[vtkusers] VTK pipeline & rendering strategies

[David Lonie]

+1 to what other Dave said. I'd also point out that you can simply add all
of the potential scalars you'd like to use to the point/cell/field data as
named arrays, and then use the vtkMapper::SelectColorArray method to select
the property to use for visualization.

This way, the threshold/DSSF pipeline would only execute once and will
produce a dataset with all of the property arrays filtered to just the
active set. Changing the visible scalars at the mapper level will not cause
the pipeline to re-execute.

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:
>
> 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*"?
>
> 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:
>
> 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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