Re: [Plplot-general] [plplot:support-requests] #43 Colormap log

[Alan W. I. <ir...@be...>]

On 2017-05-28 10:35-0000 l1q1d wrote:

> ---
>
> ** [support-requests:#43] Colormap log**
>
> **Status:** open
> **Group:**
> **Created:** Sun May 28, 2017 10:35 AM UTC by l1q1d
> **Last Updated:** Sun May 28, 2017 10:35 AM UTC
> **Owner:** nobody
>
>
> Hi, do you have any idea how to have log color in colormap/plshade/colorbar?

I have CC'd my response to your support request to the plplot-general
mailing list.  Since that is our preferred support mechanism (to
collect support requests and the discussion of them all in one place),
I ask that you subscribe to that mailing list (see directions at
<https://lists.sourceforge.net/lists/listinfo/plplot-general>) and
follow up there with your (likely inevitable) supplementary PLplot
questions rather than on this deprecated support request tracker.

That is an interesting question concerning color map 1 (N.B. not color map
0 since that is for discontinuous colors).  The independent variable of
cmap1 ranges from 0 to 1 (inclusive).  We have no switch to interpret
cmap1 values as logs, but that is not necessary since you
can do that conversion yourself.  For example, suppose you have an
array z of 3rd coordinates which you would like to represent as a cmap1
color.  Then try this transform:

c = log(z/zmax)/log(zmax/zmin) + 1

That c array is proportional to log z as you require, but due to
the linear transformation all values of that array are in the correct cmap1 range from
0 to 1 as you can see by substituting z=zmax where c = 1 and z=zmin
where c = 0 on the RHS.

So use that transformed c array or value anywhere (e.g., plshade,
plshades, plcol1, plcolorbar) that a cmap1 color map value can be
used, and you should be OK.  However, as far as I know nobody has ever
tried this so I would appreciate you following up (on the
plplot-general mailing list) with your results (whether good or bad)
for the benefit of both our developers and ordinary users.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

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).
__________________________

Linux-powered Science
__________________________