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 22:51:45 GMT
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Viewed:
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5245 times
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On Aug 22, 2007, at 10:05 PM, Travis Cobbs wrote:
> That's true. However, unless I'm badly mistaken, in addition to
> the xyz1 being
> at the end, you also need the other parameters to be in the proper
> direction in
> order to get the results that the LDraw library wants. The
> following two
> matrices are equivalent:
>
> | a d g 0 | | a b c x |
> | b e h 0 | | d e f y |
> | c f i 0 | | g h i z |
> | x y z 1 | | 0 0 0 1 |
>
> The first is "column major", the second is "row major".
Those matrices would be wrong in both cases (assuming xyz is the
translation), you probably swapped those with the next matrices. :)
> However, the following matrices are NOT equivalent to the two above
> (are they?):
>
> | a d g x | | a b c 0 |
> | b e h y | | d e f 0 |
> | c f i z | | g h i 0 |
> | 0 0 0 1 | | x y z 1 |
The first one here is a column major and the second is a row major
matrix, and you are right, they are not the same as the previous 2.
From a programmer's point of view, a column major and a row major
matrix have the same layout in memory: a b c d e f g h i x y z, so
that's why OpenGL does not care what convention you are using.
> Without saying where the xyz go with respect to the other values, I
> don't see
> how this is enough. However, my proposal of just providing the
> first two
> matrices above as examples avoids saying that it's in column major
> or row major
> form.
Saying something like lines "1 col x y z a b c d e f g h i name"
have a matrix with translation = x y z, X axis = a b c, Y axis = d e
f and Z axis = g h i should be enough and would avoid going into the
whole column/row major issue.
If you really want to include a 4x4 matrix like the ones you
posted then you need to clarify if it is a column or row major
matrix. I'd prefer to go with a more neutral way of displaying a
matrix like simply: [a b c 0] [d e f 0] [g h i 0] [x y z 1], this
would avoid the entire row/column issue because you can read it as a
column major or row major matrix.
I believe there's no need to mention column/row major at all in
the specs, and it would be great to avoid it to reduce confusion.
Leonardo
PS: Why does it take 6+ hours to see my messages posted? :(
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