Polygon winding order & holes vs outer rings

[mourner]

Do geometry rings have a defined winding order in vector tiles? E.g. counter-clockwise for outer rings and clockwise for rings? Or should I expect any order of any particular ring?

Winding order isn't mentioned anywhere in spec, but some polygon algorithms depend on a certain winding order. Also, some algorithms (especially triangulation) need to know if a polygon ring is a hole or an outer ring, which could also be derived from winding order if geometry is flattened.

[mourner]

OK, so Artem says we pass winding order as is, so there's no easy way to derive ring status... Could you consider winding order correction for a future release? It's pretty easy, and given that geometries are flattened, it would be extremely useful. Usual accepted order is CCW for outer rings, CW for rings.

[mourner]

Note that closing #53 will also close this issue, since polygon clipping procedure also resets winding order. Although fixing just the latter is much easier.

[pnorman]

Could you consider winding order correction for a future release?

Wouldn't the spec need to change for rings to have a defined winding order, or are you suggesting giving an order that wasn't there in the tile when parsing one?

[mourner]

@pnorman yes, I'm suggesting changing the winding order of polygon rings depending on whether it's a hole or an outer ring. I don't see any problems with this since the original winding order is pretty much random and has no practical use.

[jfirebaugh]

👍 for having a specified winding order.

[pnorman]

@pnorman yes, I'm suggesting changing the winding order of polygon rings depending on whether it's a hole or an outer ring. I don't see any problems with this since the original winding order is pretty much random and has no practical use.

I don't see an issue with it either, but I'm just questioning if it's a mapnik-vector-tile issue or an issue that the specification doesn't require a winding order.

[jfirebaugh]

The fix for this would be to update both the specification (to require a winding order), and mapnik-vector-tile (to output spec-compliant windings).

[mourner]

Another thing to note is that Mapnik VT output doesn't have outer1, inner1, inner1, ..., outer2, inner2, ... order — it can have outer rings and holes mixed up in random order, which is a problem when you're trying to figure out which is which.

[mourner]

Another approach is to just split MultiPolygons into several Polygon features. This is how Mapzen solved the issue: tilezen/mapbox-vector-tile#4