I got an idea today: Make the style language support layers, and allow
the user to specify which layers to generate and in which output files.
This would have the benefit that you could generate all output layers
from a map in a single run, reducing the parsing and multipolygon
processing overhead, and possibly allowing more parallel processing.
The default layer (say, layer 0) would always be generated, to keep
compatibility with the current behaviour.
If we extended the default style so that it would generate overlay lines
for bridges and tunnels on a non-default layer, these lines would be
omitted by default. Someone with a fancy TYP file could invoke mkgmap
with special options to generate these overlay lines, either in the same
output tile, or in a separate output tile, so that the map user can
enable or disable bridges and tunnels by selecting map sets on the
device.
Another application would be generating one map family per route
relation type, so that you could choose which routes you want to see on
the device: ski, bicycle, bus, road network, etc. My current
--style=routes lumps all types of route relations together, not very
useful.
Let me illustrate this rough idea with an example. The syntax is not
settled yet; I would appreciate some feedback on this. I think that I
should be able to implement this, if there is no serious opposition to
this idea.
Consider the default style with the following addition to the beginning
of the lines file:
waterway=* [layer 1]
boundary=* [layer 2]
By default, all waterways and boundaries would be omitted from the map,
because they are not in the default layer. Here is an example of such a
default invocation, in the mkgmap.args format:
family-id: 42
mapname: 63240001
description: City
input-file: 63240001.osm.gz
mapname: 63240002
description: Suburb
input-file: 63240002.osm.gz
Now, let us generate separate map sets for layer 1 and 2:
family-id: 42
mapname: 63240001
description: City
layer1-family-id: 1
layer1-mapname: 73240001
layer1-description: City Waters
layer2-family-id: 2
layer2-mapname: 83240001
layer2-description: City Boundaries
input-file: 63240001.osm.gz
mapname: 63240002
description: Suburb
layer1-family-id: 1
layer1-mapname: 73240002
layer1-description: Suburb Waters
layer2-family-id: 2
layer2-mapname: 83240002
layer2-description: Suburb Boundaries
input-file: 63240002.osm.gz
We could also merge multiple layers to a single output tile by
specifying
layer3-merge: 2
or even ask mkgmap to merge everything to the default output tile:
layer1-merge: default (or 0)
layer2-merge: default
What do you think? This scheme is only useable when it is acceptable to
use the same map tiles for all map sets. If you need smaller or larger
tiles for certain map layers, you would need to invoke mkgmap once for
every set of input tiles.
Best regards,
Marko
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