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 and , 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 bottom-left)

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 !