On Tue, Mar 03, 2009 at 08:31:05AM -0800, Dan Little wrote:
> There are two steps to a tilecache...
> Step 1: Seeding it.
> Step 2: Querying from it.
>
> Metatile usefulness is best seen while seeding the tile cache. If you look
> at the attached graphic (a rough estimation of a 3x3 meta-tile setup) then
> you'll see as the tilecache seed iterates through the map, it "oversamples"
> the map and uses the "center" image to represent the tile ("A"). When moving
> onto the next tile ("B"), it performs the same over-sampling. Since the map
> is then never at the "edge" of the generation you do not see errors with
> off-map lines and labelling at the edges.
This is incorrect.
Instead, what happens, is that when you request, A, B, or C, the 3x3
grid centered around B is rendered. when you request tile D, E, or F, the
3x3 grid around E is rendered.
In each case, a metaBuffer sized pixel boundary on all sides is created.
So, a 3x3 metaTile is actually 768 (3*256) + 20 (2 * 10) -> 788 pixels
big. This way, things near the edges of tiles are still included, and
you get fewer missed labels becasue the labels that san across the edges
of A and B are included, where they wouldn't have been before.
Regards,
--
Christopher Schmidt
MetaCarta
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