Cropped geometry vs. actual edge

How does OSciM's data format distinguish between a cropped polygon and the actual edge of the polygon (or line) if x or y are 0 or 4095?

this is not really defined at the moment. the renderer requires tiles to overlap a few pixel though to properly draw elements that are on the edges. so a line on the edge will partly be rendered on both tiles (a pixel on screen belongs to one tile to avoid overdraw of alpha parts)