interpolateHcl and achromatic colors

[danburzo]

Interpolating red and white in Lab and HCL:

Due to the particular HCL hue of achromatic colors (~ 158.2° in the current implementation of d3-color), when interpolating a chromatic and achromatic color, the non-zero Chroma of the interpolated color will pick up various hues.

In comparison, chroma.js will not interpolate the hue in this case, with a more predictable (and analogous to Lab) outcome:

Should interpolateHcl behave the same?

[mbostock]

The bug here (if there is one, and I think there is) is that the hue for white (or another achromatic color) is a number rather than NaN. If the hue is NaN then the interpolator will do the right thing.

Previously I thought that the non-NaN (NNaN) hue for white was an artifact of the D65 referent but it seems I was mistaken. We should fix this as part of your work to make d3.lab and d3.lch consistent with other implementations.

(Also would be great to implement chroma clamping…)

[mbostock]

Fixed in d3-color 1.0.4.

https://beta.observablehq.com/@mbostock/achromatic-interpolation-test