[Paraview] Colormap opacity / transparency

[Utkarsh Ayachit]

Ah! Now you are getting into murky waters :). Volume rendering
together with translucent geometry is difficult thing to do with
OpenGL. You need a full fledged ray-tracer for that to work correctly.
You are correct, in VTK/ParaView, the geometries are rendered
(including translucent geometry) before volume. Since translucent
geometry does not affect Z-buffer as opaque geometry (what's the depth
for a translucent pixel, really?), the volume mapper cannot correctly
determine when to stop the ray tracing and hence the issues.

Utkarsh

On Fri, Jun 24, 2011 at 4:41 PM, Gary Strangman
<gary.strangman at gmail.com> wrote:
> Hi Utkarsh,
> Half-progress. So, I can display a variable-transparency volume rendering
> (in this case of a head MRI, set so that the 0s around the head are
> transparent). I can then add a .vtk surface, and I can sorta see "both"
> datasets when opacity on the surface is set to 1.00 ... the grayscale volume
> peeks through at various points of the overlying (colorized) surface. I'd
> like to be able to see more of the underlying volume, so I reduce the
> opacity value for the surface. However, for any opacity value < 1.00,
> paraview renders only the /back/ side of the surface (the part of the
> surface on the other side of the head from the camera viewpoint) ... the
> foreground portion disappears completely. After playing around with
> translations, I'd guess that for opacity<1.00 the volume is entirely
> rendered last, overwriting what should be foreground surface pixels. Does
> that make any sense? Perhaps I need to show a picture ...
> -best
> Gary
>
> Gray,
>
> Currently ParaView supports changing the opacity in colormaps only for
> volume rendering. When rending slice or points as surface or anything
> other than volume, opacity will be fixed to single value that can be
> set on the Display tab.
>
> Utkarsh
>
> On Mon, Jun 20, 2011 at 4:30 PM, Gary Strangman
> <gary.strangman at gmail.com> wrote:
>> Hi again all,
>> I solved my linked-colormap problem when rendering multiple volumes
>> (thanks
>> Utkarsh, using separate variable names worked great!). My next issue is
>> this: can someone explain when colormap transparency is enabled? In some
>> of
>> my data views, the opacity option is grayed out (for example, in slice
>> renderings, and also in points data). I kinda understand the points case,
>> but I was surprised by slice renderings. I suspect there is an underlying
>> principle I'm missing here ...
>> -best
>> Gary
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