[vtkusers] Re: Re: Slow rendering of derived class of vtkImplicitFunction (David.Pont at ForestResearch.co.nz)

[#LIM LAY NA#]

Hi, thanks for the advice. I've tried vtkLODActor but it doesn't seem to
have much effect. The same goes for the specialised contour filters,
vtkMarchingContourFilter, vtkMarchingCubes, vtkKitwareContourFilter.
I've also tried

vtkTriangleFilter and vtkStripper. I need to keep the sampling at least
50*50*50 because at anything less than that, the

rendered shape is very distorted. Are you suggesting that I write my own
SampleFunction? I'm not very sure how to do that.

Can I view some sample code that you used for implicit polygonization?
Would really appreciate it. Thanks. 

 

Hi,

   My guess is that vtkSampleFunction is the part that takes the most t=
ime. You use SetSampleDimensions to define the number of samples, for
exampl= e 50*50*50 =3D 125,000 samples. Your implicit function
EvaluateFunction w= ill be called for each of these, and I see your
EvaluateFunction uses exp and sqrt. This volume is then contoured to
produce a surface, the contouring algorithms are relatively efficient,
but this might be the next slowest part.

 

If you use smaller SampleDimensions you will note a dramatic
(exponential) increase in speed, but you will also notice a poor
representation of the surface. The problem with vtkSampleFunction is
that in order to sample = the surface at high resolution you have to
sample a lot of dead space away = from the surface.

 

A basic design philosophy in vtk is to favour generality in algorithms,
for good reason. Producing a volume (structured points) from an implicit
function is smart because there are many vtk classes that can be used on
this type of data. Check the literature and you will find examples of
'fast' algorithms for creating implicit surfaces (Google 'implicit
polygon'). I played around with implicit polygonization by Velho (source
code available off the internet) some years back, but never had a real
need for it. The vtk philosophy would encourage you to implement a
faster but more specialised algorithm... (and would encourage you to
contribute it back=

 

;-P). Take a look at the documentation for vtkContourFilter and note th=
e 'See also' for an example of a generic algorithm and its more
specialis= ed friends. So how are your C++ skills? they look just fine
from your example code.=  You could implement a new vtk class to do fast
polygonization of implicit surfaces in a few days... and have
interactive blobs.

 

The only other way around might be to use vtkLODActor and feed it one (=
or more) low resolution (low SampleDimensions) pipelines and one at the
desired resolution... ??

 

regards

   Dave P.

 

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