Re: [Lib2geom-devel] FW: [Inkscape-devel] Broken feImage and lib2geommatrix inversion

[<J.B...@ew...>]

Looking at the code, it seems that it used to be possible to specify the
"numerical tolerance". Shall we put this back in?
 

> -----Original Message-----
> From: J.B...@ew... 
> [mailto:J.B...@ew...] 
> Sent: maandag 31 augustus 2009 9:57
> To: lib...@li...
> Cc: tav...@fr...
> Subject: [Lib2geom-devel] FW: [Inkscape-devel] Broken feImage 
> and lib2geommatrix inversion
> 
> Hi guys,
> 
> See the email below. 
> 
> > -----Original Message-----
> > From: Tavmjong Bah [mailto:tav...@fr...]
> > Sent: Sunday, August 30, 2009 13:10
> > To: Inkscape Devel List
> > Subject: [Inkscape-devel] Broken feImage and lib2geom 
> matrix inversion
> > 
> > 
> > 
> > Hi,
> > 
> > 	I've traced the problem with the broken feImage filter 
> primitive (see
> > https://bugs.launchpad.net/inkscape/+bug/382313 ) to a problem with 
> > lib2geom. More specifically,
> > Geom::Matrix.inverse() returns an incorrect matrix. I run 
> this simple 
> > test program:
> > 
> > #include <iostream>
> > #include <2geom/matrix.h>
> > 
> > int main() {
> > 
> >   Geom::Matrix test_matrix1( 0.003333, 0.000000, 0.000000, 
> 0.003333, 
> > 0.083333, 0.083333 );
> > 
> >   Geom::Matrix test_inverse1 = test_matrix1.inverse();
> > 
> >   std::cout << "Test input" << std::endl;
> >   std::cout << test_matrix1 << std::endl;
> > 
> >   std::cout << "Test output" << std::endl;
> >   std::cout << test_inverse1 << std::endl;
> > 
> >   Geom::Matrix test_matrix2( 0.003077, 0.000000, 0.000000, 
> 0.003077, 
> > 0.076923, 0.076923 );
> > 
> >   Geom::Matrix test_inverse2 = test_matrix2.inverse();
> > 
> >   std::cout << "Test input" << std::endl;
> >   std::cout << test_matrix2 << std::endl;
> > 
> >   std::cout << "Test output" << std::endl;
> >   std::cout << test_inverse2 << std::endl; }
> > 
> > The output I see is:
> > 
> > Test input
> > A: 0.003333  C: 0  E: 0.083333
> > B: 0  D: 0.003333  F: 0.083333
> > 
> > Test output
> > A: 300.03  C: -0  E: -25.0024
> > B: -0  D: 300.03  F: -25.0024
> > 
> > Test input
> > A: 0.003077  C: 0  E: 0.076923
> > B: 0  D: 0.003077  F: 0.076923
> > 
> > Test output
> > A: 1  C: 0  E: 0
> > B: 0  D: 1  F: 0
> > 
> > You can see that the first matrix inversion is correct but 
> the second 
> > is wrong. This was tested with SVN as of yesterday on Linux.
> > 
> > A bit more info.
> >
> > In Geom::Matrix::inverse() the matrix determinate is calculated and 
> > then compared to the constant EPSILON which is defined as 10^-5 (in 
> > coord.h).
> > If it is less than EPSILON the matrix is set to unity. This 
> EPSILON is 
> > clearly way too large for use in inverse(). For some reason 
> a value of
> > 10^-18 in commented out in coord.h. My guess is that the 
> value 10^-5 
> > was selected for comparing pixel positions to each other but it is 
> > being used for other purposes in the lib2geom library.
> >
> > Tav
> > 
> 
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