[Gnuplot-info] Different behaviour of palette in X11, postscript and png terminals

[Mike B. <mi...@mb...>]

Hi,

I have encountered some unexpected behaviour in gnuplot while trying to 
produce a surface plot using 'splot' and a particular grayscale palette. 
The plot that I'm trying to generate involves the aspect (or azimuth) 
angles of land surface slopes in a digital terrain model. These angles 
run from 0 degrees (north), through 180 degrees (south), to 360 degrees 
(north again). Therefore, the palette that I want to use ideally needs 
to run from black (0 degrees) to white (180 degrees) to black again (360 
degrees). The problem that I have with the grayscale palette is best 
illustrated by the following commands, which gives the output exactly as 
required in an X11 terminal...

set term x11
set palette gray rgbformulae -13,-13,-13
test palette

(a screenshot of the resulting output is attached; test_palette.png).

However, when I try to produce the same thing using either the epslatex 
terminal (my preferred format) or the postscript terminal, i.e.

set term epslatex  "default" 10
set palette gray rgbformulae -13,-13,-13
set output "test.eps"
test palette
set output

the output grayscale seems to revert to simple black (0 degrees) through 
to white (360 degrees).

Hmm! So, am I doing something wrong? do the rgbformulae in fact vary 
from one terminal type to another? is this a limitation of postscript 
graphics? or is it a bug in gnuplot?

Even more curiously, when I tried to output the same figure directly as 
a PNG image (as opposed to via a screen grab) using the following commands

set term png
set palette gray rgbformulae -13,-13,-13
set output "test.png"
test palette
set output

I don't get the grayscale wedge at the bottom of the image at all.

For the record, this is the output of 'show version' on my system.

      G N U P L O T
       Version 4.0 patchlevel 0
       last modified Thu Apr 15 14:44:22 CEST 2004
       System: Linux 2.6.13-15.10-default

Any advice gratefully received.

Mike Barnsley