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[mkgmap-dev] [PATCH v3] - provide alternative sea drawing mechanism

From Mark Burton markb at ordern.com on Wed Dec 30 22:02:16 GMT 2009

v3 - generate land poly even if tile doesn't contain any sea (when
--generate-sea=no-mp is specified) so that GPS units that ignore the
colour of the background poly (0x4b) don't the wrong background colour.


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


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.

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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