[mkgmap-dev] [PATCH v2] - round coordinates when zoomed out

Sun Nov 22 17:15:46 GMT 2009

Hi,

I'm the coder of the original dp code. Some comments to it:
>   
>> BTW - Is it possible that there is a difference with the douglas peucker 
>> filter whether a road is routable or not?
>>     
>
> Yes, looking at the code, it seems that it doesn't change points that
> are routing nodes so routable ways will be "less smooth" than
> non-routable ways.
>
>   
This is true.
The reason for it laid in some possible inaccuracies at nodes.
Take for example a T-crossing. This means two lines.A straigth (long) 
one and one ending at the node. In my first attempt the DP algorithm 
handles all nodes indifferent. But this means, that the long line could 
be straighened out at this node, whereas the ending line will never move 
its end point. So the end node laid not on the line. This error could 
become visible at some resolutions.

The only possibility to change this behaviour was to never move a node 
connecting lines. This would affect all routing lines.
>> I think it is possible that non routable ways are smoothed heftier than 
>> routable ways. This has nothing to do with this patch though, but the 
>> difference only seems to become really visible on resolution <20
>> (maybe resolution 24 is filtered if not routable, but not filtered if 
>> routable. Then the more filtering is applied on each successive level, 
>> the bigger the difference).
>>     
>
> It think the DP filter does nothing at res 24 whether the way is
> routable or not.
>   
Yes, as far as I can remember, there was some errors with the routing 
tables, if I enable it at res 24. Also all other filters was disabled at 
res 24. So I disabled it too.
>   
>> Also I noticed that cgpsmapper maps, seem to have much higher smoothing 
>> at lower resolutions compared to mkgmap (using 2.6). At resolution 24 
>> and 22 it seem to be mostly identical though. Maybe we could also 
>> increase the douglas peucker severity on resolution <20 compared to 
>> right now. Even with very few lines mkgmap maps seem slow on GPS on 
>> resolution 16 or 18.
>>     
>
> That's worth experimenting with. 
>
> Perhaps Johann can comment on this, he's the original author of the DP
> stuff.
>
>   
Yes, at resolution 16 most of the nodes should be allowed to be 
removable, as the connecting streets disappear at this level. But at the 
time of writing I had no reasonable idea of how to get this information 
inside the dp filter.

Maybe in the meanwhile this information is available?

Regards,
Johann