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

[Philipp K. J. <ja...@ie...>]

On Fri, 28 Nov 2014 20:41:06 +0100
Karl Ratzsch <ra...@un...> wrote:

> 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.

[snip]

> 
> > 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.

Well, I am questioning the purpose of the warning,
in particular if it is implemented using straight
floating-point equality. (I'd even argue that 
a floating-point comparison using '==' amounts to
a "bug".)

I think that there are in fact two options:
- the close-it option you suggest (which would be a
  new feature)
- suppressing the warning if the length missing 
  vertex is "small" compared to numerical accuracy
  (which would be proper floating point programming)

> 
>   Karl
> 
> 
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