[Proposal] An option for FeatureGeometryLayers to wrap geometries over the terrain

[airnez]

Displaying geometries over a terrain with FeatureGeometryLayers requires to set the base_altitude style parameter.
This is complicated for the end-user, and does not refresh based on terrain refinement.
To me, most use-cases imply setting the object base altitude at ground level. I think that it could be a nice to have to be able to wrap any feature (Point, Line, Polygon) over the terrain by enabling a flag in FeatureGeometryLayer config.

The wfs 2.5d example does that by cheating a little bit with elevation offset for point and lines:

Context

If we look at the source_stream_wfs_25d example, here is how the the bus stop points are instanciated :

var lyonBusStopLayer = new itowns.FeatureGeometryLayer('lyon_tcl_bus_stop', {
    source: lyonBusStopSource,
    zoom: { min: 4 },
    style: {
        point: {
            base_altitude: altitudeLine,
            color: 'DarkTurquoise',
            radius: 30,
        }
    }
});

The altitude_line function loosely provides the desired "wrapping effect". It does not refine when terrain subdivides and is not very handy for the end-user

Description of the proposal

We could provide an option flag like that instead :

var lyonBusStopLayer = new itowns.FeatureGeometryLayer('lyon_tcl_bus_stop', {
    source: lyonBusStopSource,
    zoom: { min: 4 },
    clampOnTerrain: true,
    style: {
        point: {
            color: 'DarkTurquoise',
            radius: 30,
        }
    }
});

Concerns about performance and implementation complexity

Looking at some of iTowns competitors, it seems that no one wrap lines or polygons over terrain.
Points are usualy rendered as oriented textures in 3D space, and lines and polygons are rasterized on terrain. I make the hypothese that wrapping vector-rendered lines and polygons over the terrain is functionnaly and technically difficult. It might also introduce performance overhead. This would anyway be very useful for points, 3D objects or extruded polygons.

We could maybe take inspiration from these solutions and build some kind of hybrid layer bundling rasterized lines/polygons and vector points. I presume this will be the choosen solution in the end. @jailln I remember talking about this idea with you, but I can't find a proposal about it. Did we write one ?

Here are the reasons of this digression :

Mapbox rasterises lines and polygons on terrain. See this example. It can also "deflate terrain" when zoomed-in to switch to vector rendering. --> See geojson.io

Google earth does not apply elevation when using vectorial 'Map' style :

Satellite (with terrain) for comparison :

[Desplandis]

Thanks for your proposal. You have my vote!

I personally agree with the need to propose an API that can clamp 3D features and labels (which we could consider as 3D points in the future) to the terrain. Current API issues aside, I think the best argument for this proposal is that not all vector datasets provide an elevation component and it is unfortunate that we can't use them when the terrain is enabled...

However, as you already described in you proposal, we should only clamp to a single tangent point of the terrain (this is already the case with base_altitude). We should not try to cut lines/polygons to clamp to them to polygons of the terrain (perf wise and because this is best served with a raster layer).

I have no idea yet how to technically implement this feature, but I can participate in such technical discussions. =)