Re: Problem with griddata and contourplot

[Ben Abbott]

On Dec 9, 2011, at 11:53 AM, Walter White wrote:

> 2011/12/9 Ben Abbott <address@hidden>
> 
>> On Dec 9, 2011, at 11:04 AM, Walter White wrote:
>> 
>> > Hello,
>> >
>> > I came across a problem and hope that you can help me.
>> >
>> > I was trying to produce a contour plot of scattered x,y,z data,
>> > therefore I was using griddata. So far everything works fine,
>> > but when my mesh is too fine the following error message
>> > appears and the script quits:
>> >
>> > gnuplot (as of v4.2) only supports 3D filled triangular patches
>> >
>> > Since the data is from a FEM software I have to plot it
>> > using a fine mesh to see the details.
>> > Does someone know a solution or a workaround to that?
>> >
>> > Kind regards,
>> > Walter
>> 
>> Can you tell us what commands you used?
>> 
>> And, if possible, provide a simple example for us to try ourselves?
>> 
>> Ben
>> 
>> <my_FEM_matrix.dat><FEM_matrix_stepwidth_1.00.png><FEM_matrix_stepwidth_0.50.png><FEM_matrix_stepwidth_0.30.png><FEM_matrix_stepwidth_0.10.png><FEM_matrix_stepwidth_0.05.png>
> 
> Thanks for the quick reply!
> 
> I used
> 
> ----------
> FEM_matrix = load('./my_FEM_matrix.dat');
> stepwidth = 0.03 %in m
> x_density = min(FEM_matrix(:,1)) : stepwidth : max(FEM_matrix(:,1));
> y_density = minFEM_matrix(:,2)) : stepwidth : max(FEM_matrix(:,2));
> 
> [XI,YI] = meshgrid(x_density,y_density);
> 
> x = FEM_matrix(:,1);
> y = FEM_matrix(:,2);
> z = FEM_matrix(:,3);
> 
> ZI = griddata(x,y,z,XI,YI);
> 
> contourf(XI,YI,ZI,50)
> 
> ylabel('y-axis [m]')
> xlabel('x-axis [m])')
> 
> title('FEM matrix')
> 
> axis([min(FEM_matrix(:,1)) , max(FEM_matrix(:,1)) , min(FEM_matrix(:,2)) , 
> max(FEM_matrix(:,2)) ]);
> 
> caxis([0 5])
> hd = colorbar;
> set(get(hd,'title'),'String','[ radian ]');
> 
> print('some_file_name.png', '-dpng','-S1024,768')
> --------------------
> 
> I also tried to disable the call to axis(...), but this does not change a 
> thing.
> The function crashes with the error message at about stepwidth = 0.02.
> 
> ---------
> error: gnuplot (as of v4.2) only supports 3D filled triangular patches
> error: called from:
> error:   /usr/share/octave/3.2.4/m/plot/__go_draw_axes__.m at line 567, 
> column 6
> error:   /usr/share/octave/3.2.4/m/plot/__go_draw_figure__.m at line 92, 
> column 3
> error:   /usr/share/octave/3.2.4/m/plot/gnuplot_drawnow.m at line 99, column 5
> error:   /usr/share/octave/3.2.4/m/plot/print.m at line 707, column 7
> -------
> 
> Another strange thing are the white areas on the plots that appear
> is a finer mesh is used. I attached some pictures.
> 
> Kind regards,
> Walter

There were some syntax problems with the script. In case anyone else would like 
to take a look, I've corrected them (see below).

I've verified that Octave and Matlab produce the same ZI and that both gnuplot 
and fltk produce similar results (different than matlab). The comparative plots 
and the corrected script can be found at the link below.

        http://files.me.com/bpabbott/rd3bjp

The link will expire in 30 days.

I also compared the plots when contourf() is replaced by contour(). The 
non-filled contour plots look essentially the same for gnuplot, fltk and Matlab.

It appears that this is a bug in Octave's construction of the filled contour.

I've filed a bug report.

        https://savannah.gnu.org/bugs/index.php?35033

Ben