[vtkusers] Odd opacity issue

[Kim Rosenbohm]

Hi Gib.

I'm a newbie to VTK and haven't used the DepthPeeling class myself, so I 
probably can't help you much... :/
I guess if you used OpenGL rendering, get no errors and the class tells 
you that "renderer->GetLastRenderingUsedDepthPeeling();" it should work.
Maybe you have to fiddle around with the settings a bit. Or the 
implementation in your VTK version is broken...

Best of luck,

Kim

Am 10.08.2014 04:37, schrieb Gib Bogle:
> My card uses OpenGL 4.1 and supports all the required extensions.  
> Depth peeling does not work though.
> ------------------------------------------------------------------------
> *From:* Kim Rosenbohm [kim.rosenbohm at posteo.de]
> *Sent:* Sunday, 10 August 2014 11:57 a.m.
> *To:* Gib Bogle; vtkusers at vtk.org
> *Subject:* Re: [vtkusers] Odd opacity issue
>
> Hi Gib.
>
> Proper blending of transparent objects is only possible by rendering 
> them in the correct order.
> This can be tricky and if objects or polygons overlap you even won't 
> get away with sorting them...
> You probably want depth peeling for order-independent transparency: 
> http://www.vtk.org/Wiki/VTK/Depth_Peeling 
> <http://www.vtk.org/Wiki/VTK/Depth_Peeling>
>
> Best Regards,
>
> Kim
>
> Am 10.08.2014 01:48, schrieb Gib Bogle:
>> The problem is simply demonstrated with the opacity example:
>> http://www.vtk.org/Wiki/VTK/Examples/Cxx/Visualization/Opacity
>>
>> Adding this line:
>>   sphereActor->SetPosition(1.0,0.0,0.0);
>> and setting the opacity of both actors to 0.99 gives an unexpected 
>> and unattractive result.
>>
>> I see that there have been posts about problems with 
>> opacity/transparency as far back as 2006.  Maybe there is a solution...
>> ------------------------------------------------------------------------
>> *From:* vtkusers [vtkusers-bounces at vtk.org] on behalf of Gib Bogle 
>> [g.bogle at auckland.ac.nz]
>> *Sent:* Sunday, 10 August 2014 11:25 a.m.
>> *To:* vtkusers at vtk.org
>> *Subject:* [vtkusers] Odd opacity issue
>>
>> Hi,
>> I'm using vtkRenderWindowInteractor (VTK 5.10) in Qt, rendering 
>> spheres.  I'm trying to implement a degree of transparency, like this:
>>
>> actor->GetProperty()->SetOpacity(opacity);
>>
>> applied to all the spheres.
>>
>> If opacity is less than 1 the image immediately changes in unexpected 
>> ways.  See the two attached images, with opacity = 1.0 and 0.99.  It 
>> looks as if spheres have had their positions and sizes changed.  But 
>> when I rotate the scene it becomes apparent that something else is 
>> happening.  Some of the "foreground" spheres move from left to right 
>> (for example) while others apparently in the foreground are moving 
>> right to left.  It seems that the order of rendering of the spheres, 
>> which normally places the most remote first and the nearest last, has 
>> somehow been messed up, so that spheres at the back are being 
>> rendered last so that they appear to be the closest.  Maybe the order 
>> has been simply reversed, but the visual effect is made more 
>> confusing by parallax - my brain is interpreting the bigger spheres 
>> as closer.
>>
>> Is this a well-known issue, and is there a simple way to fix it?
>>
>> Thanks
>> Gib
>>
>>
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