How to include zero contour

Hi,

In the .m file attached, I have a plot of a contour.

The plot for the contour is on line 111. Please how can I include the contour at imag(taumin)=0.

Answer 1

[Walter Roberson]

taumin is not complex, so imag(taumin) is zero everywhere. There is no contour to create.

You initialize

taumin=ones(nu,nxi);

which is not complex.

You set

        taumin(i,j)=min(tausolR);

where

       tausolR=imag(tausol_found);

You extract the imaginary component from tausol_found so the result is a strictly real number, and real numbers always have an all-zero imaginary component.

If the task is to include 0 in the level list then use

  LL =  union(0, min(min(taumin)):10000:max(max(taumin)));
  contourf(U,Xi,taumin,LL);
  

Beware though, that min(min(taumin)) is -4.32538271227367e-07 and max(max(taumin)) is 1, so min(min(taumin)):10000:max(max(taumin)) is -4.32538271227367e-07:10000:1 which is just -4.32538271227367e-07

[University]

Okay, thank you for your explanation.

[University]

Hi Walter,

I tried contourf(U,Xi,taumin, [-1, 0]); just to see if the zero contour will apppear but it couldn't work? From the attached fig. I you see is only the -1 contour that showed. I was expecting to see the zero contour too. Please is the rationale behind this?

[Walter Roberson]

Why does your code only produce a single taumin() value per inner loop, but plots everything for every iteration ?

[Walter Roberson]

Extracting the data from the contour plot, I see that the maximum Z data for the plot is -2.61427978861032e-08 -- so there is no zero to be plotted.

[University]

Hi Walter,

Thanks for your question. I'm only interested in the minimum value of tau for each inner loop.

I have also checked that the mintau = -1.051870082314296e+05 and maxtau =-2.614279788610316e-08. I think this is inline with what I in mind.

Thank you for your assistant.

[Walter Roberson]

Yes, but -2.614279788610316e-08 is less than 0, so you cannot contourf() the 0 level

You might be interested in using

surf(U, Xi, taumin); view(3)

[University]

Thank you for assisant @Walter. I have tried using the surf function as suggested but I don't see how the surf plot will help us virtualize the data better. I don't know why you suggested the surf plot anyways...

[Walter Roberson]

N=[0:0.1:1];
map = [0.95*(1-N') 0.95*(1-N') N'];

fig = openfig('cont1.fig');
fig.Visible = true;

ax = fig.Children(2);
C = ax.Children;
X = C.XData;
Y = C.YData;
Z = C.ZData;

figure();
surf(X, Y, Z); view(3)
colormap(map); colorbar;

That gives me a much better understanding of the data than the contour plot does. The contour plot gives no feeling for the sharp trenches.

[University]

Okay, thank you so much for your help. I get what you mean.

Answer 2

[Torsten]

You want to plot imag(taumin(U,xi)) = 0.

Thus use fimplicit:

https://uk.mathworks.com/help/matlab/ref/fimplicit.html

[University]

Hi Torsten,

Thank you for your response. I have checked the link above but it seems the example posted in the link is when dealing with a function. In case, my taumin is 61 by 61 matrix not a function.

[Torsten]

But in principle, you can compute "taumin" given U and xi ? This would define a function.

Maybe this method is easier to implement:

https://de.mathworks.com/matlabcentral/answers/86070-elevate-a-single-isoline-in-a-contour-2d-plot

[University]

Okay Torsten. Thank you for your repsonse. I will try it.