Subject:
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Re: LDraw File Format Spec 1.0 DRAFT - Call for Public Comments
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Newsgroups:
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lugnet.cad
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Date:
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Wed, 22 Aug 2007 16:10:43 GMT
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Viewed:
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4709 times
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In lugnet.cad, Rob Ross wrote:
> I came across the *current* LDraw file spec here:
>
> http://www.ldraw.org/Article45.html
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> and the matrix example is using column-major ordering. So now I'm really
> confused. Does the current spec use column-major, and the new spec is proposing
> to change it? Or is the current spec wrong in using that column-major example?
> If it's really a change in the spec, won't that break everyone's existing code?
From the standpoint of OpenGL, it uses row-major ordering. There are two ways
to draw a transformation matrix. OpenGL uses the following (array offsets shown
after):
A D G X 0 4 8 12
B E H Y 1 5 9 13
C F I Z 2 6 10 14
0 0 0 1 3 7 11 15
Sometimes it's done like so, though:
A B C 0
D E F 0
G H I 0
X Y Z 1
(The 0 0 0 1 can of course be other numbers, but generally aren't in 3D.)
Both forms work; one requires a 4-tall vector for points, and the other requires
a 4-wide vector for points.
Given the following part line:
1 16 X Y Z A B C D E F G H I stud.dat
The OpenGL matrix will look like so:
A B C X
D E F Y
G H I Z
0 0 0 1
--Travis
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