Re: [Openexr-devel] extractScaling from matrices doesn't work with negative scales?

[Benoit Leveau]

Hi Florian,

Yes, this makes sense. Thanks for the detailed answer!

Cheers,
Benoit

Florian Kainz wrote:
> Hi Benoit,
>
> extracting scaling and rotation from a matrix is ambiguous:
> a rotation by 180 degrees around one axis is equivalent to
> scaling by -1 in the other two axes.  The matrix contains
> no information about whether you meant to express rotation
> or scaling when you assembled your matrix.
>
> The Imath functions that extract scaling and rotation are
> written such that you get what you'd expect when your
> matrix has been assembled from rotations and all-positive
> scale operations.  When negative scaling is involved, there
> is no single right answer; the result you get from Imath
> will be a correct representation of the original matrix,
> but it may not be what you expect.
>
> For consistency, Imath::extractScaling() will return the
> same scale factors as Imath::extractSHRT().  In your example,
>
>     M44f m (-1, 0, 0, 0,
>              0, 2, 0, 0,
>              0, 0, 3, 0,
>              0, 0, 0, 1);
>
>     Imath::extractSHRT (m, scale, shear, rotation, translation)
>
> produces
>
>     scale =       ( -1, -2, -3)
>     shear =       (  0,  0,  0)
>     rotation =    (-pi,  0,  0)
>     translation = (  0,  0,  0)
>
> The 180-degree rotation around x compensates for negative y
> and z scale factors.
>
> Hope this helps,
>
> Florian
>
>
>
> Benoit Leveau wrote:
> > Hi,
> >
> > I'm probably doing something wrong, but it looks like if you have a 
> > negative scaling in your matrix the extractScaling function (and other 
> > functions like removeScaling that use it)
> > doesn't work properly.
> >
> > I just tried the following very simple example, and got the same 
> > incorrect result using OpenEXR 1.4 or 1.7.
> >
> > Thanks for your help,
> > Benoit
> >
> > ----------------
> >
> > #include <OpenEXR/ImathMatrixAlgo.h>
> > #include <iostream>
> > int main(int argc, char **argv)
> > {
> >     Imath::M44f m;
> >     Imath::V3f vec;
> >     for (int i=0;i<4;++i)
> >         for (int j=0;j<4;++j)
> >         {
> >             m.x[i][j] = 0;
> >         }
> >     m.x[3][3] = 1;
> >
> >     m.x[0][0] = 1;    m.x[1][1] = 2;    m.x[2][2] = 3;
> >     std::cout <<"matrix = " << std::endl << m << std::endl;
> >     if (extractScaling(m,vec,true))
> >         std::cout <<"extracted scaling:" <<std::endl << vec << std::endl;
> >
> >     m.x[0][0] = -1;    m.x[1][1] = 2;    m.x[2][2] = 3;
> >     std::cout <<"matrix = " << std::endl << m << std::endl;
> >     if (extractScaling(m,vec,true))
> >         std::cout <<"extracted scaling:" <<std::endl << vec << std::endl;
> >
> >     return 0;
> > }
> >
> > ----------------
> > Output:
> > -------
> >
> > matrix = (  1.000000e+00   0.000000e+00   0.000000e+00   0.000000e+00
> >   0.000000e+00   2.000000e+00   0.000000e+00   0.000000e+00
> >   0.000000e+00   0.000000e+00   3.000000e+00   0.000000e+00
> >   0.000000e+00   0.000000e+00   0.000000e+00   1.000000e+00)
> >
> > extracted scaling:
> > (1 2 3)
> > matrix = ( -1.000000e+00   0.000000e+00   0.000000e+00   0.000000e+00
> >   0.000000e+00   2.000000e+00   0.000000e+00   0.000000e+00
> >   0.000000e+00   0.000000e+00   3.000000e+00   0.000000e+00
> >   0.000000e+00   0.000000e+00   0.000000e+00   1.000000e+00)
> >
> > extracted scaling:
> > (-1 -2 -3)
> >
> >
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