Add support for polygons and extrusions

[pzingg]

I notice that in ianmackenzie/elm-geometry you have Polygon2d.triangulate. Was wondering how hard it would be to add two more primitives to Scene3d:

1. polygon (with or without holes) - more generalized than quad, just a planar triangulated mesh
2. polygonExtrusion (with or without holes) - like cylinder or block with planar triangulated meshes on top and bottom, and a strip mesh around the extruded sides.

I would like to be able to add extrusions other than blocks and cylinders (I really enjoyed rebuilding my hand crafted WebGL scene using this library, BTW).

Suggestion: rather than the way quad is defined by four unconstrained 3d points, it would probably be better to define these two new mesh primitives using Polygon2d points projected onto a sketch plane, to enforce planarity. The polygonExtrusion would also add a length property.

[ianmackenzie]

Hi Peter, that's very much the next step once elm-3d-scene 1.0 is out! The rough plan is:

• Add Curve2d, Curve3d and Surface3d types to elm-geometry to represent parametric curves and surfaces
• Add Region2d (a region bounded by a set of Curve2ds) and Body3d (a body bounded by a set of Surface3d) types to elm-geometry
• Add extrusion, revolution and similar methods to elm-geometry so you can do things like construct a Body3d by extruding a Region2d along an axis or revolving it around an axis, construct a Surface3d by placing a Region2d on a SketchPlane3d, or construct a Surface3d by extruding/revolving a Curve3d
• Add Scene3d.curve, Scene3d.surface and Scene3d.body functions to elm-3d-scene to render the corresponding types to within a given accuracy (or eventually a set of accuracies, with dynamic level of detail handled automatically)

My ultimate goal for elm-geometry, elm-3d-scene etc. is indeed to have a really nice ecosystem for building up 3D shapes with Elm code (roughly "CAD with code instead of a mouse"), which sounds pretty in line with what you're suggesting.

Polygons and polygon extrusions would then be a special case of regions and bodies - for example you might have a function like Region2d.polygon : Polygon2d -> Region2d which you could then extrude as you describe. But I also really want to support defining regions and bodies with curved boundaries, where conversion to triangles only happens implicitly when rendering.

Hope that makes sense - definitely happy to talk through these ideas as this is the area I'm really excited about using Elm for!

[ianmackenzie]

By the way, the entire reason I added Polygon2d.triangulate was in fact to be able to (eventually) render Body3d values - since ultimately rendering a body will involve converting various Region2d values to polygons (with some desired accuracy) and then triangulating the resulting polygons so they can be rendered with WebGL.