Special:
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[DAT] (requires LDraw-compatible viewer)
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Subject:
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It's all clear now...
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Newsgroups:
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lugnet.cad.dev
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Date:
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Tue, 5 Sep 2000 15:28:19 GMT
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Viewed:
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653 times
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This is one is for parts authors, I suppose...
I never understood the relation between the original sloped cylinder
primitives--1-4cyls.dat, 2-4cyls.dat--and the newer 1-4cyls2.dat. Both
sets of files are the result of intersecting a cylinder with an angled
plane. The difference is the angled plane.
(Suggestion: open both 1-4cyls.dat and 1-4cyls2.dat in L3Lab. Turn up the
zoom to 12800, deselect BFC, select Show Axes, and choose random colors.)
For x-4cyls, the intersecting plane is Y = X - 1.
For 1-4cyls2, once the primitive is rotated around Y -90degrees (into the
X+/Z+ quadrant), the intersecting plane is Y = Z.
No big mathematical insights here. Just two relationships to note:
1. 1-4cyls and 1-4cyls2 are complements. If you put them together the
right way, they make a 1-4cyli. Like this:
1 15 0 0 0 1 0 0 0 1 0 0 0 1 1-4cyls.dat
1 14 0 1 0 -1 0 0 0 -1 0 0 0 1 1-4cyls2.dat
Which is pretty cool, and it means that for any cylinder-plane intersection
(where the plane is parallel to the X or Z axis), these primitives will fit
the intersection.
2. This is sort of a corollary. You can use multiple copies of 1-4cyls and
1-4cyls2 to make a 4-4cyls. Like this:
1 14 0 0 0 1 0 0 0 1 0 0 0 1 1-4cyls.dat
1 4 0 1 0 1 0 0 0 1 0 0 0 1 1-4cyls2.dat
1 3 0 1 0 1 0 0 0 1 0 0 0 -1 1-4cyls2.dat
1 2 0 0 0 1 0 0 0 1 0 0 0 -1 1-4cyls.dat
1 1 0 0 0 0 0 -1 0 1 0 1 0 0 2-4cyli.dat
So 1-4cyls2 is hidden in 2-4cyls -- it's the upper section of the sloped
area. Like this:
1 16 0 0 0 1 0 0 0 1 0 0 0 1 1-4cyls.dat
1 16 0 0 0 0 0 -1 0 1 0 1 0 0 1-4cyli.dat
1 14 0 1 0 1 0 0 0 1 0 0 0 1 1-4cyls2.dat
Maybe this was discussed before, and I've forgotten since then. I stumbled
across it while making the minifig skeleton leg, and I thought I'd post it,
in case anyone else finds it useful.
Steve
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Message has 3 Replies:
 | | Re: It's all clear now... (how to use 1-4cyls2.dat)
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| (...) Thanks for explaining this again Steve - I am sure I tried to explain this when I created this primitive in Oct 1998. But it is really hard to get ones head around - it took me some time playing with the *cyls.dat files before I realised I (...) (24 years ago, 5-Sep-00, to lugnet.cad.dev)
| | | Re: It's all clear now...
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| Boy, talk about timing. I'm currently in the process of adding primitive substitution to LDView, and the ?-4cyls* primitives were next on my list. However, while I could definitely tell they were different (one slope appears concave; the other (...) (24 years ago, 6-Sep-00, to lugnet.cad.dev)
| | | Re: It's all clear now...
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| "Steve Bliss" <steve.bliss@home.com> wrote in message news:qn1arso26cua6dm...4ax.com... (...) Like Chris, I spent probably an hour rotating, inverting, skewing the cyls.dat primitives before coming to the conclusion that I needed something else. I (...) (24 years ago, 11-Sep-00, to lugnet.cad.dev)
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