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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Thu, 23 Aug 2007 00:04:02 GMT
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Viewed:
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5604 times
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In lugnet.cad, Timothy Gould wrote:
> 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
If I were new to 3D graphics (and I am ) I would be confused by this, I would
have to ask someone else how to interpret the spec. I think Travis' proposal to
show the two different matrices that can be built from the line of values is
more clear than the equation. Like they say, a picture is worth a thousand
words, or in this case, a big equation ;)
Rob
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