Latest news: I've just discovered, that the error has something to do with the zoom level! If I zoom out to 120m then the pink line draws correct.
Also if I zoom in so far that the connection between aa and bb is not visible on the screen, then the line drawing is correct.
The theory is that at high zoom level, the route is drawn using the highest detail data from the polylines in TRE/RGN, which is linked to the routing graph via "extra bits" and NET.
At low zoom levels, the route is drawn using just the data in NET/NOD ("level 1" in GDF, see http://libgarmin.wiki.sourceforge.net/Contribute).
It seems, the error occurs only in very rare conditions: + Node in osm O Node in Routing graph The pink line should go through 6,2,1,7 |8 5 4 3 2 1 | dead end---+---+---O---+---O | | |6 |7 | O The error occurs only if segment 2 is to be drawn. The error occurs not if... ... if I zoom out enough, that segment 2 is only ~4 pixels long ... if I zoom and move the screen, so that segment 2 is off the screen ... if I generate a new map with an additional routing node between 2 and 1 ... if there is a end node at the dead end (from mark) ... if the direction of the way is inverted (from mark) ... if routing starts or ends in segment 1 or 2 As far as I understand the situation, the drawing algortihm has problems to find the correct segments to draw pink. If it has to draw all segments between two routing nodes (e.g. 1 and 2) then it will draw also 3,4,5. Seems like some search function didn't stop when it should. Maybe the node between 2 and 3 should have some special flag set. I will stop further investigation for now and will try the newest commits.