A common problem facing POVers is how to difference (ouch, that 
shouldn't be a verb!) a bunch of CSG objects from another one, without 
taking hours and hours to render.

Mike Williams has a great tutorial ( 
http://www.econym.demon.co.uk/holetut/ ) that talks about two of the 
methods (subdividing the object and using isosurfaces).  Another handy 
method is to use blobs with negative strength component.

In the first and second pictures I created a blob with a big, flat 
sphere and a bunch of negative cylinders, and then intersected the 
result with a box.  Render times were under 2 minutes, and, as the 
second picture shows, an advantage is that you don't need regularly 
spaced holes (as you would with the subdivision method.)

The third picture shows why I was needing a quicker difference.  I've 
been modeling "milling processes," where a milling tool rotates and cuts 
pieces out of a block.  If the block is a cylinder or a box, the 
subdivision method works fine, with a little prep work, but when you're 
carving out of some more irregularly shaped thing, this negative blob 
method is much easier.  In the fourth picture, I put the milling union 
back in (slightly smaller) as a separate blob, to see how the carving 
was done.

If anyone's interested, I can post the sources to PBSF or PTSF.

Dave Matthews

Post a reply to this message

Attachments:
Download 'nt007b.jpg' (31 KB) Download 'nt007c.jpg' (20 KB) Download 'nt006e.jpg' (24 KB) Download 'nt006d.jpg' (24 KB)

Preview of image 'nt007b.jpg'

Preview of image 'nt007c.jpg'

Preview of image 'nt006e.jpg'

Preview of image 'nt006d.jpg'

nos### [at] nospamedu news:41b0a858@news.povray.org

> A common problem facing POVers is how to difference (ouch, that 
> shouldn't be a verb!) a bunch of CSG objects from another one, without 
> taking hours and hours to render.

Btw, another good idea is to divid object into small sectors, each 
sector heaving only few holes.

-- 
http://www.raf256.com/3d/
Rafal Maj 'Raf256', home page - http://www.raf256.com/me/
Computer Graphics

Post a reply to this message

Rafal 'Raf256' Maj wrote:

> Btw, another good idea is to divid object into small sectors, each 
> sector heaving only few holes.
> 

That would be the "first method" in Mike Williams' tutorial that I 
mentioned ;-)

The difficulty with that method, practically speaking, is that you need 
to figure out beforehand, which negative objects interact with which 
parts of the positive object.  By putting the positive object "inside" a 
blob sphere with holes already in it, in the shape you want, you can 
save some rather daunting calculations.

Dave Matthews

Post a reply to this message