So I have black lines.
The black lines appear with 8 tiles inbetween. They appear in at tile boundaries. They're always the thinnest the can be 1 pixel. Edting my tiles bitmap so they show they're boundaries it's clear the tiles also overlap! They only appear horizontally.
Black lines between tiles and overlapping change as the camera moves. My camera's has a perspective projection.
I've spent a lot of time hunting down various culprits. After a bit the bullet and changed the camera to Orthographic mode -> they remained (;_;). When I went in fullscreen mode it showed a one black line gap between every tile. At least we're getting some consistancy!
So I thought maybe it's my tile layout algorithm. (even though tile X,Ys are changed rarely - and black lines seem to appear depending on the camera. I think it's two problems that are together producing one result. One is the perspective projection the other is inaccurate tile positioning.)
Therefore I add 0.001 to the tiles Y position. Now I get big thick meaty lines between map boundaries but lines between tiles seem to have dissapeared - looks like I'm finally chasing down a solution.
(I was also worried they could be cracks or T-junctions which would have really made me cry!)
Okay they're gone. Now and again I can spy tile underlap but it's not a big problem. In both resolutions I have lovely tiling again with craziness. I also changed the camera to orthographic mode while I was at it. Now I've pushed past this road block I can trundle on again.
I always thought a really useful resource would be if some one went through all the adventures ever made and formalized all the puzzles and solutions. These abstract rules could then be used to bolster procedural content. Well there's the basic makings of something like that over at the Interactive Fiction Wiki. Of course it's extremely early days at the moment.