Re: possible bug in gnuplot backend when using 3d patches

[Ben Abbott]

On Mar 30, 2012, at 7:59 PM, Martin Helm wrote:

> Dear all,
> 
> I would like to have some feedback on the following problem (octave 3.6.1)
> 
> This code works as expected when using the fltk toolkit
> 
> patch ("Faces", [1 2 3; 2 3 4; 3 4 1; 4 1 2], \
> "Vertices", [1 0 0; 0 1 0; 1 1 0; 0 0 1],\
> "FaceVertexCData", [32; 32; 32; 32])
> 
> and shows as expected a tetrahedron with color 32 of the current colormap.
> 
> The same code when used with the gnuplot backend leads to this error
> 
> error: horizontal dimensions mismatch (4x3 vs 2x1)
> error: called from:
> error:   /usr/share/octave/3.6.1/m/plot/private/__go_draw_axes__.m at
> line 752, column 37
> error:   /usr/share/octave/3.6.1/m/plot/private/__go_draw_figure__.m at
> line 167, column 19
> error:   /usr/share/octave/3.6.1/m/plot/__gnuplot_drawnow__.m at line
> 86, column 5
> 
> while it works somehow when I feed rgb tripplets to the patch
> colormap()(32,:)
> ans =
> 
>   0.46825   1.00000   0.53175
> 
> patch ("Faces", [1 2 3; 2 3 4; 3 4 1; 4 1 2], \
> "Vertices", [1 0 0; 0 1 0; 1 1 0; 0 0 1], \
> "FaceVertexCData", [ 0.46825   1.00000   0.53175;\
>  0.46825   1.00000   0.53175;\
>  0.46825   1.00000   0.53175;\
>  0.46825   1.00000   0.53175])
> 
> But also not as I would expect, there is some spurious red-brown
> triangle there which I cannot explain.
> 
> Has anyone an idea if that's a bug or some feature I do not understand?

The problem is that the patches are 3D, and gnuplot has limited ability to 
render 3D patches.

h = findobj (gca (), "type", "patch");
get (h, "zdata")
ans =

   0   0   0   1
   0   0   1   0
   0   1   0   0

Take a look at how David Bateman implemented isosurface.m

        demo isosurface 1
        h = get (gca (), "children")
        get (h, "type")
        ans = patch

There remains some minor problems with isosurface. The edge color is [0 0 0], 
but should be "none".  However, when the edgecolor is an RGB value, gnuplot 
does not render the edge.

Ben