projection.copy?

[mbostock]

It’d be nice to have a method to create a copy of a projection with identical settings (that, or change projections to be immutable, and every method that modifies the projection instead returns a new projection).

[tinius]

+1 for immutable projections. I might even be able to help with this

[Fil]

Should we try and do this for d3-geo@2 d3-geo-projection@3 d3-geo-polygon@2 ?

I'm really not sure about immutable projections — they would mean rewriting almost all map interactions everywhere; but projection.copy() would be super useful in many instances.

Related (maybe), I very often find that I would need the projections to carry some metadata (for example & at the very least, a name; but also a flag indicating if it needs clipping to the sphere, etc).

[nirazul]

This would come in very handy when implementing rotation calculations as I currently need to set the rotation to [0, 0] and reset it to the original state after having calculated a point with attitude.

For example:

function getProjectedPoint(projection, event) {
  const originalRotation = projection.rotate()
  const pt = pointers(event, event.sourceEvent.target)[0]
  const result = projection.rotate([0, 0]).invert(pt)

  projection.rotate(originalRotation)

  return result
}

[Fil]

Agree, but I don't have the beginning of an idea how to implement this—projections are a very complex object and there's no easy "deep clone" operation. Maybe I'm missing something? Contributions are welcome.