Re: [Plplot-general] rgb colourscale interpolation

[Alan W. I. <Ala...@gm...>]

On 2018-07-25 10:13+0100 Phil Rosenberg wrote:

> On 24 July 2018 at 20:19, Alan W. Irwin <Ala...@gm...> wrote:
>> [.... O]one concern I have about the current documentation, is
>> the documentation of the plscmap1la and plscmap1l alt_hue_path
>> argument.  That argument makes a lot of sense in HLS space, but does
>> it make any sense at all if you are doing all interpolation in RGB
>> space?  That is, should that argument be completely ignored if you are
>> using RGB space interpolation?
>
> If I remember correctly, the point of alt_hue_path is specifically to
> determine the direction around the colour wheel for interpolation once
> the rgb values have been converted to hls. When supplying colours in
> hls space, you can specify the direction by using hue values outside
> the range of 0-360, for example specifying the two hues 0 and 240
> would go from red to blue via yellow, green and cyan, whereas
> specifying the two hues 0 and -120 would go from red to blue via
> magenta. However if you specify rgb colours [1,0,0] and [0,0,1] then
> you can use the alt_hue_path to tell plplot to interpolat backwards
> round the colour wheel.

Hi Phil:

I believe your interpretation of what goes on with this argument for
the HLS case is correct.

> So moving to rgb interpolation for rgb values makes this variable
> defunct really.

I am pretty sure that is the case, but, of course, you should check for the
RGB case in your update to the code that this argument is completely ignored.

> I guess the best thing is to simply state this in the
> documentation?

Yes.

> Can we deprecate it somehow and in a future release
> remove it altogether?

This would require splitting each of the current plscmap1l and
plscmap1la functions into two in a backwards-incompatible way with one
for the HLS case (where alt_hue_path would be in the argument list)
and one for the RGB case (with no alt_hue_path in the argument list).
Thus, I think our current C API is the best choice so long as you
document that alt_hue_path is ignored for the RGB case.

> I have made the code changes and am about to test them. I will then
> make the documentation changes and commit the results today.

I look forward to seeing that commit.

Alan
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Alan W. Irwin

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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