A problem with the pdf cairo backend was found in inkscape, see bug https://bugs.launchpad.net/inkscape/+bug/320646
The attached program reproduces this behaviour. It does basically the following:
draw a circle
draw another circle at same place as first
One would expect that both circles are shown at the same place, which is not the case. It depends on the page size, whether the second circle is at the correct position or not.
Created attachment 27055 [details]
Program to verify bug
Just an update: the error is a non-trivial rounding of surface sizes when mixing the device-transform and cairo-to-pdf matrices. My thoughts towards a solution lie both along improving the internal interfaces to communicate non-integer extents (though, those extents tend only to be related to rasterisation/pixels anyway...) and converting pdf to setup its device-transform more appropriately (which doesn't immediately address this issue, but might be a step towards a fix).
I've added cairo/test/group-unaligned.c to capture this bug. Thanks.
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