Foreground 3D extrusions missing in output tiles

[CloudNiner]

Issue is the same as experienced in mapbox/mapbox-gl-js#7767:

We aren't generating tiles with PostGIS so we're likely experiencing a similar issue somewhere in the JTS + GeoTrellis + GenerateVT. The linked issue notes that:

Uncaught Error: Vector Tile has POLYGON with first ring clockwise. It is not valid according to v2 of VT spec.

Might be a good first place to look, and ensure that our tiles are exporting geometries that are correctly wound clockwise.

We'd require further testing to determine if this issue is present in the new VectorPipe RC2+ releases.

[jpolchlo]

JTS has no winding requirement. If you need to reverse a ring, there is a way to extract the coordinate sequence from a polygon (and, I presume, its holes) and reverse it and build a new poly from the result. This gets to be non-trivial, though, if there are "islands" inside holes (i.e., a polygon with a hole with another positive area inside the negative space). You'll have to build the hierarchy of rings yourself and then check them for winding manually (assuming valid geometry, this should just be a bunch of contains checks). An easy winding test is described here.

[CloudNiner]

Oof, that certainly seems like the cause. The MapBox Vector Tile spec states:

Any polygon interior ring must be oriented with the winding order opposite that of their parent exterior ring and all interior rings must directly follow the exterior ring to which they belong. Exterior rings must be oriented clockwise and interior rings must be oriented counter-clockwise (when viewed in screen coordinates).

Thinking about this more generally, and we can move the conversation, does an option to re-wind ring geometries make sense as something that the new VectorPipe API provides? Otherwise it appears that any attempt to use extrusion will render poorly.

[jpolchlo]

It makes sense. I've always wondered why JTS isn't picky about winding order, since it's very common throughout computer graphics that polygons have CCW winding. (Come to think of it, it's weird that the VT spec wants a CW exterior winding...)

At any rate, this is a thing VP should provide. Go ahead and create an issue for it? Include that quote from the spec, if you would.

[CloudNiner]

👍