[vtkusers] VTK pipeline & rendering strategies

David E DeMarle dave.demarle at kitware.com
Mon Nov 28 09:40:00 EST 2016

Responses inline. Mostly though, make sure you are using the "OpenGL2"
rendering backend, which is the default for 7.0 and 7.1.

David E DeMarle
Kitware, Inc.
R&D Engineer
21 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-881-4909

On Mon, Nov 28, 2016 at 7:35 AM, Andrea Gavana <andrea.gavana at gmail.com>

> 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

> 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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