Boolean operations move object position

[primexx]

When using any of the path tools (e.g. union, difference, etc.), after doing the operation the remaining object is moved downward (Y-axis) by 0.013.

Windows 8.1x64 Inkscape 0.48.5 fresh install. Attached is image of steps to reproduce.

[ScislaC (scislac)]

Reproduced with 0.48.4 & Inkscape trunk. It seems to be worse with the 0.48 series than trunk. It also seems to be dependent on the object location on the page as to how much it will offset after the operation in 0.48.x.

If the object is at 0x0, in 0.48.4 it will move Y -.013, in trunk it will move Y -.001. In 0.48.4 if I move the objects a little above the page, it seems to move Y -.02.

[ScislaC (scislac)]

Attaching test file.

[Alvin Penner (apenner)]

all of the boolean path operations depend upon the idea that the original object will be "snapped" onto a grid before performing the boolean operation. In the case of the x coordinate this may not be a problem, if the original x origin was an integer. In the case of the y coordinate this normally will be a problem because the y coordinates get inverted during the calculation, and if the page height is not a perfect integer then the inversion will lead to round-off error.
    The latest reference I could find on this is:
http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/40786
   and bzr rev 12420.
    The original size of the grid was 1/32 pixel = 0.03125
    The current size of the grid, in trunk, is 1/512 pixel = 0.00195
    These numbers look to be consistent with the above comments.