Subject:
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Re: Unusual View matrices in LDRAW.INI
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 5 Oct 2000 20:58:00 GMT
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Viewed:
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575 times
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"Don Heyse" <dheyse@hotmail.spam.go.away.com> wrote in message
news:G1xC6x.7nK@lugnet.com...
> OK now we're getting somewhere. Thanks for providing the exact • mathematical
> representations of those numbers. Now I'm gonna have to figure out how • you
> calculated them. :-) (I just had the computer do the work and print out • the
> decimal representations)
To be perfectly honest, I can't really remember how I came up with them. I
think I actually started with the numeric values and--going on the
assumption that they had more mathematically precise definitions--just
figured out what those definitions were. I had some clues to work with;
namely the fact that I expected all the numbers to involve either square
root or a sine/cosine of some even fraction of pi.
> But we still don't know where those "oblique" numbers came from and what
> they're supposed to represent. Should I even bother with them for
> compatibility?
Well, they obviously represent a skew (obvious when you rotate them,
anyway). It may well be that they were just stumbled upon, and don't have
any higher meaning. They produce pleasing results, and they only start to
produce weirdness when you start assuming they are a set of rotations (which
they of course aren't).
> By the way, the row-column order for the matrix came straight out of • ldraw.ini
> and is the way ldlite (and I suppose ldao) represent them. I haven't even
> bothered to check if they're the same as ldraw. I'm far too lazy.
I looked at my code again, and it's even worse. I actually rotate 180
degrees aroung Z, then 180 degrees around Y, and only THEN apply the
rotation matrix (with rows and columns swapped from yours). If I don't
perform the two 180 degree rotations, it ends up looking at a different side
of the model. Since you are using OpenGL, we SHOULD have the same starting
point, I've obviously gone through hoops that you didn't have to. What
exactly do you apply these matrices to?
In LDView, I first translate back along the Z axis a certain distance, then
multiply by my rotation matrix (which is computed as mentioned above with
the two 180 degree rotations), then draw the model. This works fine, but
has the disadvantage that my starting point matrix is different from what
LDraw and the other tools expect, not to mention having the odd 180 degree
rotations. I'm assuming that you are doing something different.
--Travis Cobbs (tcobbs@san.REMOVE.rr.com)
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Message is in Reply To:
 | | Re: Unusual View matrices in LDRAW.INI
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| OK now we're getting somewhere. Thanks for providing the exact mathematical representations of those numbers. Now I'm gonna have to figure out how you calculated them. :-) (I just had the computer do the work and print out the decimal (...) (24 years ago, 4-Oct-00, to lugnet.cad.dev)
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