Subject:
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Re: It's all clear now... (how to use 1-4cyls2.dat)
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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 17:56:31 GMT
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Viewed:
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1319 times
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In lugnet.cad.dev, Steve Bliss writes:
> 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
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 needed something different. The understanding is not helped by the
inaccurate information currently on my primitive reference at
http://www.ldraw.org/dev/primitive.html. I'll try and fix this.
What is really strange is that I created 1-4cyls2.dat specifically for the
minifig helmet visor (2447.dat), yet the current released version does not use
this primitive, even though my copy of what I submitted does. Now things have
gone full circle and the revised 2447.dat in the current voting does correctly
use this, allowing better rendering by l3p, etc, which is good news.
I agree that the L3Lab random colours option is a good way to see what is going
on but it is still tricky to figure out how to rotate this (and the other cyls
primitives correctly). What I do during authoring is to massively scale up the
primitive and rotate it until it looks right, before scaling it down to the
required size.
Definitely a topic for the parts authoring FAQ.
Chris
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Message has 1 Reply:
 | | Re: It's all clear now... (how to use 1-4cyls2.dat)
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| (...) I kind of figured you had, but I totally didn't remember that you had. (...) Hmm. I wonder if the current 2447.dat was inlined at some point, because 1-4cyls2.dat hadn't been made official yet... (...) Yep. Or, you can copy the primitives to a (...) (26 years ago, 6-Sep-00, to lugnet.cad.dev)
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Message is in Reply To:
| | It's all clear now... [DAT]
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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 (...) (26 years ago, 5-Sep-00, to lugnet.cad.dev)
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