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.C.Engelen@...1578... [mailto:J.B.C.Engelen@...1578...] Sent: maandag 31 augustus 2009 9:57 To: lib2geom-devel@lists.sourceforge.net Cc: tavmjong@...8... Subject: [Lib2geom-devel] FW: [Inkscape-devel] Broken feImage and lib2geommatrix inversion

Hi guys,

See the email below.

...

-----Original Message----- From: Tavmjong Bah [mailto:tavmjong@...8...] 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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