Re: [Plplot-general] rgb colourscale interpolation

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

On 2018-07-24 13:35-0000 Arjen Markus wrote:

> Hi Phil,
>
>> -----Original Message-----
>> From: Phil Rosenberg [mailto:p.d...@gm...]
>>
>> I feel that if someone provides colours in rgb values we really should interpolate in
>> rgb values. Or at the very least we need to make it very clear in the documentation
>> that we always interpolate in hls space even if colours are provided as rgb.
>>
>> Any thoughts?
>>
> I agree: if you specify the colours in RGB, then you are not interested in the HLS colour space. Indeed it would be very awkward to select the RGB coordinates in such a way that you get intermediate RGB colours, when the interpolation is actually done in HLS.
>
> Maybe there are good reasons to do it like this, but then I sure would like to hear them. I tend to think that it is simply an oversight.

To Phil and Arjen:

I agree as well.  The documentation at
<http://plplot.org/docbook-manual/plplot-html-5.13.0/color.html#color-map-1>
clearly implies that if you choose RGB, the interpolation is in that
space, and the documentation of plscmap1la and plscmap1l implies the
same.  However, 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?

Anyhow, if the code currently does not follow the documentation
(subject to any small documentation changes relevant to the above
concern), then I think it is an oversight, and my thanks to Phil for
being willing to sort this all out.

Alan
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Alan W. Irwin

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