[Octave-bug-tracker] [bug #54545] Images plotted with imshow are too bright with Gnuplot 5.0 and earlier

[Shane]

URL:
  <http://savannah.gnu.org/bugs/?54545>

                 Summary: Images plotted with imshow are too bright with
Gnuplot 5.0 and earlier
                 Project: GNU Octave
            Submitted by: sffc
            Submitted on: Tue 21 Aug 2018 06:46:24 AM UTC
                Category: Plotting with gnuplot
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Regression
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.2.2
        Operating System: GNU/Linux

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Details:

Originally reported here: https://savannah.gnu.org/bugs/?52599#comment10

I'm using gnuplot 5.0 and Octave 4.2.2.

imshow was working fine with gnuplot in Octave 4.2.1.  However, in Octave
4.2.2, images plotted with imshow or imagesc appear too bright (in fact, 4x
too bright).  I've determined that if I reverse patch posted by Dan on #52599,
images one again plot correctly.

#52599 was to make Octave work with gnuplot 5.2 and higher.  However, gnuplot
versions prior to 5.2 are still widely found in the wild, even on up-to-date
distributions.  Version 5.2 made it into Ubuntu 18.04 LTS, but the latest
Debian and Fedora are on 5.0, and the latest CentOS is on 4.6.

Here is a session: 


$ gnuplot --version
gnuplot 5.0 patchlevel 2
$ ./run-octave --no-gui
GNU Octave, version 4.2.2
...
octave:1> graphics_toolkit fltk; figure(); imshow(imread("~/mandrill.png"));
libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast
octave:2> graphics_toolkit gnuplot; figure();
imshow(imread("~/mandrill.png"));


Screenshot of the figures: https://snag.gy/ljNxYJ.jpg

The file mandrill.png is a standard sample image from here:
http://pngnq.sourceforge.net/pngnqsamples.html

This is with an unpatched Octave built from source on GNU/Linux from the tag
"release-4-2-2". Most other packages are installed from the system
repositories (CentOS 7).

I have not tested whether this problem still exists in Octave 4.4, but it
likely does since I believe Dan's patch is still present in 4.4.




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