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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Wed, 4 Oct 2000 20:00:14 GMT
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Viewed:
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576 times
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I believe the matrix happens to produce a standard isometric view, but
doesn't do so in the traditional 3D graphics way of rotation. This means
that in addition to not looking right after rotation, it also doens't
produce correct results when you apply a perspective projection to it
(AFAIK). The others most likely also produce "standard" projections
visually, but don't necessarily represent something that is possible in the
real world.
The default matrix I use in LDView is equivalent to your oblique one (but
for some odd reason, with rows/columns swapped, assuming you wrote your
matrices in the same order that LDRAW expects, ie column major). I actually
had a difficult time getting the model rotated around to this angle.
If anyone cares about maximum accuracy, the irrational numbers in the matrix
are:
.707104 = sqrt(2) / 2
.353553 = sqrt(2) / 4
.866025 = sin(pi / 3)
.612372 = sqrt(1.5) / 2
--Travis Cobbs (tcobbs@san.REMOVE.rr.com)
"Don Heyse" <dheyse@hotmail.spam.go.away.com> wrote in message
news:G1ws1M.1ty@lugnet.com...
> Hey, does anybody out there know the origin of the "Oblique",
> "UpsideDown", and "Natural" viewing matrices in LDRAW.INI. I'm not
> sure whether these came from LDAO, LDLITE, or perhaps even LDRAW?
> Wherever they came from, they don't seem to be pure rotation matrices
> fit for a viewing transformation. When I multiply one of these with
> another rotation matrix the model starts to look sheared. Is there a
> projection transform in there or are they just plain wrong? I believe
> someone previously mentioned that there was something unusual about
> these matrices but I can't remember what.
>
> Anyways, the point is I finally got that pesky mouse-drag model
> spinning trick to work in ldglite but I had to replace these three
> view matrices to do it.
>
> Also I noticed at one point that L3LAB seemed to shear the model
> sometimes during spinning. Does/Did it perhaps use the view matrices
> from LDRAW.INI as well?
>
> Here are the original matrices.
> Oblique=1,0,1,0.5,1,-0.5,-1,0,1
> UpsideDown=-1,0,1,-0.5,-1,-0.5,1,0,0
> Natural=0.625,0,1.075,0.5375,1.25,-0.3125,-1.25,0,2.5
>
> Here are the replacement matrices.
Oblique=0.707104,0,0.707104,0.353553,0.866025,-0.353553,-0.612372,0.5,0.6123
72
UpsideDown=0.707104,0,0.707104,-0.353553,-0.866025,0.353553,0.612372,-0.5,-
> 0.612372
> Natural=0.5,0,0.866025,0.433013,0.866025,-0.25,-0.75,0.5,0.433013
>
> My Oblique matrix is a 45 degree rotation about y followed by 30 about x.
> My UpsideDown matrix is a 45 degree rotation about y followed by 210 about • x.
> My Natural matrix is a 60 degree rotation about y followed by 30 about x.
>
> Don
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Message has 1 Reply:
 | | 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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Message is in Reply To:
| | Unusual View matrices in LDRAW.INI
|
| Hey, does anybody out there know the origin of the "Oblique", "UpsideDown", and "Natural" viewing matrices in LDRAW.INI. I'm not sure whether these came from LDAO, LDLITE, or perhaps even LDRAW? Wherever they came from, they don't seem to be pure (...) (24 years ago, 4-Oct-00, to lugnet.cad.dev)
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