[vtkusers] efficient vtkViewport::WorldToDisplay() ?

[Paul Harris]

Hi all,

I have written a vtk Picker that works with some rendered points that I
display.
I previously tried all sorts of techniques like ray tracing etc but I've
discovered the most accurate way is to simply check every point I plot
against the pixel location, by converting each point to the Display
coordinate with WorldToDisplay()

Unfortunately, that means 100k+ calls to WorldToDisplay(), which is pretty
damn slow.

How can I make this faster?  I assume I could get the WorldToDisplay
conversion matrix and then just multiply each point by the matrix.  That
would be a lot faster... at the moment WorldToDisplay() seems to reconstruct
the matrix every single time.

thanks,
Paul
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