Subject:
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Re: primer/tutorial on BFC?
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Newsgroups:
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lugnet.cad.dev
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Date:
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Sun, 4 Jan 2004 21:21:20 GMT
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Viewed:
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1998 times
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In lugnet.cad.dev, Andrew Westrate wrote:
> In lugnet.cad.dev, Steve Bliss wrote:
> > In lugnet.cad.dev, Chris Dee wrote: • [snip]
> > Your explanation was very good in all respects except for one small point:
> >
> > > ... although for cylinders it
> > > is possible to turn them inside out by negating the transformation matrix, and
> > > adjusting the position).
> >
> > Actually, negating the transformation matrix should not invert any object; and even
> > if it does, it's not a valid technique for BFC'ing.
> >
> > Steve
>
> For the two-dimensional primitives (disc, ndis, ring, chrd) you can do the
> following:
>
> Say you have a disc in the x-z plane, like:
> 1 16 0 0 0 10 0 0 0 1 0 0 0 10 4-4disc.dat
>
> If this has the wrong BFC winding, you can just flip (mirror) the primitive by
> changing the matrix to:
> 1 16 0 0 0 10 0 0 0 -1 0 0 0 10 4-4disc.dat
>
> Any reason why that isn't valid? (I do it all the time)
>
> Using a mirror matrix doesn't work for the 3d primitives, like the cylinder
> Chris mentions.
I just wanted to add one more post in an attempt to clear up any confusion that
may have been caused in this thread. What Andy says above is correct, and my
earlier post indicating otherwise was incorrect.
The two dimensional primitives in the X-Z plane can all be flipped by negating
the non-zero Y term of the rotation matrix. The BFC INVERTNEXT meta-command
produces the same effect, but as Steve pointed out in another post, this ties
the two lines together, which add complications. (Note that he was endorsing
the idea of rotating the primitive 180 degrees--instead of flipping it--when he
said that.)
Steve's suggestion of rotating 180 degrees will also work for symmetric
primitives, but not for asymmetric ones. Given that mirroring does produce the
desired effect (and correctly so, not due to a bug in L3Lab), I personally agree
with Andy that this is the best way to go.
--Travis Cobbs
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Message is in Reply To:
 | | Re: primer/tutorial on BFC? [DAT]
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| (...) For the two-dimensional primitives (disc, ndis, ring, chrd) you can do the following: Say you have a disc in the x-z plane, like: 1 16 0 0 0 10 0 0 0 1 0 0 0 10 4-4disc.dat If this has the wrong BFC winding, you can just flip (mirror) the (...) (21 years ago, 30-Dec-03, to lugnet.cad.dev)
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