[vtkusers] Contouring through constant value areas of dataset

[Burlen Loring]

Gerrick, right, it just looked odd to me at first. Did you see my follow 
up post?

http://www.vtk.org/pipermail/vtkusers/2008-September/096843.html

To summarize:

If all nodes of a dataset (or in your case block) have a constant value 
then no contour lines get generated because this configuration of cells 
never results in a contour line intersecting any of the cell's edge's. 
See p 166 and 167 of the vtk text for a better explanation. You'd end up 
with all the cells having the configuration of "Case 15" in fig 6-5 
where no contour line is drawn.

Burlen

Gerrick Bivins wrote:
> The dataset is a mutliblock dataset with discontinuous scalars. I've used
> the solution mentioned in a previous post to accomplish this( using
> vtkShrinkFilter with value of 1.0 to explode the dataset and duplicate point
> info).
> However, I've also run across this issue with a dataset that has continuous
> regions. 
> As David mentioned below, this issue can be reproduced using any source with
> a constant scalar data array assigned to it.
> Gerrick
>
> On 9/6/08 10:46 AM, "Burlen Loring" <burlen.loring at kitware.com> wrote:
>
>   
>> Gerrick,
>>
>> There is something suspect with the data you are contouring.
>> http://www.nabble.com/file/p19338704/constantValues-surface.png
>>
>> Let me explain why I think this. Contouring uses interpolation along
>> cell edges, making use of nodal data. In my experience, when rendering
>> nodal data vtk interpolates so that in the rendered image you never see
>> a discontinuity across adjacent cells when adjacent cells share nodes
>> (it will always show as a steep but smooth transition). In your images
>> we can see discontinuities between the regions, therefor I wonder if
>> those cells are actually sharing nodes with their neighbors. ? In other
>> words to vtk they might not be connected regions.
>>
>> Just a guess.
>> Of course it would be easier to help if you posted your data.
>>
>> Burlen
>>
>> David E DeMarle wrote:
>>     
>>> Ah, so presumably one of those isocontours happens to hit the
>>> contoured value exactly and you expect the whole thing to be drawn but
>>> it is not.
>>>
>>> You can reproduce the behavior with any Source, using Calculator to
>>> give every point the same constant scalar value, and then contouring
>>> on that value.
>>>
>>> You would expect everything to be drawn, but nothing is. I don't know
>>> why it does that.
>>>
>>> anyone?
>>>
>>> On Fri, Sep 5, 2008 at 4:38 PM, Gerrick Bivins
>>> <gbivins at objectreservoir.com> wrote:
>>>   
>>>       
>>>> Right. But for instance, if I were visualizing this data via volume
>>>> rendering, the areas with this constant value would be displayed rather than
>>>> not.
>>>> Am I misunderstanding the intent of contouring? If so, is there another
>>>> filter I can use to accomplish what I'm describing?
>>>> Gerrick
>>>>
>>>>
>>>> On 9/5/08 3:29 PM, "David E DeMarle" <dave.demarle at kitware.com> wrote:
>>>>
>>>>     
>>>>         
>>>>> Umm, because the values are constant? Which means they contain only
>>>>> one value, and thus either nowhere contain the value that a contour
>>>>> displays, or everywhere contain it.
>>>>>
>>>>> I am not sure I understand your question otherwise.
>>>>>
>>>>>
>>>>> On Fri, Sep 5, 2008 at 4:17 PM, Gerrick Bivins
>>>>> <gbivins at objectreservoir.com> wrote:
>>>>>       
>>>>>           
>>>>>> Hi All,
>>>>>> When generating contour values (isosurfaces) with vtkContourFilter, I've
>>>>>> noticed that areas in the dataset with constant values ( probably not the
>>>>>> correct terminology here) don't produce contours.
>>>>>>
>>>>>> For example, here is the original multiblock dataset in Paraview with
>>>>>> areas
>>>>>> of constant value:
>>>>>> http://www.nabble.com/file/p19338704/constantValues-surface.png
>>>>>>
>>>>>> Here is the contouring with about 100 values:
>>>>>> http://www.nabble.com/file/p19338704/constantValues-100-contours.png
>>>>>>
>>>>>> Here is the contouring with ALOT of values:
>>>>>> http://www.nabble.com/file/p19338704/constantValues-many-contours.png
>>>>>>
>>>>>> Can someone explain why the square area(bottom left of the elliptical
>>>>>> dataset) and triangle area (bottom right) don't produce any contours? The
>>>>>> only similarities are that the values in those areas are constant in those
>>>>>> areas.
>>>>>> Gerrick
>>>>>>
>>>>>>
>>>>>> --
>>>>>> View this message in context:
>>>>>> http://www.nabble.com/Contouring-through-constant-value-areas-of-dataset-t
>>>>>> p19
>>>>>> 338704p19338704.html
>>>>>> Sent from the VTK - Users mailing list archive at Nabble.com.
>>>>>>
>>>>>> _______________________________________________
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>>>>>>
>>>>>>         
>>>>>>             
>>>>     
>>>>         
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>>>       
>
>   


-- 
Burlen Loring
Kitware, Inc.
R&D Engineer
28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x137