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Archive Notice: This thread is old and no longer active. It is here for reference purposes. This thread was created on an older version of the flipcode forums, before the site closed in 2005. Please keep that in mind as you view this thread, as many of the topics and opinions may be outdated.
 
Led

May 04, 1999, 11:40 AM

I'm working on a realtime shadowcasting-system (Just as Jacco :)) using the winged-edge structure. My intention is to do create a shadowmap using countouring of a mesh.

There's not much to find about the subject of contouring - all the doc's tell me it can only work on convex objects, 'cause a convex object always has a convex contour.
I just haven't seen any proof of why it couldn't work on concave objects - with a spansystem I think it just *might* be possible.
Anyone got some interesting thoughts about the subject - or a good expl. why it could never work on concave meshes..?

(Jacco, seems like a nice one for you again..? :))

 
Conor Stokes (aKa DirtyPunk)

May 05, 1999, 07:10 AM



Led wrote:
>>I'm working on a realtime shadowcasting-system (Just as Jacco :)) using the winged-edge structure. My intention is to do create a shadowmap using countouring of a mesh.
>>
>>There's not much to find about the subject of contouring - all the doc's tell me it can only work on convex objects, 'cause a convex object always has a convex contour.
>>I just haven't seen any proof of why it couldn't work on concave objects - with a spansystem I think it just *might* be possible.
>>Anyone got some interesting thoughts about the subject - or a good expl. why it could never work on concave meshes..?
>>
>>(Jacco, seems like a nice one for you again..? :))

I use my visibility set to first remove invisible polygons, then I put test the object against
the precalculated shadow beamtree of the light. Then, if the polygons are visible to both
the light + user, then I procedure to enter them into the tree. I use geometry based shadowing
though, which is rather easy to implement.

Conor Stokes

 
Phantom

May 06, 1999, 07:04 AM

Hey,

Led wrote:
>> I'm working on a realtime shadowcasting-system (Just as Jacco :))

Actually, I'm working on visibility at the moment. But shadows and
vis info are closely related. :)

>> using the winged-edge structure. My intention is to do create a
>> shadowmap using countouring of a mesh.

Wow. Tricky business.

>> There's not much to find about the subject of contouring - all the
>> doc's tell me it can only work on convex objects, 'cause a convex
>> object always has a convex contour.
>> I just haven't seen any proof of why it couldn't work on concave
>> objects - with a spansystem I think it just *might* be possible.
>> Anyone got some interesting thoughts about the subject - or a good
>> expl. why it could never work on concave meshes..?

If you use convex objects and a winged edge structure, you can find
contours by checking the winged polygons: If a winged polygon is
not facing the camera, the edge you just tested is a contour edge.
This does not work for convex shapes, as the invisible polygon might
again have an edge to a visible one, and this visible one might even
be in front of a previously found contour edge. I guess that's the
problem.
You could still break your concave objects in convex ones, though.

>>(Jacco, seems like a nice one for you again..? :))

Hey, I'm back. Till tomorrow, that is, I'm having a little vacation
(two weeks) as of tomorrow.

 
Led

May 06, 1999, 07:20 AM

>>Wow. Tricky business.
Yep :)

>>If you use convex objects and a winged edge structure, you can find
>>contours by checking the winged polygons: If a winged polygon is
>>not facing the camera, the edge you just tested is a contour edge.
That's the easy part :)

>>This does not work for convex shapes, as the invisible polygon might
>>again have an edge to a visible one, and this visible one might even
>>be in front of a previously found contour edge. I guess that's the
>>problem.
Hmm... I know what you mean, but I'm just not sure if there can't
be a solution to that problem - thought of some sort of boolean spanner or something...
but it just doesn't work :)

>>You could still break your concave objects in convex ones, though.
Thought of that too. I just want to know for sure that contouring concave-objects is impossible... And up to now I haven't seen any airtight proof it can't be done - just some serious hunches - that's what bothers me :)

>>Hey, I'm back. Till tomorrow, that is, I'm having a little vacation
>>(two weeks) as of tomorrow.

Enjoy, have fun and don't dare to think about computers in the meantime ;)

 
Steph

May 07, 1999, 04:09 AM


If your object is not convex but "closed", with no holes ( no torus or things like that),
let's say a "nice" object, you can get one or more contour, and then you can do your shadowmap
using those contours (add them geometrically or directly on your shadowmap).
I think that you can extend this to objects with hole, transparency ... but it should be a bit
harder.
Steph

 
colas

May 07, 1999, 05:14 AM




>>You could still break your concave objects in convex ones, though.

Some1 know a doc/url to fing a good (the best) alogorithm to cut a concave object in convex ones ??
It dont need to be fast because we can cut the object during a preprocesing.
But, if a fast one exist, i prefer :-)

Colas/Move


 
colas

May 07, 1999, 05:14 AM




>>You could still break your concave objects in convex ones, though.

Some1 know a doc/url to fing a good (the best) alogorithm to cut a concave object in convex ones ??
It dont need to be fast because we can cut the object during a preprocesing.
But, if a fast one exist, i prefer :-)

Colas/Move


 
David

June 03, 1999, 06:02 PM

I am not sure the specifics of the algorithims that you are using, but in our current game I do dynamic lighting using the contours of objects. With most meshes you will end up with several contour loops, but this is essential for self shadowing to be accurate.

David

 
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