[vtkusers] VTK pipeline & rendering strategies

Andrea Gavana andrea.gavana at gmail.com
Mon Nov 28 10:12:58 EST 2016

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:


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
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.

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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