Re: Do SoViewingMatrixElement and SoViewVolumeElement break bounding box cache?

[Chester Liu <cht@xxxxxxxxxxx>]

I see!
That explains it.  Is there a way to look at an
element without creating a cache dependency?
Like a <peek>.

I performed the following experiment.  SoCache has
a method called isValid().  Basically it checks whether
the elements in the cache are consistent with the state.
I put in a bit of code which prints the dependent elements.

Interestingly, if I don't include the SoViewVolumeElement::get(...),
then the cache reports that there are 0 cache dependencies.
However, if I put in the viewvolume statement, then
it reports the following dependencies:

the element is: SoOverrideElement
the element is: SoShapeStyleElement
the element is: SoViewVolumeElement

CACHE DEBUG: cache(0x109a2b20) not valid because element
SoViewVolumeElement does not match:
------
Element in state:
------
Element in cache:



So, to answer Gavin's question,
without the offending code, I can move the camera around
and the bounding box cache doesn't break.  (It shouldn't break,
because the stuff beneath the separator doesn't change, only
the camera above it.) 

My simplified scenegraph is like this:

camera
  |
separator (render culling turned ON)
  |
 shape

The shape needs to know what the viewvolume is, and only
change itself (rarely) if certain criteria are met.  Most
of the time it's static and this should NOT break the separator's
cache.  For example, a level-of-detail node needs to know
the viewvolume, but it only switches its children infrequently.

Thanks for more insights!

Chester











Thanks,
Chester


Paul Strauss wrote:
> 
> Gavin Andresen wrote:
> 
> >
> > >      const SbViewVolume & viewVol =
> > >              SoViewVolumeElement::get(action->getState());
> > >
> > >      const SbMatrix & viewMat =
> > >SoViewingMatrixElement::get(action->getState());
> > >
> > >
> > >The separator reports that the bounding box cache (used for
> > >render culling) is being invalidated.  If I comment out these
> > >lines of code, it's fine.
> > >
> > >Why?
> >
> > The ViewingMatrix and ViewVolume change whenever the camera moves, so if
> > your GetBoundingBox method relies on them, then the bbox cache SHOULD be
> > invalidated.
> 
> One of the keys to answering Chester's question is that calling the
> "get()" methods on the elements results in a cache dependency being
> created on those elements. So, yes, just looking at the values of the
> elements (during cache construction) will change the behavior of the
> cache.
> 
> ----
> Paul S. Strauss           pss@xxxxxxxxx        Pixar

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::                                                         ::
::  Chester Liu                          3D Graphics Guru  ::
::  cht@xxxxxxxxxxx        Weather Services International  ::  
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