[vtkusers] Polydata filtering in real-time

[Nil Goyette]

HI all, I tried more things and I can finally call it a success. 
Modifying the vtkPolyData when the cube moves is a bad idea for 
real-time rendering. A simple polydata->Modified() takes 3-4 seconds to 
render the next frame. There's too much fibers and not enough time in a 
second.

I had some good results using another vtkPolyData, building it with the 
small subset of fibers passing by the cube and displaying it with a 
normal mapper. It was rendering in real-time most of the time, but not 
in the part with the most fibers.

After a lot of trials and errors to make it fast, I discovered the 
OpenGL list functions in vtkOpenGLPolyDataMapper.cxx: glGenLists, 
glNewList, glEndList and glCallList. I understand why they were used in 
the code (compiling a list of commands to make them fast *after*), but I 
had to remove them to render in real-time. Now moving around in the 
scene is slower, but rendering the filtered fibers is as fast as I 
wanted it to be.

In summary, the solution is to modify the vtkPolyDataMapper, not the 
vtkPolydata, in order to draw only the fibers you need in OpenGL.

Le 2014-05-23 13:58, Nil Goyette a écrit :
> While waiting for an answer, I tried coding the third solution I had: 
> "Extend vtkPolyData to add the concept of visibility". I took 
> vtkStructuredGrid as an example, noted where this->CellVisibility 
> appears and extended vtkPolyData with the important methods: 
> Initialize, GetCellType and GetCell_1-2, added Blank, UnBlank, etc.
>
> Of course, it didn't work and I'm not sure why.  It seems GetCell and 
> GetCellType are never called. I was sure that one method somewhere 
> during the rendering process called them but it's not the case. What I 
> don't understand then is, if all methods related to CellVisibility are 
> not called, how can it works for vtkStructuredGrid? There's no example 
> using vtkStructuredVisibilityConstraint. Do I need to pass the result 
> of polydata->GetCellVisibilityArray() to another method?
>
> Le 2014-05-21 10:43, Nil Goyette a écrit :
>> Hi,
>>
>> I'm searching how to recreate the FiberNavigator [1] using vtk and 
>> I'm stuck on one problem. As you can see in my screenshot [2], we use 
>> a little teal cube to filter which fibers we want to see. The cube 
>> will be a vtkBoxWidget and the fibers are a vtkPolyData. The problem 
>> is that there's no concept of cell visibility in a vtkPolyData. I 
>> found this feature in vtkStructuredGrid but I believe my data doesn't 
>> fit the class' description. I found three ways to do what I want:
>>
>>  1. Use vtkExtractSelectedPolyDataIds to extract only the cells I
>>     need. I think this would be super slow.
>>  2. Add/delete cells when the cube pass by. I think this would also
>>     be slow.
>>  3. Extend vtkPolyData to add the concept of visibility
>>     (vtkStructuredVisibilityConstraint). This looks like the best
>>     option to me but I may be wrong.
>>
>> I wonder what's the best way to filter the fibers. Do you have any 
>> suggestions for me? Thanks for your time.
>>
>> [1] https://github.com/scilus/fibernavigator
>> [2] http://imgur.com/1edZpWT

-- 
Logo Imeka <http://imeka.ca/> 	Nil Goyette, M.Sc.
Programmer
www.imeka.ca <http://imeka.ca/>

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