[mkgmap-dev] Empty rectangles in map

Wed Apr 28 10:14:39 BST 2021

Hi Ticker,

please create two separate patches for method 1 and 2. It's not obvious to me what changes are needed to get method 1.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
Gesendet: Dienstag, 27. April 2021 13:11
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Empty rectangles in map

Hi Gerd and others

The cause for this unrendered area is as follows:

A large multipolygon (larger than the maximum size for a level 1
subdiv) exists, and is broken into some similar size pieces and some
smaller pieces to expose inners/holes. The same problem can be caused
by any polygons having these size properties.

Option --order-by-decreasing-area is not in effect (with this option
the problem goes away).

At the outermost level, the large pieces are split into the grid of
subdivs necessary to be within size limits. Small pieces that fit
within a subdiv are allocated to the correct one. Intermediate pieces
that extend out of the nominal subdiv bounds, but don't exceed the
actual addressing limits, are allocated to an existing subdiv based on
their centre. Larger pieces that fit in a subdiv but would cause size
problems if added to an existing subdiv are given a subdiv of their
own.

At the next level down, each of the above subdivs will required
splitting to fit within the addressing limits. The logic that did this
used the subdiv bounds and split this into a grid. Then every element
in the outer MapArea is processed as above (ie split, safely placed,
placed with overlap or own subdiv)

The problem is that the splitting algorithm simply goes through the
child subdiv, clips the polygon to this size and saves the bits that
are within the area. Polygon parts that are outside the defined outer
level never get noticed!

There are various possible solutions:

Method 1: use the rigorous polygon splitting into subdivs that --order
-by-decreasing-area uses for levels above level 0. This is simple to
implement, safe, mergeShapes might recombine more cut fragments as they
are all together. Disadvantages are that, for a normal map, more
polygons might be split, leading to larger RGN size (but not for bad
-map-split.osm.pbf). The overall size is still less that with --order
-by...

Method 2: When the child subdiv grid is created, use any expanded size
of the parent subdiv. This stops overhanging bits of polygons being
lost. Again, simple to implement. It results in overlapping subdivs at
lower levels and I don't know this causes other problems, but probably
not; this used to be done and still does for some sizes of polygons and
lines. Some shape-merging won't happen because the joining bits are in
a different subdiv.

Method 3: Allocated oversize polygons to their own subdiv. Again,
simple to implement, but I see no advantage over Method 2. Even more
shape-merging might not happen.

Method 4: Detect when an object will have bits missed and semi-cut
rather than clip them to continue having an oversize part in the child
subdiv. This is more complicated to implement but, if done, could
replace the clipping code and be more efficient. If a parent subDiv is
divided into 2 children, it already happens like this.

Although I prefer Method 1, I suspect Method 2 is what should be done
in the short-term and so I've attached a patch that is configured for
Method 2, but, with a couple of edits to change some flag handling,
does Method 1.

Ticker