[mkgmap-dev] [Patch v1] improve filters to reduce artifacts in shapes at lower resolutions

Wed Jul 27 15:10:13 BST 2016

Hi all,

attached is a patch that addresses the following problem:

When you use e.g. GPSMapEdit to look at the polygons produced by

mkgmap for resolutions < 24,

you may notice e.g. white triangles in the ocean where there should be

water.

I am not sure if they are visible in Basecamp or MapSource or a Garmin

device, but they are not intended and they sometimes "confuse" the

optimization filters.

They are produced when the Douglas-Peucker-Filter removes points

which are somehow important.

Important points are

1) Those that lie on the bounding box of the polygon

2) Points which connect a hole with the outline

Another problem was

The patch changes the code so that these points are preserved

before line simplification is applied. It also makes sure that

the result doesn't depend on the order in which the shapes

are processed.

You may see small changes in img size, sometimes size is decreased,

sometimes increased (more likely). If you see changes > 10 kB on a single img file

compared to r3683, please post a link to the corresponding input file.

If I hear no complains I'll commit this patch next Monday.

A binary for testing is here:

http://files.mkgmap.org.uk/download/303/mkgmap.jar

Gerd
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