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[mkgmap-dev] [PATCH v2] - provide alternative sea drawing mechanism

From Mark Burton markb at ordern.com on Wed Dec 30 20:57:35 GMT 2009

Firstly, I would like to thank everyone who responded to my
"Understanding the sea" post. From those posts and my own study, I
finally worked out what it does. And yes, it works pretty well inasmuch
that it generates some useful polygons. The "sea sector" heuristic
doesn't work for me so I have been disabling it.

v2 - avoid generating sea poly when unclipped tile contains coastline
but the clipped tile contains no coastline.


The problem is, the end result is "butt ugly". Our MP implementation
really can't do a good job on zillions of little islands. 

After frigging around for a couple of days trying to improve the
visual quality, I tried the obvious alternative technique of not using
a MP but, instead, just having a sea background and then put the land
on top using plain polygons. Guess what? It works very nicely as the
attached pics show. I now have a splendid map of the Baltic area (OK
it's weird at the edges but that's not a problem for now).

Here's the (future) commit comment for this patch:

    Provide an alternative to using a multipolygon when generating the
    The --generate-sea option can now take a comma separated list of
    no-mp - don't generate the sea using a multipolygon - instead,
    generate a background sea polygon and then individual land polygons
    with tag natural=land. This requires a suitable land polygon type
    to be defined in the style file (suggested type is 0x010100) and
    the polygon must be defined in the TYP file as having a higher
    drawing level than the sea polygon type.
    no-sea-sectors - disable the generation of "sea sectors" when the
    coastline fails to reach the tile's boundary.
    With no values specified, the --generate-sea option should work as

Oh yes, this patch doesn't include it, but I am also disabling the DP
code for polygons as it makes them weird.

Cheers and thanks again for the helpful postings.

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