queryRenderedFeatures feature request

[kelvinabrokwa]

A feature that returns all features rendered at a certain lon/lat (within a radius for points).

See queryRenderedFeatures in Mapbox GL.

If this is of interest I can take a shot at implementing it.

[perliedman]

Hi, and thanks for the suggestion!

I think this could be useful, and a proof of concept implementation would be very nice.

Some words of caution though: as I'm sure you already know, performance is important for Leaflet.VectorGrid, so it's of course important to make sure that this implementation does not affect rendering performance or memory consumption considerably.

[IvanSanchez]

We already have something like this - the L.Canvas.Tile logic does search for a (leaflet) layer at a given pixel, and with L.SVG.Tile one can already use document.elementsFromPoint(), as everything are DOM elements and the browser can handle queries.

I'm not sure about the use cases for such a feature, though. We already handle event propagation to the features AFAIK.

[ssontag55]

@IvanSanchez Can you provide an example? Say you have two features, one on top of the other, how could you identify both features?

[IvanSanchez]

@ssontag55 With a SVG implementation and document.elementsFromPoint(), one can get a reference to the individual SVGElements. Storing a reference to the Leaflet layer in every such SVGElement makes things easy.

For Canvas/WebGL implementations, the typical implementation is to encode a unique ID (the layer's _leaflet_id in this case) as RGBA values in a separate canvas. A query for a pixel would then return the internal _leaflet_id and then do a lookup for the layer instance.

[ssontag55]

Can't really query a pixel if they are both right on top of each other, right

[IvanSanchez]

For Canvas/WebGL implementations, this means one can only query the feature with the top-most symbol.