Subject:
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Re: OK here's the real problem! (Was: Re: Finding a rotation matrix for a triangle)
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Newsgroups:
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lugnet.cad
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Date:
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Tue, 6 Dec 2005 16:59:31 GMT
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Viewed:
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1836 times
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In lugnet.cad, Niels Bugge wrote:
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Well, I think I’ve tried to explain what I need to do 3-4 times already,
which is where I think the language barriers have played a pretty big role...
The reason I tried to cook the problem down, is to reduce confusion, but I
guess that was a bad idea if my ideas went in the wrong direction ;-)
Regarding the formula you suggested, I’m afraid that I can’t use it: I can’t
just add the primitive via text, because I need to turn it around the
y-axis before I get to the problem I posted about here. And although
possible, scaling afterwards would be difficult.
I think I need to adopt a more direct approach, so I’ll try to explain
excactly what I need to do:
1 14 -30 4 10 0 0 1 0 -5 0 -1 0 0 stud3.dat
4 4 -24 4 -16 -24 20 -46 -36 20 -10 -36 4 20
2 2 -36 4 20 -24 4 -16
1 2 -30 4 10 -3.69552 0 -1.53073 0 1 0 1.53073 0 -3.69552 1-4edge.dat
I have this stud3 that passes through the wonky quad (line 1 and 2). I need
an edge between the two of them, and want to use the 1-4edge for that
(line4).
Note that I’ve already scaled and turned the edge 22.5 degrees around the y
axis.
What I need now, is to turn the edge down at the same angle as the surface of
the quad, and since I don’t want to loose the previous two steps (scaling and
first angling), I have to do it via the rotation vector command in MLCAD “on
top” of them.
As far as I can see, what you’re suggesting is simply to enter a new rotation
matrix via notepad, but that will of course delete the two other nessecary
steps, unless you use a much more complex formula that includes the effect of
all three steps.
I hope that it’s more clear now, and that I got some of the worms back inside
the can...
Cheers NB
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I think my interpratation from the last time was halfway there and this explains
it much better so we’re definitely getting somewhere faster now :).
You should be able to use the formula I gave in the last reply instead of an
XYZ+angle in MLCAD as they are both in the same command box (without the
translation bit which you can then do by hand). If you can’t use one, then the
other one won’t work so simply either I suspect.
Another trick would be to multiply the two matrices by hand (or a little script)
and enter that. The problem is that XYZ+angle vectors aren’t additive under most
circumstances (ie. you can’t just add two of them and get the same result as
performing one and then the other).
The short answer is that there is no short answer to your problem. Rotations in
3D are rarely simple due to the non-commutative nature of them (ie. doing one
rotation and then another is not always the same as the reverse order).
I’ll have more of a think about this specific problem and see if I can come up
with some sort of formula but I fear it will be a one use solution.
Tim
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