[R] Drawing a colour wheel - bug in hcl?

[Achim Zeileis]

Hadley:

> I'm trying trying to draw a colour wheel (a slice of hcl space) in R.
> Running the code below doesn't give me what I expect - there's some
> oddly bright colours of the wrong hue around c(0, 0) and I see three
> coloured circles (a small magenta, a medium sized yellow and a large
> cyan).  Am I doing something wrong or is there a bug in the hcl code?

All of these have been fixed and do not correspond to HCL colors.

> hcl <- expand.grid(h = seq(0, 360, by = 2), c = 70, l = seq(0, 100, by
> = 2), fixup=FALSE)
> hcl <- transform(hcl,
>  angle = h * pi / 180,
>  radius = l / 100
> )
> hcl <- transform(hcl,
>  x = radius * sin(angle),
>  y = radius * cos(angle)
> )
> hcl$colour <- hcl(hcl$h, hcl$c, hcl$l)

If you replace this by
   hcl$colour <- hcl(hcl$h, hcl$c, hcl$l, fixup = FALSE)

> with(hcl, plot(x, y, col=colour, pch=20))

Then, the resulting "wheel" has three overlapping "holes" corresponding to 
the colors previously fixed.

Also, as hue/chroma correspond to polar coordinates in the UV plane of 
CIELUV, it makes more sense to plot wheels in the hue/chroma plane for 
fixed luminance. To draw a hue/luminance plane for fixed chroma is harder 
to interpret geometrically.

hth,
Z

> -- 
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