[vtkusers] Picking points from multiple surfaces.

[An Jiye]

>
> Not that it is appropriate for your application, but for others who may
> not be satisfied with the behavior of their current picker, there also
> is a vtkPropPicker class whose behavior we prefer for our applications
> (SureFit and SuMS, http://brainmap.wustl.edu):
>
> 	http://www.vtk.org/doc/nightly/html/classvtkPropPicker.html
>
> Donna
>

Thank you, Donna.

I have tried this picker and it works well in my situation. I mean it will
always return the y-axis value of 20, 19.xxx or 20.xxx no matter how I
rotate or zoom the camera by mouse inut. That's Great!

But I have some questions about this picker.

If my actor's opacity is not 1.0, 0.3 for example. It seems I can not get
the correct coordinate. The resulting coordinates is strange. And when I
rotate the actor, I can not get a constant y-axis value as before. But if I
set the opacity to 1.0 again, everything goes OK. Can you explain me how
opacity influnce the picking result?

In practice, I have multiple actors. Let's consider two of them, with one
inside another. When I set the opacity of the outer surface to 0.3, while
keeping the opacity of the inner one 1.0, and also set the PickableOff of
the inner actor. Then the picking results will be:

    1.On the surface of the outer actor, the y-axis will be 31.5, actually
it should be 1.0. This is the same problem as above. It dues to the opacity.

    2.Since the outer one is semi-transparent, I can see the inner actor in
the overlap region of the two actors. When I perform picking in that region,
the resulting y-axis will be 20. It is the inner surface itself. But I have
already turned it PickableOff. Leaving alone the opacity problem, the
resulting y-axis value from the outer surface should all be 31.5, though 1.0
is correct.

Are these caused by the rendering procedure of graphics hardware? I know
vtkProPicker uses graphics hardware/rendering system to pick. My major is
Biomedical Engineering, and I have no Computer Graphics background. I will
learn it later.

Thank you in advance.

Best Regards,
An Jiye