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 22:05:52 GMT
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Viewed:
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5063 times
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In lugnet.cad, Leonardo Zide wrote:
> The API doesn't care if you use column major or row major matrices, it just
> expects an array of 16 floats with the translation at the end. The column major
> notation you see on the manual is just an example that a lot of people take as
> the only way to do things when it's not.
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".
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 same is true for LDraw, it's not really column or row major. All you need
> to explain is that line type 1 is an array of floats that form a matrix and
> you're using a column major notation and post multiplying the matrices for
> reference in the spec but the LDraw standard is not column major, you can get
> the same exact results by pre multiplying using row major matrices.
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.
--Travis
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