From: Simon C Mills (simon++at++wgs.estec.esa.nl)
Date: 01/22/2001 01:41:28
Brian Furtaw wrote:
>
> Could you give some more information about what you are trying to do. If
> you have doorways or windows in buildings that are modeled transparently
> it will require a different solution then trying to shoot through trees.
> What are you intersecting with and why? In Performer each pfNode carries
> an isect mask so you can have classes of objects that behave differently
> in isect queries. You are most likely going to need to add some extra
> information to the scenegraph to solve your problem.
OK. I have a model of the International Space Station which is had lots
of thin truss structures. To model these we use transparency in textures
a lot to represent geometry. This is the model I'm intersecting with.
I'm drawing a nice sun and lens flare effect (to make the sun look
really bright and for camera simulation) and to test if the sun is
occluded I'm using an intersection test. The test uses a vector from the
eye point in the sun direction and the effect is drawn depending on the
intersection result. Now, because intersections don't take the purely
transparent areas into account, I get anomalies where the sun is in view
but the intersection returns true and turns off the effects.
So, I'm looking for a way around this problem without having to model
the whole ISS in solid geometry (_lots_ of polygons required for this!).
One thought I also had was to use stencil tests since the areas causing
the anomalies are purely transparent (alpha = 0). By setting alpha draw
function to alpha > 0 these pixels are not draw. At the same time I
could update the stencil buffer and use this then as an additional test
after by intersection test. This relies on the fact that the
intersection uses the eye point and would allow me to combine the 3D and
2D (pixel space) techniques. In fact any 2D technique that indicates if
any part of the sun's disk is not occluded would be sufficient.
Thanks for your interest,
Simon
_______________________________________________________________________
Simon Mills
Silicon Worlds S.A.
c/o Modelling & Simulation Section (TOS-EMM) Tel: +31 (0)71 565 3725
European Space Agency (ESA/ESTEC) Fax: +31 (0)71 565 5419
Postbus 299, 2200AG Noordwijk e-mail: simon++at++wgs.estec.esa.nl
The Netherlands http://www.estec.esa.nl/wmwww/EMM
_______________________________________________________________________
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