I stumbled across this problem recently recently and found it somewhat
intriguing.
My setup is an Openlayers client talking directly to mapserver in tiled
WMS mode. My map data is a polygon coverage, being rendered as a
single-pixel wide line. I noticed in several places, where the edge of a
polygon was nearly parallel to and crossed the edge of the map tile, it
is possible for a piece of the line to be missing. Ie. the tile on one
side rendered the first 1/3 of the line, the tile on the other side
rendered the last 1/3 of the line, but the 1/3 of the line in the middle
went un-drawn by either tile.
I see that someone else has already documented something closely related
to this issue:
http://crschmidt.net/blog/archives/154/mapserver-rendering-bug/
And that it has already been solved in tilecache by adding a buffer
around the meta-tiles during rendering - but I don't think that solution
will help with the single-pixel-wide line. Meta-tiling and/or increasing
the size of the tiles would reduce the likelihood of this happening, but
I don't think they could prevent it. Rendering oversize tiles/meta-tiles
and cropping them wouldn't have any effect, unless you were using
anti-aliasing, in which case the problem might be reduced to the line
getting more faint as it crossed the edge, but still I don't think that
the line would be properly rendered.
Without looking at the code, I suspect the heart of the problem is
two-fold, (1) the clipper is rounding to the nearest pixel (possibly
differently for each tile) and (2) the line rasterizer cannot and will
not draw exactly the same pixels for lines that are clipped as it would
for drawing the whole line (especially if they are clipped differently
in each tile).
I suppose this might be considered a minor issue because of how rarely
one would expect to have their lines be nearly parallel to and cross the
tile boundaries, but I've actually seen that it can happen on just about
any line that crosses a tile boundary at an acute angle.
It seems to me that the only solution is to not clip lines on the edge
of the image, and instead keep the entire line to the next point (but
not the entire linestring or polygon). This obviously has an impact
(probably minor) on performance, so perhaps it should be an option to
turn it on/off. Perhaps I'm completely off-base here, and maybe no-one
cares how good non-anti-aliased, single-pixel-wide lines look. But the
solution seems simple enough that it might be worth implementing.
Chris
