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

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

Forwarding HBB's response to the list...

(and my response below)


Begin forwarded message:

Date: Fri, 28 Nov 2014 19:31:46 +0100
From: Hans-Bernhard Bröker <HBB...@t-...>
To: "Philipp K. Janert" <ja...@ie...>
Subject: Re: "Polygon not closed" warning - rounding error?


Am 28.11.2014 um 18:58 schrieb Philipp K. Janert:
> If I issue the following two commands (in a fresh 5.0rc3 session):

FWIW, 4.6pl6 Win32 does the same.

> 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

> then gnuplot complains that the second polygon is not closed (and
> that gnuplot is adding a vertex).

> Is this due to internal roundoff, so that the endpoint != startpoint
> in floating point arithmetic?

Somewhat obviously: yes.  This is a relatively clear example of

	In computing, 10 times 0.1 is hardly ever 1.0

> It did confuse me for a while (did I do something wrong?). Might it
> be possible to suppress such warnings?

Only at the cost of getting a non-closed polygon, which would be a good 
deal worse.


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
not look at the implementation), then it would 
be possible to suppress such warnings if the 
length of the "missing vertex" is sufficiently
small (compared to the rest of the polygon?).

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