The 'bad' nodes are points that are co linear with a previous segment on the
same contour. When this happens, the even-odd algorithms used for clipping
only see one intersection instead of two. So, as a scanline is evaluated,
if we were currently outside a poly, we would then determine we were inside,
when it should have gone back to outside.
In papillon's terragear repo, we have integrated clipper, which is an
actively supported clipping routine that has far fewer errors, and is faster
as well.
I still hit these bad nodes often still, however. I am experimenting with
nudging these nodes perpendicular away from the segment they touch instead
of removing them completely. Will update when if I get consistent results.
On Oct 16, 2011 7:22 AM, "Maxime Guillaud" <m...@mguillaud.net> wrote:
> On Sat, 15 Oct 2011 08:53:45 +0000 (UTC)
> Martin Spott <martin.sp...@mgras.net> wrote:
> > I think the only solution is to make GPC obsolete - either by replacing
> > GPC by something different but functional equivalent or "simply" (TM ;-)
> > by avoiding any polygon clipping in 'fgfs-construct' overall.
>
> For what it's worth... I am currently trying to generate custom scenery for
> all of Europe
> based on Corine + OSM data (similar to what I did for France some time ago:
> http://wiki.flightgear.org/Custom_France_Scenery), and I am also getting
> lost in the
> intricacies of fgfs-construct and crashes in GPC.
>
> I managed to make a few changes in the code that improved the situation:
> - conversely to what is suggested in README.gpc, I did not change the
> GPC_EPSILON constant
> (left it defined as DBL_EPSILON)
> - I completely disabled the removal of so-called "bad nodes" in
> poly_support.cxx
>
> However I did a lot of preprocessing of the data in Postgis before feeding
> it to
> fgfs-construct, hopefully removing the need for the hacks mentioned above.
>
> So far this configuration has proved very stable for me, and I was able to
> generate
> more than a hundred of 1x1 deg tiles without any crash.
>
> So far the only problems that remain seem to be located around the 0deg
> longitude area -
> there, I am still running into the problem of the NULL pointer in the GPC
> merge_right().
>
> What I noticed so far lends me to think that there is a lot of code in
> terragear that is
> supposed to deal with imperfect input data - and should maybe be
> deactivated.
> Can anyone shed some light on what the "bad node" detection is supposed to
> achieve, for
> instance ? Also, the suggested increase of GPC_EPSILON is bound to
> introduce
> inconsistencies in the geometric routines - much better to clean up the
> input data).
>
> Unfortunately, there are many places in terragear where some arbitrary
> "epsilon" constants
> are hiding with unexpected effects - just run "grep -r '0\.0000'
> terragear-cs" and count
> the matches if you don't believe me.
>
> So maybe the problem is not in GPC after all but in what we feed it....
>
> Maxime
>
>
>
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