I am not certain that I understand your last comment, but perhaps the following may be of interest:
https://docs.python.org/2/faq/design.html?highlight=self#why-are-floating-point-calculations-so-inaccurate
https://docs.python.org/2/tutorial/floatingpoint.html#tut-fp-issues

-u



---- On Sat, 07 May 2016 21:41:20 +0000 Brynn <brynn@...3089...> wrote ----
Interesting! Yes, there are several situations where you can make Inkscape
perform a little better by adding extra nodes ;-)

Well, in this case, I didn't need absolute precision. I would have liked
it. But that was good enough for this project. (I just use Inkscape as a
hobby.)

This is another subject, but I recently learned that snapping is somewhat
less than precise. Apparently snapping can be off by 0.00001 to 0.0001.
Maybe it's what I've seen referred to as "rounding errors", I'm not sure.

Anyway, thanks again!

___________________
From: ugajin
Sent: Saturday, May 07, 2016 4:29 AM
To: Brynn
Cc: Inkscape User Community
Subject: Re: Re: [Inkscape-user] Difference difference


I am surprised, you were satisfied with the work around. The 'answer' turns
out to be one those things that seem obvious after the fact. If you double
the number of node you get a better result! Stricly speaking you only need
to double the node count for the intersecting segments. Also, it may or may
not not surprise you, but you need to add nodes to both objects.

See grab, as proof of concept.

-u




---- On Sat, 07 May 2016 04:07:19 +0000 Brynn <brynn@...3089...> wrote ----

Hi u,
No, I don't need a solution for the Difference problem. I already
made it work using your first idea.
But, I'm always interested to learn new things! What did you find?

brynn
_________________________
From: ugajin
Sent: Wednesday, May 04, 2016 4:53 PM
To: Inkscape User Community
Cc: Inkscape User Community
Subject: Re: [Inkscape-user] Difference difference




Finally found an answer to this. Do you still need a solution?

-u


---- On Sun, 24 Apr 2016 20:33:31 +0000 ugajin <ugajin@...3218...> wrote ----

I hadn't noticed the small remaining overlap as it was hidden by the
stacking order.

-u


---- On Sun, 24 Apr 2016 17:58:39 +0000 Brynn<brynn@...3089...> wrote ----

Hi C R,
I just realized something. I use a custom default doc with
landscape orientation. Could this be related to the transform attribute
issues?
I was one second away from posting the bug report when I thought of
it. Do you think it should still be posted, given the new info about custom
default doc? Or would that explain it?

Btw, ugajin is right about reversing the subpaths. It improves the
situation, resulting in less overlap. But there's still a tiny bit of
overlap.

(And C R, in my previous message about positioning, I think I probably
misunderstood you. Both rings still sit at the same x and y coordinates as
before the Difference. So nevermind about that comment :-))

Thanks,
brynn
_____________________________
From: C R
Sent: Sunday, April 24, 2016 9:27 AM
To: Inkscape User Community
Subject: Re: [Inkscape-user] Difference difference


Heh, interesting! I'm getting a new and different offset error in inkscape
trunk.
It does not appear to be a problem with the difference boolean op,
(fortunately) but rather the position of the result is off by a little.


I'm not sure what causes this, but I suspect it's a roundoff error to do
with the precision.
To fix, simply erase the shapes directly behind your result, or use snapping
to snap the


It is a bug though. Whether it's one that can be easily fixed, I have no
idea. :)


I say file a bug report for it.


My 2p.
-C




On Sun, Apr 24, 2016 at 3:19 PM, Brynn <brynn@...3089...> wrote:

Thanks C R.

That was the first thing I thought too, at first. The bottom object must
have a stroke. But as I said, they are both filled with no stroke.

Here's link to SVG file: http://inkscapecommunity.com/jd/imgs/diff-diff.svg
. It has the rings already duplicated. So to reproduce, select the top
green and top gold rings, and do Difference.

Thanks again :-)
brynn

___________________________________
From: C R
Sent: Saturday, April 23, 2016 12:05 PM
To: Inkscape User Community
Subject: Re: [Inkscape-user] Difference difference



Could not duplicate the error, however if you have a stroke applied to your
shapes that is the same colour as your fill, that may explain what we are
seeing here. :)


Boolean shape ops are done to fills only, and do not take into account line
thickness on the resulting paths (this is intentional).


-C


On Sat, Apr 23, 2016 at 6:21 PM, Brynn <brynn@...3089...> wrote:

Hi Friends,
It seems like I should know the answer to this. But I don't recall
hearing about it.
Starting with these 2 rings, which have fill and no stroke:
http://inkscapecommunity.com/jd/ss/diff1.png
I duplicate both, and then Difference. At 100% zoom they look ok.
But zoom in (in this case, all the way, for best view) and you can see that
the Difference wasn't correct. http://inkscapecommunity.com/jd/ss/diff2.png
It's not a huge amount, but I would expect there shouldn't be any
difference at all. This on Windows 7, 64-bit, sp1, Inkscape 0.91, 32-bit,
exe installer.
Is that a bug? Or is there some reason it's expected not to be
precise?

Thanks for your help :-)
brynn


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