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 23:54:08 GMT
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Viewed:
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5158 times
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In lugnet.cad, Travis Cobbs wrote:
> In lugnet.cad, Rob Ross wrote:
> > What I meant was, I am confused by the (apparent) difference in what the
> > *current* spec says on this issue, and what the new proposed spec says.
> >
> > If you will look at the current spec and scroll down near the end to the Line
> > Format header, you'll see this:
> >
> >
> > Line Format:
> > 1 colour x y z a b c d e f g h i part.dat
> >
> > Fields a through i are orientation & scaling parameters, which can be used in
> > 'standard' 3D transformation matrices. Fields x, y and z also fit into this
> > matrix:
> >
> > | a d g 0 |
> > | b e h 0 |
> > | c f i 0 |
> > | x y z 1 |
> >
> >
> >
> > Which seems to be an example showing column-major ordering.
>
> Yes, but look where the x, y, and z are. The example matrix is transposed vs
> the "standard" one shown in OpenGL text. If you swap the rows and columns in
> the above matrix, you'll get the OpenGL one. It's an alternate way of
> expressing a 3D matrix that assumes a different layout for the vectors that get
> transformed, but the results are the same. That's what I was saying. In my
> response to Leonardo I provided a proposed clarification. Let me know what you
> think of it.
>
>
> > So my question about this is, is the spec really changing the ordering of all
> > matrices, OR was this just wrong in the original spec?
>
> Neither. It's just a question of wording. As long as you put the X, Y, and Z
> in the appropriate place (and the 0s in the other appropriate spots), and as
> long as you do your transformations correctly, both ways produce the same
> results.
>
> --Travis
I think a full worked through example would do the specs a lot of good. If I
understand right the line
1 c x y z a b c d e f g h i part.dat
transforms any point by the operation
(u, v, w)->(x+a*u+b*v+c*w, y+d*u+e*v+f*w, z+g*u+h*v+i*w)
With it spelled out explicitly like that a programmer can use whatever internal
format they like.
Tim
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