[Talk-us] What's causing rendering artifacts in the Southeast?

[David ``Smith'']

On Fri, Nov 27, 2009 at 2:18 AM, Dale Puch <dale.puch at gmail.com> wrote:
> It sounds like tile at home needs to trim data at the tile border, and then
> close open ways along the border of the tile...
> Just my uneducated opinion.

Let's say you have a tile with two ways crossing it, and they don't
intersect in the tile.  Both of these ways are "outer" members of the
same multipolygon relation, and they don't share any nodes.  There's
no way to determine whether to color in the space between the two
ways, versus the rest of the tile except for that space, unless the
renderer knows whether the ways wrap around the polygon clockwise or
counterclockwise.  One might think that it's safe to assume (as with
coastlines) the answer is clockwise, but when multipolygons are used
to map adjacent features with shared ways, that condition cannot be
true for both/all multipolygon relations; so that assumption must be
thrown out.  In order to paint the polygon correctly, the entire
polygon must be known, which means loading the entire multipolygon
relation*.

Trimming at the tile border is therefore ineffective at solving this
problem.  It could be done optionally (after the entire multipolygon
is loaded, so the trimming can be done correctly) which would result
in a label for the feature in each tile.  That side-effect may
actually be a good thing, with some tweaking to avoid
label-overkill...

(* For very large features, this would impose undue hardship on the
rendering client.  If they are water features, they should probably be
represented by coastlines rather than multipolygons.  Most other
features that I can think of with the potential to cover a large area
are administrative boundaries, and because they are typically not
rendered as filled polygons**, the rendering can be done with only the
consituent ways passing through the area being rendered.)

(** I would actually like to see some kind of four-(or more-)color
shading scheme implemented for administrative areas.  This would
involve a number of technical and practical challenges, however.)

-- 
David "Smith"
a.k.a. Vid the Kid
a.k.a. Bír'd'in

Does this font make me look fat?