[mapserver-users] Mapserver images doesn't fit well on a tiling scheme???

[Christopher Schmidt]

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