Hi Maxime,
Maxime BOUCHER schrieb:
Hi,
I have a lot of questions for you!
But, there is one really more critical than the others.
Let me just introduce the problem :).
(I am sorry if I bother, but I searched on the net, in the doc and examples
and didn't find answers. I apologize if this is because I badly searched.)
I want to cut my primitives along the frustum of a camera.
1)
To do this I first have to get my planes equations. I have found a page dealing about this problem in OpenGL:
http://www2.ravensoft.com/users/ggribb/plane%20extraction.pdf
I think I'll use it, replacing in OSG conventions.
I also found this:
http://markmail.org/message/eeolcuruqpsbgrgu#query:osg%20frustum+page:1+mid:eeolcuruqpsbgrgu+state:results
From famous Paul ;).
Well, it draws the frustum, but it can be used to get the planes equations.
(for whom it may interest)
2)
Then I have my equations.
Thus I can declare many osg::Plane.
Question
If the plane is a Vec4d (a,b,c,d) does it well represent the equation
ax+by+cz+d = 0 ?
The plane uses the above plane equation.
(If you are familiar with projective spaces, the Vec4 can be seen
as the projective normal in homogenous coordinates:
(a,b,c,d) dot (x,y,z,w) = 0, where w=1 for affine points )
Note the the representation is only unique up to a
factor (you can always multiply the equation with a
factor != 0.0)
I would like to use a PolytopeIntersector. In the doc it's written planes
normals have to point the inside of the polytope.
Question
How are the normals set when the plane is initialized?
The normal is given implicitly by the representation. Given the above
equation, the normal is (a,b,c). You can easily change the direction
of the plane normal unsing its flip()-method.
Question
Is there any problem declaring a non-closed (also called opened) polytope? I
don't mind about the far plane.
No problem at all. PolytopeIntersector uses open polytopes internally
when the geometry lies to one side of a plane.
3) Once I have a polytope, I would like to intersect the polytope with the
scene.
Thus I declare a PolytopeIntersector I associate to an IntersectionVisitor.
The BIG Question
How do I associate the IntersectionVisitor to a model?
I saw for a LineIntersector it is possible to associate it to a view via computeIntersections(), but it doesn't seem possible to do the same with polytope.
You call the method accept(osg::NodeVisitor&) of your model (which is a
osg::Node).
See the example osgkeyboardmouse.
4)
Question
Having the intersected points, is it possible to "easily" get the primitive or
do I have to use this (it is in french, sorry):
http://www.enseignement.polytechnique.fr/profs/informatique/Philippe.Chassignet/03-04/INF_431/td_16.html
(exercise 4)
This exercise is about polygons not polytopes!
As for the PolytopeIntersector: It will give you all points,
lines, triangles and quads which truly intersect the polytope volume,
given by the primitive index.
Thank you a lot!
Cheers,
Maxime
Cheers,
Peter
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