Re: Fw: "Polygon not closed" warning - rounding error?

[Karl R. <ra...@un...>]

Am 28.11.2014 um 19:38 schrieb Philipp K. Janert:

> Am 28.11.2014 um 18:58 schrieb Philipp K. Janert:

>> set object 1 polygon from -0.9,0.9 rto 0.8,0 rto 0,-0.8 rto -0.8,0
>> rto 0,0.8 set object 2 polygon from  0.1,0.9 rto 0.8,0 rto 0,-0.8 rto
>> -0.8,0 rto 0,0.8

Why not leave out the last step? The polygon closes itself by itself.

>> then gnuplot complains that the second polygon is not closed (and
>> that gnuplot is adding a vertex).
> ....
> PKJ: I disagree. If the warning is in fact due
> to round-off (which I don't know - it might be
> due to a bug in the code, for instance; I did

It´s rounding :

gnuplot> print 0.1 + 0.8 - 0.8 - 0.1
-2.77555756156289e-017

> After all, using straight equality in floating
> point arithmetic is rarely the right thing to do.

True. That´s why a polygon should get closed by returning to the origin,
not to a place that would be the origin if we had infinite precision.

So i´d say there´s nothing to be done about this, except that "set
object polygon" could get an additional "close it" option, that
suppresses the extra-vertex warning.

  Karl