[x3d-public] Picking Objects [was: review of various X3D statement implementations for X3DOM]

[Leonard Daly]

On 12/20/2017 11:16 AM, Andreas Plesch wrote:
> Hi Michalis,
>
> yes, that is how DEF/USE should be implemented. The point I am trying
> to understand is if there is absolutely no distinction between the
> original DEF node and a USE node which references the DEF node, how
> can the USE node be identified, for example during picking ? The USE
> node still needs to have at least a distinct ID, right ?

Andreas,

There is no distinction as it is not a separate node. For example:

<Transform position='5 0 0'>
   <Shape DEF='PickShape'>
     <Box DEF='PickGeometry'/>
     <Appearance>
       <Material diffuseColor='1 0 0'/>
     </Appearance>
   </Shape>
</Transform>

<Transform position='0 5 0'>
   <Shape USE='PickShape'/>
</Transform>

<Transform position='0 0 5'>
   <Shape DEF='PickShape2'>
     <Box USE='PickGeometry'/>
     <Appearance>
       <Material diffuseColor='0 1 0'/>
     </Appearance>
   </Shape>
</Transform>


Will display a scene with 3 boxes, one at 5 units from the origin on 
each coordinate axis. The X & Y boxes are colored red, the Z box is 
colored green. Picking (from the X3D scene) will generate an event on 
either PickShape or PickGeometry node, no matter which one is picked. If 
internally OpenGL identifies the boxes as different, it is reduced to 
the single instance when the picked objected is returned.

[I know the code above won't exactly work for picking (assuming 
TouchSensor or equivalent), but it should be clear enough to get the 
point across.]


Leonard Daly


>
> A conceptual scene graph structure may look like this:
>
> group1
>    node 1: DEF "original"
> group2
>    node 2: USE "original"
>
> There are still two nodes. node 2 is not a copy of node 1 but is still
> a separate node which happens to reference node 1.
>
> Or
>
> group1
>   node 1: DEF "orginal"
> group2
>   node 1 (inserted a second time during scene graph construction)
>
> Closer to spec. but makes it hard (impossible?) to distinguish the two
> node 1 instances from each other.
>
> So I was thinking something like an additional ID may have to exist:
>
> 1. group1
> 2.  node 1: DEF "orginal"
> 3. group2
> 4.  node 1 (inserted during scene graph construction)
>
> ?
>
> -Andreas
>
>
>
>
> On Wed, Dec 20, 2017 at 11:03 AM, Michalis Kamburelis
> <michalis.kambi at gmail.com> wrote:
>> 2017-12-20 15:51 GMT+01:00 Andreas Plesch <andreasplesch at gmail.com>:
>>> (DEF/USE in x3dom: There are a several x3dom issues about picking a
>>> USE Shape. Since x3dom literally places the referenced DEF Shape in
>>> the scene graph, the picking actually reports the node of the
>>> referenced DEF Shape when the USE Shape is picked. This is because
>>> x3dom actually does not keep a distinct representation of the USE node
>>> at all. Except for the fact that the DEF node now has two parents.
>>> This may be too literal an interpretation of the DEF/USE concept, and
>>> I wonder if in the spec. there is a section which explains how the
>>> node which has a USE reference is still a distinct node. I suspect
>>> that keeping such a distinct node around is how most x3d browsers work
>>> ?
>> This DEF / USE (in x3dom) treatment is required by the X3D
>> specification, and it (should) be the same in all VRML / X3D browsers.
>> I know it is the same in view3dscene and Castle Game Engine. When we
>> encounter a "USE Xxx" then internally we *do not* make a copy of the
>> "Xxx" node, instead we make an additional link to the "Xxx" node that
>> in effect has 2 parents. This implementation is most natural and
>> practically required by the X3D specification:
>>
>> http://www.web3d.org/documents/specifications/19775-1/V3.2/Part01/concepts.html#DEFL_USESemantics
>>
>> """
>> The USE statement does not create a copy of the node. Instead, the
>> same node is inserted into the scene graph a second time, resulting in
>> the node having multiple parents (see 4.3.5 Transformation hierarchy,
>> for restrictions on self-referential nodes).
>> """
>>
>> And that's a very good concept, in my opinion: it allows to use DEF /
>> USE mechanism for various optimizations. Since you have only one copy
>> of the node, you should also be able to have only one copy of various
>> internal things (like OpenGL VBO data) associated e.g. with Shape that
>> is USEd multiple times. This is really powerful, you can construct a
>> large scene by USEing the same shapes multiple times, and it will
>> still render fast and not eat too much memory:)
>>
>> In contrast, expanding the PROTOtypes makes a copy on the contents.
>>
>> Regards,
>> Michalis
>
>

-- 
*Leonard Daly*
3D Systems & Cloud Consultant
LA ACM SIGGRAPH Past Chair
President, Daly Realism - /Creating the Future/
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