Subject:
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Re: rotation matrices
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Newsgroups:
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lugnet.cad.dat.parts
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Date:
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Tue, 17 Oct 2000 21:07:14 GMT
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Viewed:
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999 times
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> > Hello,
> > Does anyone have more information about those 9 numbers that make up the
> > rotation matrix for a part in the dat file?
>
> Could you be more specific?
Well, I copied the coordinates of one side of the axlehol8 dat. This gives
me a plane that is perpendicular to the Y-axis. I then had to position this
plane on points of the curves and perpendicular to that curve. To get the
direction I substracted the coordinates of 2 following points wich gives me
a [a b c] vector to which the plane should be parallel. The rotation matrix
I created was then A=[b/l -a/l 0; a/L b/L c/L; -a/l*c/L -b/l*c/L l/L] where
l=square root(a^2+b^2) , L=square root(a^2+b^2+c^2) and ;means next row. I
don't really know if this is correct but the results seem to be right.
Now, can I use these same numbers given by this matrix to position a part
the correct way?
(I kind of derived my matrix from the top half of
http://mathworld.wolfram.com/EulerAngles.html , so is this the correct way
to do it in Ldraw . Or is there an other kind used since the axes are
rotated ?)
In the Ldraw faq it also says something about scaling. How is put in to the
matrix?
> Have you checked your file for duplicate or extraneous codes? That can
> happen with big files.
I think the code is correct, one line gives me one triangle and all the
triangles are different.
> A couple of things you might be able to do to control filesize:
>
> 0. Make each segment *really* short, and just use axlehol8.dat for each
> segment.
I don't really like this solution since it will create an awfull lot of
triangles wich would take it a larger time to render.
And I like spinning things 'round and 'round in Ldview a lot, so I can't do
this.
> 1. Put repeated segments into a subfile. If you got a section of the part
> that's got a fixed angle, each segment should be the same shape. If you
> turn this segment into a subfile, you can reuse the subfile as much as you
> want, while only adding one line to the main file.
There are no repeated parts since I don't actually use axlehol8.dat , I just
use its dimensions and then draw triangles along the outside of the figure
between two following figures. This gives me a nice and smooth axle wich
keeps on looking good even when zoomed in .
I could of course use subparts for the begin and end piece and for those 6
cilinders that are located at the beginning and end.
Can I put these in the same file some way? If yes: how?
I guess putting a cilinder on top of the axle wouldn't cause any problems.
> 2. One thing I've done with bent cylinders is to build each straight
> segment out of two sloped cylinders with a regular cylinder in the middle.
> Each segment ends up looking something like this:
> _________
> \ | | /
> \|___|/
>
> Except the sloped parts aren't usually that extreme. After each segment • is
> constructed, it's a simple matter of arranging the segments so the angled
> ends meet cleanly.
>
> This approach could work on a crossaxle, except that you would need to
> construct the 'sloped crossaxle' subpart. And as long as you are either
> bending in 2 dimensions, or you have a very regular curve.
>
> Steve
I can't figure the drawing out, it must be deformed by my newsreader.
but are you perhaps meaning to make subparts like those used in the
hose2.dat
on http://www.math.uio.no/~fredrigl/technic/ldraw-mode/ .
here every piece fits the next as long as the curve isn't too bend.
I could see the same thing for the ribbed things, where a large and a small
cilinder could be used , with the small cilinder continueing into the large
one so it also becomes a closed shape.
I think that for this curved axle (and sortlike airtubes and flexcables) I
will stick to the large files, since the difference when zoomed into is
pretty large with the one on Fredrik Glockner 's page.
thx
Koen
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Message has 1 Reply:
 | | Re: rotation matrices
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| (...) I briefly scanned the page you mentioned, it looks like it should be OK for LDraw. Paul Gyugyi uses a general rotation method for LDLite's ROTATE meta-statement, and I've implemented an equivalent in LDAO's LDL2DAT translator. But I don't know (...) (24 years ago, 18-Oct-00, to lugnet.cad.dat.parts)
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Message is in Reply To:
| | Re: rotation matrices
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| (...) Could you be more specific? (...) Like what? Again, could you be more specific. (...) There's no general limit. (...) That looks very nice. It's not too surprising you're getting a big file. Have you checked your file for duplicate or (...) (24 years ago, 17-Oct-00, to lugnet.cad.dat.parts)
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