[vtkusers] reticle / crosshair

[Marty Humperdink]

I'm working on a project to test the pointing accuracy
of a 3D tracking system and am using VTK to visualize
results. I'm rendering a 3D scene where the actors
remain fixed (in world coordindates) but I move the
camera based on measured data to create frames for an
animation. I'd like to include a fixed reference
feature (crosshair or reticle) in each frame that
stays in the same positon wrt the camera and borders
of the image frame and shows what is "in the line of
fire" of the camera. 

I first tried vtkCursor3D to create an actor that
moves with the camera in order to produce crosshairs
in each frame, but haven't been satisfied with the
look of the results.

It seems like another apporach would be to overlay a
2D image onto the rendering of the 3D actors. To do
this I'm guessing I would need to create a 2D image of
crosshairs with an alpha channel then use
vtkImageBlend to overlay this onto each frame, but am
not certain how to do this. Do I need to first use
vtkWindowToImageFilter to make a 2D image of the
rendered scene then blend this with a image of the
crosshairs?

Many thanks for suggestions or opinions.

MH

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