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To: [email protected]
Subject: Re: [Opensg-users] search for hidden polygons
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[email protected] schrieb am 25.05.05 17:49:23:
Hi Dirk,
>Hi Andi,
>On Wed, 2005-05-04 at 09:29 +0200, =?iso-8859-1?Q? Andreas=20Br=FCning ?
> Thanks Dirk,
>
> that's a nice idea. But is it faster? I don't have special viewports,
> the user can look at the mode! l from every site. I search for the
> poygons, which are kompletly invis ible. I only load a model and show
> it. Now I try to take less rays and find the neighbors of the
> neighbors of the founded visible polygon. (and so on...) It's a very
> tricky problem. May be there is a good combination.
>
>Well, completely invisible is pretty hard to do, but you won't be able
>to do that totally correct with rays either. Image an object enclosed by
>a sphere with a small hole. One of your rays would have to hit the hole,
>to decide that the object is visible, and the chances of that are
>proportional to the size of the hole, which can be pretty much
>arbitrarily small.
>You can think of the rendering approach as sending a whole lot of rays
>from a single point into the scene (or parallel rays if you use an
>orthographic projection), it's just faster than sending individual rays,
>and you don't have as much control over every single ray.
>
>Given that it's hard to decide the area-area visibility beforehand,
>current hardware supports a method to do it on the fly. Look for
>'occlusion culling' as a keyword to find papers about it.
>
>Hope it helps
>
>Dirk
thanks for your help. I know, it is very tricky to find all hidden polygons.
For my work, i have a good solution. At first i use a polygon reducer (VIzup).
Then I shoot my rays around the modell. I use polar- coordinates and with phi
and theta i can decide, how many rays i want to shoot. After that comes the
neighbourhood. For every visible polygon, i select the polygons around too.
Furthermore i can choose, if want to select the neighbours of the neighbours
... . With that, i need less rays, but it select some hidden polygons. The
program finds a lot of hidden polygons and it´s ok for my work. To speed up the
iterations, i create several multimaps.
Before i started to write my program, i read a lot about occlusion culling. I
decided, that it doesn´t helps me, because i want to create a new object
without hidden polygons and write it into a vrml- file.
many thanks for your help
Andreas
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