Can't pick property after changing display setting

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Can't pick property after changing display setting

Hey guys, I'm a newbie in VTK. I got some display problem when trying to
change the style of displaying image.
I have a renderwindow and an observer to pick pixel when mouse hovering, the
observer setup is imitated from link below.

[CASE 1]
current display function is as below:
/* begin */
/* end */
and this is the result, which is great.

[CASE 2]
Afterward, I try to change the style of image by SetOpacity(0.5). (Actually,
I use <vtkLookupTable> and <vtkImageMapToColors>, but the SetOpacity
represent the same situation)
Than, I can't get the coordinate from observer anymore.
The Only line of code I added was
and this is the result, can't pick prop anymore. (the annotation at the

I found out that in observer callback (exactly same as the example
PickPixel2), "this->Picker->GetPath()" always return NULL in [CASE 2], which
means I didn't pickup any prop with my mouse events.
The only change I make is setting opacity, how could this make any different
in prop-picking?

Any idea?
Thanks in advance for any help !

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