Map-tile edge rendering issue

[Chris Hodgson]

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