[Octave-bug-tracker] [bug #42561] gnuplot colors incorrect for 'demo trimesh'

[Dan Sebald]

Follow-up Comment #21, bug #42561 (project octave):

I'm not seeing the drastic color difference you are seeing as given in your
example fltk/gnuplot toolkit PNGs.  I'm attaching screenshots of what I'm
seeing and the cyan color is dominant in both FLTK and gnuplot toolkits.

It could be that there is a difference between the gnuplot version you are
using and the one I'm using.

So the differences I'm seeing are subtle, occasional differences in a color of
a line.  I think that is due to the manner in which OpenGL and gnuplot choose
(or is configured) to blend colors.  There are a half dozen or more ways to
construct a color for an element when given the corner colors of a mesh.  For
example, one could pick one of the corners of the mesh element, choose an
average of all corners, so on.  See the very last example of the following
page:

http://gnuplot.sourceforge.net/demo_5.1/pm3d.html

Do we want to go into that detail?  If so, we'd have to reference what Matlab
is doing.  And then are we assured we can achieve the exact same behavior?  I
mean, mesh element coloring has far more internal algorithms than the simpler
construction of a color map.  We could pick this or that blending scheme for
both OpenGL and gnuplot and still not get a perfect match (maybe we could, but
going to such lengths would mean we might as well match what Matlab is doing
as well).

Notice in the fltk_toolkit_trimesh_Screenshot_from_2017-12-13_12-27-44.png
file that the title appears in the center of the plot, suggesting that 
copyobj() didn't work completely correctly.  Are you seeing that?  If not, it
might be an issue with a race condition of when items are updated versus
callbacks or something.

(file #42642, file #42643)
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Additional Item Attachment:

File name: fltk_toolkit_trimesh_Screenshot_from_2017-12-13_12-27-44.png
Size:36 KB
File name: gnuplot_toolkit_trimesh_Screenshot_from_2017-12-13_12-26-24.png
Size:62 KB


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