Add identity function

[lucaswoj]

It is often useful to define an identity property-function. For example, when styling a 3d building, users might have a feature property called height which maps directly to the pixel height of the building. Presently a user might define such a function like:

{
    property: 'height',
    steps: [[0, 0], [1000, 1000]]
}

We could make this use case more ergonomic and safe by adding an identity function type

{
    property: 'height',
    type: 'identity'
}

The identity function may also allow using colors in property functions, which was previously impossible

{
    property: 'color',
    type: 'identity'
}

cc @andreasviglakis @scothis

[lbud]

👍 . This also takes the work out of having to figure out or guess at the max value to include in the highest stop (currently if the stops are [[0, 0], [1000, 1000]] and there are buildings higher than 1000, that highest stop will chop them all off at 1000).

[lbud]

I'm hesitant to even make the following suggestion, because it may be a bit of a slippery slope, but we may also consider something like a multiplier property, which would turn

{
    "property": "height",
    "type": "exponential",
    "steps": [[0, 0], [1000, 3000]]
}

into

{
    "property": "height",
    "type": "identity",
    "multiplier": 3
}

[scothis]

lucaswoj and I have been batting around the idea of derived values, they would address your use case and a lot more @lbud.

The exact syntax is tbd, but you could do something like:

{
    "property": ["*", { "$ref": "height" }, 3],
    "type": "identity"
}

... or convert celsius to fahrenheit.

{
    "property": ["+", ["*", { "$ref": "temperature" }, ["/", 9, 5], 32]],
    "type": "identity"
}

As more functions are introduced, the power of the concept grows. To select the the red channel of a color

{
    "property": ["rgba", ["red", { "$ref": "color" }], 0, 0, 0.5],
    "type": "identity"
}

Slippery slope archived.

[lbud]

👀