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[mkgmap-dev] Filters for overview map

From Johann Gail johann.gail at gmx.de on Mon Jun 15 23:03:52 BST 2009

> Hi,
> okay, first part.
In general i find it an good idea to split up a problem into small 
parts, but at the moment I cant see the whole thing. Why is it 
neccessary break things down to smaller rectangles to get an improvement 
in alignment?
Yes, I have seen this error and up to now i thought, there was some 
rounding error in it.

> In my testing it turned out, that the SizeFilter and the
> DouglasPeuckerFilter either dropped my small rectangles or
> converted them to a triangle.
> Removing the filters fixed this for me.
> If you think, that the generated rectangles should be large
> enough, so that they're not "cleaned":
> - If this is so, then there really is no reason for
>   calling the filters anyway, they should be NOPs in that
>   case. So we can optimize them away.
My opinioin:
I dont know the special case for the overview map. On a normal map layer 
I dont assume that all elements are large enough. If an element is to 
small to be displayed then we can drop it completely.

> - I hope to show in later patches (don't hold your breath!
>   I will go slowly step by step. There is no need to hurry
>   anyway) that smaller rects might make some sense.
As said before, I dont know the complete background. Maybe with the 
overview map it is reasonable to not drop the indisplayable things.
> So this simple patch disables both filters for the overview
> map.
> If nobody objects or I get some "Go", I'll commit it soon.
With this patch you introduce the possibility to disable filtering. I 
see nothing destructed, but (at the moment) no use in it.

But if I understand it correctly, then the filtering is switched of for 
the complete overview map layer. Wouldn't that increase file size a lot? 
I would expect at this resolution a lot of filtered small streets. More 
important: Does it increase redrawing speed at low zoom levels, which is 
possibly the main use of a overview map? Or is the overview map not used 
at the gps unit?
The main intention for the douglas peucker filter was the low drawing 
speed on my etrex handheld at low zoom levels.

Don't be discouraged by my critics. It are only my thoughts at the moment.


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