[vtkusers] Re: Re:Strange Artefacts with Surface Rendering

[Tobias Mönch]

Hi Jim,

You are right, the inside-out-look comes from the "inside-surface" which is
becomes visible when the front-surface gets transparent. But it's strange that
this happens depending on the orientation. At first everything is drawn right. I
can turn everything without problems, but at a special angle of rotation the
surfaces seem to be rendered in a wrong direction. I watched it very carefully
and noticed that the front-surface suddenly seems to get pushed behind the other
surfaces (which became visible due to transparency-changes) and the surface
which would be behind the front surface (= the inside of the opposite side of
the brain) gets drawn as if it was in front (where it isn't).

Tobias



----- Original Message -----
From: "James Carroll" <mrmaple at gmail.com>
To: "Tobias Mönch" <Tobias.Moench at Student.Uni-Magdeburg.DE>
Cc: <vtkusers at vtk.org>
Sent: Thursday, September 22, 2005 2:46 PM
Subject: Re: [vtkusers] Strange Artefacts with Surface Rendering


Hi Tobias,

This is a common 3D rendering issue.  When the surfaces are opaque,
the surfacae has a  position in the Z buffer, and if the closest face
gets drawn first, then because of the Z buffer (also called the depth
test.) things that are behind the surface aren't drawn at all.  As
soon as you have some transparency, all of the surfaces have to be
drawn, so the depth test is turned off.  Things that are behind are
blended equally with things that are in front.

I think this would cause the inside-out look of your brain surface.

This wouldn't cause the parts of the surface disappearing though...  hmm...

-Jim


On 9/22/05, Tobias Mönch <Tobias.Moench at student.uni-magdeburg.de> wrote:
> Hi vtk'ers!
>
> I've again some problems. I'm doing a simple SurfaceRendering of a brain using
a
> vtkPolyDataMapper (and some smoothing before). When I use no transparency then
> everything is fine.
> But if I set the opacity of the surface-actor to some value below 1.0 the
> following happens:
> The surface seems to turn its inside out when I rotate it in some direction
> (sometimes it looks like this). It also seems that some parts of the surface
> disappear when rotating (as it was clipped). It's no problem to rotate the
> brain-surface back and then everything is completely closed as it should. But
in
> some special positions the described things happen.
>
> Does anyone had the same problems or an idea what the problem could be?
>
> I attached a screenshot where i marked where the surface already disappeared.
>
>
> Thank you,
> Tobias
>
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