Unimplemented distance expression

[cyrilchapon]

Context

I'm trying to color my symbols based on their distance from a point on a map (a marker of my own).
I landed on the documented distance expression and naively tried to replace my icon-color with it.

maplibre-gl-js version: 2.4.0

browser: Chrome 108.0.5359.124

Steps to Trigger Behavior

1. Use the expression as below

[
   'interpolate', ['linear'],
   [
     'distance',
     {
       type: 'Feature',
       geometry: {
         type: 'Point',
         coordinates: [2, 40]
       }
     },
   ],
   100, '#E02F58',
   10000, '#000000',
 ]

2. Read the console

Link to Demonstration

PoC, working symbol with static color : https://jsfiddle.net/cpn1dkv8/
Not working symbol with distance based-color https://jsfiddle.net/cpn1dkv8/1/

Expected Behavior

Since it's documented, I'd expect it to work like so.

Actual Behavior

It's generating an error :

Error: layers.[my-layer].icon-color[2][0]: Unknown expression "distance". If you wanted a literal array, use ["literal", [...]].

Addendum

For what it's worth, the original mapbox-gl implementation seems to be blocked in a draft PR here (and the feature is also documented in mapbox docs too).

[HarelM]

It might be that this is not fully supported as you said about the draft PR, I'm not sure.
I would start with a simpler example for the distance and see if that works. I know icon-color can use the priorities from a feature (using [get, ...]) but I'm not sure about distance and the combination of these two.

[wipfli]

Distance is the only expression that is only implemented in MapLibre GL Native and not in MapLibre GL JS, see the supported SDKs table in https://maplibre.org/maplibre-gl-js-docs/style-spec/expressions/#distance

[cyrilchapon]

Thanks @wipfli for pointing this, I'm so stupid !

---

Ok so I think this features's state is inherited from mapbox-gl before the fork. I dugg into the draft PR a little bit; and apparently the feature is developed, but the bundle size bump involved is considered too high to allow a merge as-is. The feature has been marked as "postponed" in that sense.

"Bundle size bump too big" + "already implemented" sounds like a perfect fit for a plugin, in my mind. Doesn't it ?

[HarelM]

Yes, plugin is the right solution for bundle size issues.
I'm not sure how easy it will be to add an expression as a plugin, but if you manage to do it, or even create a PR that will allow to add expression as plugins it can open a world of opportunities.

[cyrilchapon]

Hey @HarelM ,

I'm not sure how easy it will be to add an expression as a plugin
That was my thought. I never encountered such a plugin, so I guess it wouldn't be that easy.

or even create a PR that will allow to add expression as plugins it can open a world of opportunities.
Indeed. That would be so much valuable.

I'm afraid I'm not familiar enough with the codebase, the cartography world and don't have enough time to give it a shot. Though I surely can help and start by documenting and researching a little bit.

Probably related :

• mapbox#9462 "Expression engine extension points"
• maplibre#1295 "Expression engine extension points"

[DevelWolf]

I'm not sure how easy it will be to add an expression as a plugin, but if you manage to do it, or even create a PR that will allow to add expression as plugins it can open a world of opportunities.

Answer moved to #1295 (comment)

[HarelM]

I've done an initial implementation of distance, full polygon support is still missing, but it's getting there...
maplibre/maplibre-style-spec#642

[HarelM]

This will probably get released in the next version within a few days.