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[mkgmap-dev] [Patch] Improve boundary splitting

From WanMil wmgcnfg at web.de on Wed May 2 21:46:11 BST 2012

Hi Gerd,
I have some question :-)

1. If the intersect algorithm is noticeably faster it should not be 
buried into the boundary section! Please have a look a the common 
Clipper classes (PolygonClipper, AreaClipper etc.). Maybe it improves 
the overall mkgmap performance if you include the new algorithm in these 

2. Wikipedia points out "Note that if the subject polygon was concave at 
vertices outside the clipping polygon, the new polygon may have 
coincident (i.e. overlapping) edges – this is acceptable for rendering, 
but not for other applications such as computing shadows." Did you test 
if this is a problem?

3. I have posted a first patch to use precompiled sea files. I want to 
(or better I do) use the boundary classes to read and write the 
precompiled files. This will require some changes to them:
- The RASTER size should not be defined by a constant. For sea files I 
want to use a RASTER size 2^n because that reduces some visual artefacts.
- Most of BoundaryUtil must be transferred to a class BoundaryLoader 
which is then no longer static.
- Maybe the BoundarySaver and BoundaryLoader can be splitted to more 
classes to implement the different formats (quadtree and raw).
So if you implement any changes to the existing classes please do not 
introduce more uses of the static methods and constants.

Have fun!

> Hi WanMil,
> attached is a patch that uses an optimized implementation of the
> Sutherland-Hodgman algorithm
> http://en.wikipedia.org/wiki/Sutherland-Hodgeman wikipedia
> which is much faster than the area.intersect. It is only used to create the
> precompiled boundaries.
> The improvements depend on the continent, for africa I see  130 secs with
> r2272 and 84 secs
> with the patched version. For asia, I see only a small improvement of ~ 10%
> On my machine, the precompilation of the boundaries is now quite often
> waiting for the disk.
> http://gis.19327.n5.nabble.com/file/n5670294/BoundarySplitter.patch
> BoundarySplitter.patch
> Gerd
> --
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