Re: Ploting a Temperature field

[John Smith]

On Wed, 15 Mar 2000, Rober Frobeen wrote:

> As the result of my calculations I got a matrix with the Temperature
> values of a x-y mesh in it.
> 
> For a first overview I would like to plot them as an Temperature field
> in a x-y plane where each temperature got a x and a y value and the
> temperature value is displayed as a colour.

> To plot this, there is the command "pcolor" in matlab. Is there a
> similar command in octave? If so, which one is it and how is it applied? 
> 

Raw Octave is not too useful here, but there are some approaches. 
Does anyone else have favourite ways to do this sort of thing?


A very crude method might be based on this sort of logic:

##
## calculate the temperature...
##
  nx = 200 ;
  ny = 150 ;
  x = ones(ny,1) * linspace(-10,10,nx) ;
  y = linspace(-10,10,ny)' * ones(1,nx);

  temp= sin(x.*x + y.*y) ./ (x.*x + y.*y) ;

##
## Display it...
##
  colormap(ocean) ;
  imagesc(temp) ;


--------------------------------------------------------------------
A better approach might be to use a little (may be well) known 
tool called plotmtv; I seem to have been using plotmtv 1.4.1. 
A semi-general function (posted here some time ago) is attached below.
You might call it like this:

##
## Calculate temperature
##
      n = 40 ;
      x = linspace( -10,10,n ) ;
      y = linspace( -10,10,n ) ;
      
      xx = ones(n,1) * x ;
      yy = y' * ones(1,n) ;
      rr = sqrt(xx.*xx + yy.*yy ) + 1e-6 ;
      
      temp = sin(rr) ./ rr ;  
      
##
## Display it
##
      mtv_ctr( x, y, temp ) ;

Then click on the "3D Plot" button in the top left corner to get
a 2D map.

There is a useful manual that comes with plotmtv that explains how 
to write the input files, to add titles and all things pretty.

Hope this helps,

  John Smith
-------------------------------------------------------------------

function mtv_ctr (t, u, v, w)
# usage: mtv_ctr(Z), mtv_ctr(Z,V), mtv_ctr(x,y,Z)  or  mtv_ctr(x,y,Z,V)
#
# where Z is a n x m matrix of values of f(x,y), x along rows & y UP cols
# i.e. it is assumed that Z has been obtained by the equivalent of
#
# [X,Y]=meshdom(x,y); Z = f(X,Y);
#
# and V is either an integer number of contours (default = 10)
#          or a vector of contour-level values (max = 98)
#
# The dimensions of the Z-mesh should both be not greater than 99.
# This uses the Plotmtv CONTOUR data format (Data Format Manual Sect. 7)
#
# Additional Plotmtv command-line options can be set in the
#        octave string variable mtv_options. (Not implemented)
#
# See also: plot, semilogx, semilogy, loglog, polar, meshdom, contour,
#           bar, stairs, gplot, gsplot, replot, xlabel, ylabel, title 

# Created by Ted Harding June 1996
# Possibly changed by John Smith 1999-2000.

# This file is part of Octave.

  if (nargin == 1), z=t; x=y=[]; V=10; endif
  if (nargin == 2), z=t; V=u; x=y=[]; endif
  if (nargin == 3), x=t; y=u; z=v; V=10; endif
  if (nargin == 4), x=t; y=u; z=v; V=w; endif

  if (is_matrix (z))
    m = columns(z); n = rows(z);
  else
    error ("mesh: argument must be a matrix");
  endif

  if (is_vector(x) && is_vector(y))
    if (columns(x)==1), x=x'; endif
    if (columns(y)==1), y=y'; endif
  else
    x = (1:m); y = (1:n);
  endif

  z = flipud(z);

  file = "/tmp/mtv_curves_dat";
  FILENUM = fopen(file,"a");
  fprintf(FILENUM,"\n\n$ data=CONTOUR\n\n");
%%
%% For non-rectangular-grid data (e.g. polar) try e.g.
%% printf(FILENUM,"\n\n$ data=CONTCURVE\n\n");
%% instead of the above line.
%%
  fprintf(FILENUM,"%% xlabel=\"\"\n");
  fprintf(FILENUM,"%% ylabel=\"\"\n");
  fprintf(FILENUM,"%% zlabel=\"\"\n");
  fprintf(FILENUM,"%% toplabel=\"\"\n");
  fprintf(FILENUM,"%% subtitle=\"\"\n");
  fprintf(FILENUM,"%% contstyle=2\n\n");

  fprintf(FILENUM,"%% hiddenline=True\n");
  fprintf(FILENUM,"%% eyepos.y=%g\n",-sqrt(3));
  fprintf(FILENUM,"%% eyepos.x=%g\n",1);
  fprintf(FILENUM,"%% eyepos.z=%g\n",2*tan(pi/18));
  if (is_scalar(V))
    fprintf(FILENUM,"%% nsteps = %d\n",V);
  else
    fprintf(FILENUM,"%% contours=(");
    fprintf(FILENUM," %.3g",V);
    fprintf(FILENUM,")\n");
  endif
  fprintf(FILENUM,"%% nx = %d xgrid=True\n",m); 
  fprintf(FILENUM," %.3g",x);fprintf(FILENUM,"\n");
  fprintf(FILENUM,"\n%% ny = %d ygrid=True\n",n); 
  fprintf(FILENUM," %.3g",y);fprintf(FILENUM,"\n");
  fprintf(FILENUM,"%% binary=True\n");
  fclose(FILENUM);
  z = fliplr(z');
  save -mat-binary /tmp/temp.mat z
  mtv_cmd = sprintf("system(\"tail -c %.0f /tmp/temp.mat >> %s\")", \
                    8*m*n, file);
  eval(mtv_cmd);

  shell_cmd("/home/john/Plotmtv1.4.1/Bin/plotmtv -3d -geom 688x572+80+0 -nodate 
-noframe \
             /tmp/mtv_curves_dat > /dev/null ; rm /tmp/mtv_curves_dat &");
endfunction














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