WebMercatorViewport supports camera meter offset

[Pessimistress]

Needed for react-map-gl to support Mapbox terrain. Names are aligned with deck.gl's Viewport class.

Change list:

• Add position to WebMercatorViewport constructor
• Add meterOffset field to viewport instance
• Docs
• Unit test

[coveralls]

Coverage increased (+0.02%) to 90.779% when pulling 197dd55 on x/mercator-viewport-position into fd96311 on master.

[ibgreen]

meterOffset not mentioned?

Perhaps a small section further down in the doc describing how meterOffset and position work, for positioning camera and for following elevation of maps?

[Pessimistress]

meterOffset is a member on the viewport instance. position is an optional prop you pass to the constructor. They are essentially the same value in this use case. Only position is user-facing.

Yes it is confusing. I am only doing this to be compatible with the deck.gl Viewport class because react-map-gl needs to support both.

[ibgreen]

This is not used?

[ibgreen]

I would prefer if meterOffset and position were kept separate and added internally. I have been viewing position as the camera's relative position in meters.

[ibgreen]

I am only doing this to be compatible with the deck.gl Viewport class because react-map-gl needs to support both.

Understood. As mentioned, I would still prefer if I could set position independently of map terrain elevation - this makes it seem like this field is being taken over for the elevation purpose when on a terrain map.

If that is not possible, perhaps some confusion could be reduced if we store it as position and offer a getter called meterOffset that just returns it?

I was assuming the meterOffset only needs to be a single number, i.e elevation? But perhaps mapbox expect a vector?

[Pessimistress]

Both position and meterOffset are vec3 like this class's deck.gl counter part. You can see how it is used in react-map-gl: visgl/react-map-gl#1483 essentially position is set to [0,0,elevation].

If you are using this class with Mapbox then it doesn't make sense to set the offset to anything else, for that will cause the cameras to go out of sync.