[vtkusers] VTK pipeline & rendering strategies

Tue Nov 29 03:39:09 EST 2016

Dear All,

On 28 November 2016 at 16:12, Andrea Gavana <andrea.gavana at gmail.com> wrote:

> Hi David,
>
>     thank you for your answer. I will put some more comments inline too.
> Any insight or suggestion is more than welcome.
>
> On 28 November 2016 at 15:40, David E DeMarle wrote:
>
>> Responses inline. Mostly though, make sure you are using the "OpenGL2"
>> rendering backend, which is the default for 7.0 and 7.1.
>>
>
>
> I am using VTK 7.0 through the Python bindings (the official ones from the
> Kitware website here:
>
> http://www.vtk.org/download/
>
>
> So I was assuming that the rendering backend was already OpenGL2... is
> there any way to check if this is really the case?
>
> Also, I have tried to load my dataset in ParaView, and ParaView seems to
> be doing some kind of black magic on my vtkUnstructuredGrid as I get these
> kind of messages when I load the vtk file:
>
> Warning: In C:\bbd\df0abce0\source-paraview\VTK\Rendering\VolumeOpenGL2\
> vtkOpenGLProjectedTetrahedraMapper.cxx, line 251
>
> vtkOpenGLProjectedTetrahedraMapper (000000000D3CAA60): Missing FBO
> support. The algorithm may produce visual artifacts.
>
>
>
>
> Which, I believe, is telling me that ParaView is not doing what I am doing
> but maybe using some kind of volume-based rendering - when I rotate the
> grid the (almost cube-shaped) cells becomes subdivided in small triangles,
> when I stop interacting with them they go back to their normal appearance.
> It would be nice to know what ParaView is doing though, and also what "FBO
> support" means :-). I also noticed that ParaView is slightly faster in
> changing the displayed property - maybe 2 or 3 times faster than my
> bare-bone script.
>
>
>

Just to add some more information: it is true that ParaView is faster in
the re-rendering when I switch to another property, but it also shows heavy
visual artifacts (i.e., cells with multiple colors in them?!? - see
attached screenshot). So, if I understood it correctly, ParaView uses some
kind of volume rendering of my vtkUnstructuredGrid (although I didn't ask
it to... should it not use the default vtkDataSetMapper?).

Based on my original pipeline:

vtkUnstructuredGrid --> vtkThreshold --> vtkDataSetMapper --> vtkActor

I have tried everything I know to speed up the re-rendering of the grid, I
actually operate directly on the output of vtkThreshold (that is still a
vtkUnstructuredGrid), so I simply throw away the original grid. With and
without ImmediateModeRendering, with and without backface culling. My best
improvements so far took the rendering time from 1.04 seconds to 0.94
seconds... so not much of an improvement :-) .

I am of course open to any suggestions anyone may have on the rendering
part - assuming that my geometry is fixed (it is not, but I'll worry about
that later...), basically only on the process:

vtkUnstructuredGrid.SetScalars(my_data)
vtkUnstructuredGrid.Modified()
vtkRenderWindow.Render()

And of course - if there are other visualization strategies I may use,
please shout :-)

Thank you.

Andrea.

>
>
> On Mon, Nov 28, 2016 at 7:35 AM, Andrea Gavana <andrea.gavana at gmail.com>
>> wrote:
>>
>>> Dear All,
>>>
>>>      I am working with some stuff coming out of CFD simulations, and in
>>> the current work the simulator produces a 3D grid (unstructured grid made
>>> of hexahedrons). The full grid is about 4 million cells, but due to other
>>> settings in the simulator the number of "active" cells in the simulation
>>> ends up being "only" 270,000. In order to visualize all this, I create a
>>> vtkUnstructuredGrid to hold the full grid, use a vtkThreshold to remove the
>>> "inactive" cells and then use a vtkDataSetMapper to visualize the resulting
>>> active grid:
>>>
>>> vtkUnstructuredGrid --> vtkThreshold --> vtkDataSetMapper --> vtkActor
>>>
>>> However, the rendering speed for the 270,000 cells grid is quite low -
>>> it takes about one second to display a new property by using SetScalars on
>>> the output of vtkThreshold. So I thought of using a vtkDataSetSurfaceFilter
>>> on the output of vtkThreshold to try and speed up the rendering. So, the
>>> current visualization strategy I have implemented is the following:
>>>
>>> vtkUnstructuredGrid --> vtkThreshold --> vtkDataSetSurfaceFilter -->
>>> vtkPolyDataMapper --> vtkActor
>>>
>>> This is still as slow as my first approach, and I also have a couple of
>>> questions - which stems from my ignorance in VTK things:
>>>
>>>
>> DataSetMapper internally does vtkDataSetSurfaceFilter->vtkPolyDataMapper
>> when given something other than PolyData, so not surprising that it isn't
>> faster.
>>
>>
>>> 1. When I load (from the simulator outputs) a new property (cell-based)
>>> and I assign its values to the original vtkUnstructuredGrid (by using
>>> SetScalars on it), do all the filters (vtkThreshold and
>>> vtkDataSetSurfaceFilter) need to be re-run? If yes, why? I am not changing
>>> the active/inactive cells nor the geometry of the grid, only assigning
>>> different scalars. And, if yes, is there any way to tell the pipeline:
>>> "look, I've only changed the scalars, there's no need to re-run all the
>>> thresholds and surface filters *again*"?
>>>
>>>
>> Yes they do, since the Executive classes' Modified time tracking is not
>> fine grained enough to know the difference, and few if any of the filters
>> would know how to update just the changed portions.
>>
>>
>>> 2. Is there any other pipeline style or visualization technique in VTK
>>> or any settings whatsoever that could bring down the rendering time (memory
>>> is not that much of a concern)? Basically, what I have a the moment - in
>>> terms of timing - is as follows:
>>>
>>>
>> Yes, use OpenGL"2". Modern OpenGL programming techniques make it up to
>> hundreds of times faster than the Legacy fixed function "OpenGL" backend.
>>
>>
>>> Reading data from simulator: 0.042 seconds
>>> Create VTK array with data : 0.002 seconds
>>> Call to SetScalars         : 0.000 seconds
>>> Create Lookup Table        : 0.001 seconds
>>> Render on screen           : about 1 second
>>>
>>>
>>> Thank you in advance for any suggestion, my apologies for the long
>>> message.
>>>
>>> Andrea.
>>>
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>>
>
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