ok, that was a numerical instability in the Ligne.cpp file.

Thanks, that one is fixed.

on my box, the second example you sent me didn't produce any errors. it might be that floating point computations are different since it's a PowerPC CPU, but still, it's intriguing

I attach a screenshot. Does it help?

I have found no more rendering problems so far. The only thing I'm concerned about is the speed. Like I said, it's not intolerably bad, but it's noticeably slower than before at high zooms. It would be nice to avoid such regression. Do you think your code can be optimized for speed? Have you tried to optimize it already?

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