# Can't pick property after changing display setting

**URL:** https://discourse.vtk.org/t/cant-pick-property-after-changing-display-setting/928
**Category:** Support
**Created:** [May 8, 2019, 3:50am UTC](https://discourse.vtk.org/t/cant-pick-property-after-changing-display-setting/928 "2019-05-08T03:50:16Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![wenyalin](https://discourse.vtk.org/user_avatar/discourse.vtk.org/wenyalin/32/387_2.png) [@wenyalin](https://discourse.vtk.org/u/wenyalin)
#### Post date: [May 8, 2019, 3:50am UTC](https://discourse.vtk.org/t/cant-pick-property-after-changing-display-setting/928/1 "2019-05-08T03:50:16Z")

</div>

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.  
([https://vtk.org/Wiki/VTK/Examples/Cxx/Images/PickPixel2](https://vtk.org/Wiki/VTK/Examples/Cxx/Images/PickPixel2))

[CASE 1]  
current display function is as below:  
/\* begin _/  
m\_ImageViewer-\>SetInputData(image);  
m\_ImageViewer-\>UpdateDisplayExtent();  
m\_Renderer-\>ResetCamera();  
m\_ImageViewer-\>SetRenderWindow(RenderWindow);  
RenderWindow-\>Render();  
/_ end \*/  
and this is the result, which is great.

 ![1](https://discourse.vtk.org/uploads/default/original/1X/66d5855f4c10b3c45e21b3c1d161fc33f28a68d0.png)

[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  
“m\_ImageViewer-\>GetImageActor()-\>GetProperty()-\>SetOpacity(0.5);”  
and this is the result, can’t pick prop anymore. (the annotation at the bottom-left)

 ![2](https://discourse.vtk.org/uploads/default/original/1X/abe68bb1b85d8b6aafb1eb28af86a56a0932c587.png)

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 !