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Archive Notice: This thread is old and no longer active.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 Please keep that in mind as you view this thread, as many of the topics and opinions may be outdated.
 
devil20

March 10, 2005, 07:04 AM

Hi there

Is qhull support concave type object in input ???

I don't know very well about Qhull but problem is when i'm giving Concave object with some faces going inside I mean by some inner surface. But problem is constructed QHull output is converting Convex type .

So anyone know which option should set-up to give input object as concave and output as alos concave. I mean by keep concave as it is. ?? Or Qhull not supporting Concave object ??


Suggestion Welcome.

devil20

 
Illco

March 10, 2005, 07:20 AM

Did you even wonder if it would make any sense to have a convex hull algorithm that works only on convex objects?

 
devil20

March 10, 2005, 07:51 AM

Hey I asked question :)

I am not well experince with qhull. please try to direct me .

thanx

 
Illco

March 10, 2005, 09:40 AM

Ok -- you're right. What I meant is that any algorithm for the convex hull must be able to deal with concave objects, because convex objects don't need to have their convex hulls computed at all.

The QHull algorithm is capable of doing that as well. However, I think you're problem is more implementation-related. Which library/package/implementation do you use? Perhaps you can show some code details or examples?

 
devil20

March 10, 2005, 10:04 AM

Hi

Thanx I'm just telling all stuff.


I'm using ConveHull package by John W. Ratcliff on flipcode.com

http://www.flipcode.com/cgi-bin/fcmsg.cgi?thread_show=11119#p72078


It is containing one example that is loading wave front .obj file. and giving out-put as convexhull and simplified mesh .

I am uploading 2 picture related my prob.

This is orignal concave object as input that i am loading. Just open link in browser :

http://www.geocities.com/devloper99/concave.JPG


So now output is like that in this link :


http://www.geocities.com/devloper99/convex.JPG



So u can find my problem inner face of mesh going totally inside is now removed and done convex object.

So this is problem if u need any thing else like code, model file (.obj) i can send it to u. Please let me know :)


thanx
devil20

 
Illco

March 10, 2005, 10:10 AM

I've checked your images, but I can only conclude: this is exactly what you want. The convex hull removes concavities and computes a... well, a convex hull of the perhaps concave input object.

To be more precise, the concavity you encircled is removed. But that was not the only concavity in the object; between the (roughly) upper left corner and bottom left corner is also a (larger) concavity which is removed by the convex hull. Therfore the smaller concavity is removed totally out of sight.

Perhaps it helps to know that not all vertices of an object lie on its convex hull.

 
devil20

March 10, 2005, 10:17 AM

Hi
So Qhull will not handel like this stuff ?? I mean it's always remove inner area ??

I checked help qhull has some option saying about not to remove concave of object but it not working anyway .


Hey u can post ur reply i'll check tommorow :) just going home .


thanx
devil20

 
Illco

March 10, 2005, 10:24 AM

I'm not sure if I understand you correctly, but if I do than the convex hull does not do that for you. It does not remove small concavities only; it removes all concavity.

So if you have some input object shaped a bit like this:

  1.  
  2. +---------------+
  3. |               |
  4. |     +---+     |
  5. |  +--/   --+  |
  6. +--/         --+
  7.  


Its convex hull will not be this, where only some concavity is gone:
  1.  
  2. +---------------+
  3. |               |
  4. |               |
  5. |  +---------+  |
  6. +--/         --+
  7.  


But this, where all concavity is gone:
  1.  
  2. +---------------+
  3. |               |
  4. |               |
  5. |               |
  6. +---------------+
  7.  


Hopefully that helps.

 
Lennox

March 10, 2005, 10:41 AM

I looks like he is getting convex hulls mixed up with mesh reduction algorithms. The convex hull behaves just as Illco said, in that it will give you a hull that does not contain any concavity (its convex).

What you're probably looking for is mesh reduction. There are a ton of methods for this, but check out : http://www.melax.com/polychop/

 
devil20

March 12, 2005, 02:10 AM

Thanx Guys For response.

I think i understand now :) i have to do another coding for concave objects.


devil20

 
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