Need ability to force a labelcache buffer around the edge of a map

[dmorissette]

We are looking at using MapServer to generated tiled maps (google-style), and
ran into some rendering issues at the edge of the map image. To work around
those issues, we want to generate our map image in MapServ er larger than the
tiles, and cut off the outside edge. This works great, except for the labels
that end up being truncated.

To solve the truncated label issue, I'd like to add a way to specify a buffer
(value in pixels) around the map image where no labels can be drawn.

[dmorissette]

Reassigned to myself.

[dmorissette]

I have committed to CVS a first implementation that takes the buffer value from
the map-level metadata "labelcache_map_edge_buffer" if set. This value is added
to the label buffer value that is passed to the labelInImage() function in mapgd.c.

Metadata may not be the cleanest way to do this, but this way the change is very
simple. Do you have any comments or suggestions Steve?

[sdlime]

I do worry that we overuse metadata blocks a bit. However, for the moment I
think that's appropriate in this case. I view this as experimental at this point
so there's little reason to go through the trouble of adding a new parameter. I
don't know that we know how to best handle labeling with Google style tiles. I'm
not sure not allowing labels along the edge will always work either. If you have
a lot of "edge" features then they'll never get labeled right?

Ideally you'd render a monster image and then tile, but that's not practical. I
wonder if sort of a moving window, rendering in chunks, might work. Were chunks
overlap half a tile? I dunno, just thinking out loud. 

Bottom line is there will be experimentation so using metadata makes sense until
it's all worked out...

Steve