[cairo] How do I...?
rael at edge.ping.de
Mon Apr 5 04:17:24 PDT 2010
I use cairo as part of my libosmsocut project (http://libosmscout.sf.net) to
draw maps based on OpenStreetMap data (after using it as possibly backend
for drawing widgets as part of libIllumination).
Cairo is very useful for this on over all I'm very happy (thanks for that
I'm now starting to implement more sophisticated draw features and have some
questions how to realize this using cairo and how much I can handle using
cairo features itself and how much of this I have to do in my code (by doing
some preprocessing for example). I'm searching for solutions that are
optimized for visual quality but even more for drawing speed (especially on
Currently ways are drawn by just stroking paths using move_to and line_to
and setting color, pen thickness as appropiate. Coordinates are a result of
the (mercator) transformed coordinates I get from OSM nodes (as part of a
This should be OK, I assume ;-)?
For outlines I simply draw a line twice with different line thickness.
Should be still OK, but is there possibly a faster/simpler solution (=> path
Now I want to further decorate the drawn way.
* First I want to have periodical markers on the way like for example arrows
that show the driving direction (oneway) or images that signal bumpers. The
markers should be centered on the line. How do I do this?
* Now, in the case that a way has multiple lanes (motorway for example), I
also want to draw lane seperation markers (at least on higher zoom level
:-)). For the case that the the way exists of two lanes this is simple
because I just can draw another line with different color and width over the
existing path. But what do I do in case where a motorway has three lanes? If
I still just draw another line over the existing way I now have to calculate
new polygon point coordinates that do the necessary "away from line center"
transformation or is there a way where cairo can do this for me, too?
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