of 
all of the child-to-parent transforms from an object to the root.   We 
could support duplication as is done in some scene representations - 
creating "virtual" copies of an object, but I don't think the complexity 
they add is justified given the rare utility of them.

SpatialObjects specifically use the itkTree datastructure - contributed 
by MITK developers.

Stephen

Miller, James V (Research) wrote:
> Julien, 
> 
> Do the SpatialObjects support a directed acyclic graph?
> 
> Jim
> 
> -----Original Message-----
> From: Julien Jomier [mailto:jjomier at cs.unc.edu]
> Sent: Wednesday, March 16, 2005 5:38 PM
> To: Miller, James V (Research)
> Cc: Dominique Belhachemi; insight-users at itk.org
> Subject: Re: [Insight-users] Cycles in SpatialObject possible
> 
> 
> Yes you cannot have cycles with SpatialObjects. SpatialObject hierarchy 
> is a designed as a tree.
> 
> Julien
> 
> Miller, James V (Research) wrote:
> 
>>Stephen and Julien could probably answer this better, but I assume that 
>>SpatialObjects are "expected" to be either a directed acyclic graph or a 
>>tree.
>> 
>>Jim
>>
>>    -----Original Message-----
>>    *From:* insight-users-bounces at itk.org
>>    [mailto:insight-users-bounces at itk.org]*On Behalf Of *Dominique
>>    Belhachemi
>>    *Sent:* Wednesday, March 16, 2005 4:41 PM
>>    *To:* insight-users at itk.org
>>    *Subject:* [Insight-users] Cycles in SpatialObject possible
>>
>>    Hello,
>>     
>>    can i assume that each instance of itk::SpatialObject is cycle free
>>    in respect of the function itk::SpatialObject::GetChildren(....) ?
>>     
>>    Can i imagine a itk::SpatialObject rather than tree or as graph?
>>     
>>    I need to know this because i want to map an itk::SpatialObject to a
>>    list of SpatialObject-Pointers. For later calculations it is easier
>>    for me to work with a list.
>>     
>>     
>>    Dominique
>>     
>>     
>>     
>>     
>>     
>>     
>>
>>
>>------------------------------------------------------------------------
>>
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> 
> 
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-- 
===========================================================
Dr. Stephen R. Aylward
Associate Professor of Radiology
Adjunct Associate Professor of Computer Science and Surgery
http://caddlab.rad.unc.edu
aylward at unc.edu
(919) 966-9695