Fwd: Re: User Transform....

[David Gobbi]

Hi Lisa,

This sounds like an option - make vtkMatrix4x4 a data object.
Unfortunately vtkRenderWindowInteractor and probably other classes
abuse the UserMatrix i.e. modify it directly and completely
disregard the fact that it might be part of vtkTransform much
less part of a pipeline.

There are a couple reasons why I started from scratch in developing
the transform pipeline instead of using existing VTK mechanisms:
1) Efficiency, both speed & memory usage 
2) Simplicity - e.g. the vtkGeneralTransformConcatenation acts as both
   filter & data in one.  This cuts down the number of lines of code
   needed to both implement the class & to use it.
3) Flexibility - only a subset of the vtk*Transforms use a vtkMatrix4x4,
   e.g. the vtkThinPlateSplineTransform obviously could not use a
   vtkMatrix4x4.

I actually have a class (not contributed yet) which will allow you
to convert a vtkMatrix4x4 object into a vtkLinearTransform object.
If the vtkMatrix4x4 was a data object, then it would be possible to
go the other way as well.


So, in conclusion:  I think that converting the transform pipeline 
into a data+filter pipeline would both add complication to and 
reduce the flexibility the transform pipeline.  Being able to do

     trans = vtkTransform()
     invtrans = trans.GetInverse()

is a much tidier way of getting a pipelined transform inverse than

     trans = vtkTransform()

     inverse = vtkTransformInverse()
     inverse.SetInput(trans.GetOutput())

     invmatrix = inverse.GetOutput()

However, having vtkMatrix4x4 as a data object could be a good thing.

 - David

P.S.  Is this list new?  A vtk-developers list is really a good idea.

--
  David Gobbi, MSc                    dgobbi at irus.rri.on.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario

On Thu, 23 Mar 2000, Lisa Sobierajski Avila wrote:

> Hello David,
> 
> I see what you are trying to do, and I was faced with the same problem at 
> one point with vtkPiecewiseFunction - I needed it to be an output of 
> something and work in a pipeline. The solution I took in that case was to 
> make vtkPiecewiseFunction a subclass of vtkDataObject. That was an ok 
> solution there, but wouldn't work (with vtk "as is") for you since 
> vtkDataObject is pretty heavy weight.
> 
> Here is a suggestion I have (and I throw this out to everyone on vtkdevelop 
> for comment). I would like to make vtkDataObject as light weight as 
> possible - remove most of the ivars. Then create a new subclass with those 
> ivars back in it, and vtkDataSet would be a subclass of this new class. 
> vtkMatrix4x4, vtkPiecewiseFunction, and whatever else needs to be would be 
> subclasses of vtkDataObject.  This way you could do 
> actor->SetUserMatrix(yourfilter->GetOutput()) where the GetOutput() method 
> of your filter would return a vtkMatrix *. vtkMatrix would be modified to 
> call Update() on its source when necessary (or do nothing if its source is 
> NULL) With these changes you could remove the SetUserTransform from vtkProp3D.
> 
> I'll probably go ahead and try this out in the next few days to see how it 
> works out. It will take a bit of thought to determine how much 
> functionality to provide - the basic data object won't support streaming so 
> it won't need to store any extent / pieces information and it won't support 
> data releasing, but can a vtkMatrix4x4 move across a input/output port? Not 
> that the ports are back in yet - but they will be soon....
> 
> 
> Lisa
> 
> 
> 
> 
> At 03:17 PM 3/22/00, you wrote:
> >The answer to the UserTransform ivar in vtkProp3D. I'm studying this to 
> >see what impact it has on the picking.
> >Will
> >
> > >Date: Wed, 22 Mar 2000 14:55:50 -0500 (EST)
> > >From: David Gobbi <dgobbi at irus.rri.on.ca>
> > >X-Sender: dgobbi at localhost.localdomain
> > >To: Will Schroeder <will.schroeder at kitware.com>
> > >Subject: Re: User Transform....
> > >
> > >Hi Will,
> > >
> > >The UserTransform serves the same purpose as the UserMatrix.
> > >That much is obvious, so here is why just doing
> > >actor->SetUserMatrix(transform->GetMatrixPointer())
> > >is different from doing
> > >actor->SetUserTransform(transform):
> > >
> > >The new set of transforms that I've implemented are pipelined.
> > >When you call  transform->TransformPoint(inPoint,outPoint)
> > >or  transform->TransformPoints(inPoints,outPoints)
> > >the class first calls its 'Update()' method before proceeding.
> > >
> > >The vtkTransform::Update() method is empty, but e.g.
> > >vtkLandmarkTransform::Update() uses this->SourceLandmarks
> > >and this->TargetLandmarks to make a 4x4 registration matrix.
> > >The vtkLinearTransformConcatenation::Update() method looks at
> > >its list of transforms and concatenates all their matrices.
> > >
> > >
> > >So, the reason that actor->SetUserMatrix(transform->GetMatrixPointer())
> > >is insufficient is that it 'breaks the pipeline', i.e. the matrix
> > >will only be updated if the user explicitly calls Update() on the
> > >transform.
> > >
> > >
> > >For example, in my lab we have a vtkFlockTracker class that interfaces
> > >to the Flock of birds.  We want to get a transform from the flock,
> > >apply various calibrations, and then pass the result on to an actor.
> > >
> > >                             flockTracker
> > >                                  |
> > >                                  v
> > >         calibrationTrans1    flockTrans    calibrationTrans2
> > >                    |             |              |
> > >                    |             v              |
> > >                    \-->  concatenationTrans  <--/
> > >                                  |
> > >                                  v
> > >                           flockActor->SetUserTransform()
> > >
> > >I'm not sure how clear the diagram is, but the idea is that when
> > >flockActor needs do 'do its stuff,' it calls calibrationTrans->GetMatrix()
> > >which (internally) calls calibrationTrans->Update() which causes
> > >calibrationTrans1, flockTrans, and calibrationTrans2 to be concatenated.
> > >
> > >
> > >So, to make a long story short, back a couple years ago when I started
> > >using VTK the data pipeline made a lot of sense... but I really wished
> > >that I could pipeline operations on transforms as well.  It wasn't
> > >until a few months ago that I started actually writing the code.
> > >
> > >  - David
> > >
> > >
> > >P.S.  In general I've been working on this more-or-less independently
> > >       and with little feedback, perhaps most VTK users simply don't
> > >       work with transforms much or take viewpoint that 'it's nice, but
> > >       can't you do that already with assemblies?'  Not that lack
> > >       of feedback would ever keep me from monkeying around with VTK
> > >       as long as I have CVS access...
> > >
> > >--
> > >   David Gobbi, MSc                    dgobbi at irus.rri.on.ca
> > >   Advanced Imaging Research Group
> > >   Robarts Research Institute, University of Western Ontario
> > >
> > >On Wed, 22 Mar 2000, Will Schroeder wrote:
> > >
> > > >
> > > > Hi David-
> > > >
> > > > Can you help me here? The UserTransform, how do you use and what's it 
> > for?
> > > >
> > > > Will
> > > >
> 
>