[vtkusers] Odd opacity issue

[Bill Lorensen]

Maybe this example will be useful:
http://www.vtk.org/Wiki/VTK/Examples/Cxx/Visualization/CorrectlyRenderingTranslucentGeometry

Bill

On Sun, Aug 10, 2014 at 6:17 AM, Kim Rosenbohm <kim.rosenbohm at posteo.de> wrote:
> 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
>
> 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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