One of the problems I’m running into with mapserver right now is related to its rendering of LINE elements which are wider than one as they run into tile boundaries at acute angles. It seems that mapserver is drawing the centerlines for these elements up to the side of the image — but in cases where a line is approaching a boundary at an acute angle, this means that the ‘outer’ edge of the rendered line stops away from the edge.
In non anti-aliased lines, this is less visible as a problem (especially if you’re not looking at the images as tiles) because the lines just stop — and visually, it’s hard to tell if it’s at the image edge. However, it becomes very obvious in cases where anti-aliasing is on because the edges of the left and right boundary are ‘tied’ together by a curving, anti-alised line: resulting in a bubbled look at tile boundaries.
I’m not sure if this is a known bug, or something that other people have run into: I’m mostly recording it here so that I have a description of it.
Right now, it seems like the workarounds are:
* Use thinner lines (so it’s less visible)
* Don’t have roads near boundaries at acute angles — although there’s not much I can do about this one!