On Sunday, August 12, 2012 at 08:02:20 (-0700) Alan W. Irwin writes:
> On 2012-08-12 00:22-0500 Maurice LeBrun wrote:
>
> > Here is the behavior I see on a plplot build (a bit old but none of this
> > stuff
> > has changed recently AFAIK). BTW H=120 is green, blue is H=240. Cf.
> > http://en.wikipedia.org/wiki/HLS_color_space
> >
> > Using a Hue range of {0,120} gives a Red - Yellow - Green shading.
> > Using a Hue range of {120,0} gives a Green - Yellow - Red shading.
> >
> > Selecting rev=1 causes the interpolation to go around "the back side" of
> > the
> > color wheel, yielding in the latter case Green - Cyan - Blue - Magenta -
> > Red.
> >
> > So from my perspective it's working as expected, aside from the "rev"
> > misnomer
> > ("back" or "wrap" would've been better -- my bad).
>
> The name of that argument can be changed without causing an API
> disruption so if we can come up with a name that is a lot better, I
> would encourage a patch making the change (and also the associated
> change to the documentation).
>
> But I am not sure "back" or "wrap" are a _lot_ better than "rev". Just
> to throw another possibility into the pot and get some discussion
> going about the ideal name, how about "large" or "larger_angle" (for
> the choice of range corresponding to the larger hue angles). I also
> thought of cc (for counter-clockwise), but that is a similar misnomer
> to rev.
"large" & such variants don't work when the hue range exceeds 180, e.g.
{0,240}. where the shorter angle is traversed by going around the back side.
I agree "back" is poor also since "front" & "back" are arbitrary
constructions. The most accurate mathematical description would be something
like "path_includes_zero" as the interpolation is defined by going through the
H=0=360 (red) point. So maybe that or something similar.
Of course, this silliness could've been avoided if I'd merely chosen a default
interpolation direction, like counterclockwise, and then used the flag to
reverse polarity. That didn't occur to me at the time. The reason for the
existing design probably reflects bias due to seeing the HLS wheel displayed
in flattened form (red on the left). In that view, a default "front side"
interpolation seems more intuitive. But so does driving on the RHS of the
road. ;)
--
Maurice LeBrun
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