Feature Request: Support meters as size unit for points

[westnordost]

I am trying to show the accuracy radius of a location around the user. Here, I ran into some problems. I think part of it is a bug, part of it is a missing feature probably.

The relevant portion of the style file:

styles:
    location-radius:
        base: points
        blend: overlay
        blend_order: 2
layers:
    streetcomplete_location:
        data: { source: streetcomplete_location }
        draw:
            location-radius:
                collide: false
                color: '#44fd6c3e'
                size: function() { return feature.radius }

and the code

locationLayer.clear();
LngLat pos = new LngLat(location.getLongitude(), location.getLatitude());
int accuracy = (int)(Math.ceil(location.getAccuracy()));

Map<String, String> props = new HashMap<>();
props.put("type", "point");
props.put("radius", ""+accuracy+"m");
locationLayer.addPoint(pos, props);

However, this gives i.e. the error:
styleParam.cpp:184: Invalid size parameter '22m'

If I set a static size in the style definition, I get no error, but it doesn't seem to show at all.

So my guess is that meters as a size unit for points is simply not implemented as there hasn't been a use case for this (until now :-P). At least it is not documented. That Tangram-ES does not report an error if you write size: 50m seems to be a bug then.
In any case, this ticket shall be a feature request to support meters as a unit for points.

Plan B for the use case I have in mind would be to use a circle-polygon for the radius, but I find that rather ugly.

[westnordost]

Perhaps another use case would be tree-nodes. (natural=tree)

[Guillaume227]

@westnordost : one other posssible workaround would be to use a circular marker together with a js function for the size. That function would back out the size in pixel from the zoom level so it remains constant in meters. I haven't tested that yet.

[westnordost]

Another use case where supporting meters for points would be helpful: Rendering mini-roundabouts and turning circles with some realistic diameter in meters on high zoom levels.

[westnordost]

Actually, nevermind my earlier algorithm, much easier is

pixels_per_meter = 2^zoom_level * 256 / (cos(latitude * pi/180) * earth_circumference)

There is also a $meters_per_pixel in the docs, which would make it trivial to workaround this via a javascript function, however in tangram-es it doesn't seem to be supported. See #2138

[westnordost]

(Ah, realized that $meters_per_pixel does actually not solve this, because it only changes on full zoom levels changes.)