add new distance metrics

[ijlyttle]

with colorio, we have access to a bunch of color spaces, including CAM16-UCS and Jzazbz, with the hope of "better" distance measurements.

The base color space for colorio is xyz100 - and we have a way to translate to it using farver::decode_colour()

Currently, the internal function .get_distance() has a signature:

.get_distance <- function(pal_luv, n, method, ...)

Here, method refers to farver::compare_colour(); we use a default value of "cie2000".

What if our method argument used a syntax like "farver::cie2000", and could also take a value like "colorio::CAM16-UCS"?

[ijlyttle]

this means that we would need a .get_distance_farver() and a .get_distance_colorio()

.get_distance_farver <- function(a, b, from_space, method, ...) {
  purrr::map2_dbl(
    a,
    b,
    farver::compare_colour,
    from_space = from_space,
    method = method,
    ...
  )
}

# this is a bit stinky, but it would get us out of jail
.get_distance_colorio <- function(a, b, from_space, method_space) {

  # method_space would have to be one of the defined color spaces
  space <- colorio::colorio[[method_space]]()

  to_space <- function(x) {

    as_ xyz <- farver::convert_color(x, from = from_space, to = "xyz")
    as_space <- space$from_xyz(t(as_xyz))

    as_space
  }

  a_space <- to_space(a)
  b_space <- to_space(b)

  sq_dist <- colorio$dist(a_space, b_space)
  dist <- sqrt(sq_dist)

  dist
} 

[ijlyttle]

obviated by #51