Re: [cgkit-user] confusion over cg-kit's mat4 format

[Nathaniel A. <nat...@gm...>]

Thanks for the reply Matthias,
After sleeping on this debacle it had dawned on me that the transpose at the
end (then asking for the rows) was pointless.  At the same time I hadn't
caught that I was copying the mat4, just to get another copy again with the
transpose().  Good catch.  Sadly, I had originally expected it would take a
10 - 20 minutes to set up a test scene and learn how to get the data in and
out of Maya...instead it took the day.  *sigh* How could something so simple
become one of my more frustrating scripting experiences.  I think this is
right up there with my pain of doing a custom grid control in wxPython, but
that's another story :)

Nathaniel




On Sun, Apr 3, 2011 at 12:59 PM, Matthias Baas <mat...@gm...>wrote:

> Hi Nathaniel,
>
> it seems you got it all working now, but just to recap:
> Whenever you deal with separate packages that use matrices to store
> transformations, you need to check what their conventions are with
> respect to how points are transformed. You can transform them by either
> multiplying them to the left or the right of the matrix:
>
> 1) Maya: transformedPoint = point*Matrix
>   (in this case, the translation part is the 4th *row* of the matrix)
>
> 2) cgkit: transformedPoint = Matrix*point
>   (in this case, the translation part is the 4th *column* of the matrix)
>
> It's straightforward to switch from one convention to the other, you
> just have to transpose the matrix.
>
> If those packages store matrices as flat lists instead of dedicated
> matrix objects, then there's an additional stumbling block. You need to
> know the order of the values, they are either in column-major or
> row-major order (which has nothing to do with the above convention about
> the order of the multiplication). If you get this wrong, you end up
> having a transposed matrix. If you get both things wrong (the above
> transformation convention and the value ordering), you are actually fine
> again as one mistake cancels out the other one (which may actually
> further increase the confusion).
>
> > def cgkit_matrix( xform_list ):
> >     """
> >     takes a 16 element list (assumed to be the result of a xform query)
> > and returns a cgkit.cgtypes.mat4 in proper form
> >     """
> >     matrix = cgkit.cgtypes.mat4( xform_list )
> >     matrix = matrix.transpose( )
> >     return matrix
>
> That looks correct to me.
>
> > def maya_xform_list( matrix ):
> >     """
> >     converts a cgkit.cgtypes.mat4 into a row list that can be used with
> > mays.cmds.xform.
> >     The original matrix is left unaltered.
> >     """
> >     #create a copy of the original matrix
> >     matrix = cgkit.cgtypes.mat4( matrix )
> >     matrix = matrix.transpose( )
> >     return matrix.toList( rowmajor = True )
>
> The transpose() method returns a copy anyway, so there's no need to copy
> the matrix at the beginning.
> Also note that this function could even be far simpler, you could just
> return matrix.toList() (using the default column-major order) in which
> case the transpose will be done implicitly by toList().
>
> - Matthias -
>
>
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