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[mkgmap-dev] Configurable flood blocker

From Felix Hartmann extremecarver at gmail.com on Sun Dec 19 18:34:06 GMT 2010

On 19.12.2010 18:17, WanMil wrote:
>>> Felix,
>>> there is nothing to check. The flood blocker works only in
>>> multipolygon mode. It is not possible to implement that for the
>>> polygon mode.
>>> Where do you think is the advantage of the polygon mode?
>>> WanMil
>> The advantage is:
>> a) Speed (if you use --transparent and then gmt to set the transparency
>> to no) you can have for land one polygon (only land area) for sea one
>> polygon (only sea area) and NO background polygon.
>> b) overlapping maps - using --transparent there is less chance that a
>> land polygon from one map, overlaps sea from another map (on GPS as only
>> there can be several maps active at the same time).
>> --- in principal there is no speed advantage - but only as long as one
>> does not attribute a color to 0x4b, however as one colors the
>> background/land to be say white, it is much quicker to have white land
>> background only, otherwise the GPS has to read much more data, and on
>> older generation GPS when panning the map, there is a notable speed
>> difference on top of flahing up background color, until it is overdrawn
>> by the real landuse/natural/... polygon.
> I don't want to put into question your final observation (polygon 
> renders quicker on GPS device) but I cannot follow your arguments. 
> Let's try to sort it out. Please correct me at any point if I am wrong.
> I have added a picture for both sea generation methods. It shows the 
> resulting tile and all polygons contained in the tile.
> * First of all the new mp code in the coast branch generates land 
> polygons. This is a new feature. So land+sea polygons=tile.
> * The polygon code creates one sea polygon covering the whole tile and 
> adds all land polygons. The style implementor must ensure that land is 
> drawn over sea. Otherwise the tile is flooded.
> * For tiles that don't contain any see one land polygon covering the 
> whole tile is created (both for polygon and multipolygon processing).
> => a): For both variants you don't need a background polygon. The 
> whole tile is covered by sea or land.
> => b): I don't understand where there should be the difference between 
> polygon and multipolygon processing.
> => in principal: Do you mean that you don't want to paint land 
> polygons but instead the background polygon? Ok, but in this case you 
> can ignore the land polygon in the multipolgon variant but not in the 
> polygon variant. Otherwise the background is painted and after that 
> the sea is painted over it and you get flooded tiles. So you need to 
> color the land polygons.
> The multipolygon processing generates much more complex polygons. I 
> think that's the reason why it is rendered more slowly on GPS devices.
> WanMil
I have not checked the speed after the change on GPS. I'll do that and 
then report back. Before definitely the quickest was --transparent and 
--polygons. (as I set the opqaque switch with gmaptool afterwards, I of 
course colored the land polygon - but the maps had no background polygon 
at all).

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