CODEWITHSUNDEEP
matlabplotcomplex-numbers
mickkk
mickkk

Reputation: 1192

Plot gradient in matlab and nothing but blank appears

I'm trying to plot the gradient of the real part of a complex function, however what I get is a blank figure. I do not understand what I am doing wrong since the code works with other functions (such as the imaginary part)

% Set up
x = -3:0.2:3;
y1 = (-3:0.2:3);
y = (-3:0.2:3)*1i;

[X, Y]= meshgrid(x,y);

% Complex variable s
s = X + Y;

% Complex function f(z)
z = s + 1./s;

figure
subplot(1,2,1);
[Dx, Dy] = gradient(real(z),.2,.5);
quiver(x,y1,Dx,Dy)
title('u(x,y) gradient, vector field');

%%Imaginary part
subplot(1,2,2)
contour(x,y1,imag(z),linspace(-10,10,100)); title('Contour of Im(f)');
xlabel('x'); ylabel('y'); %clabel(C3);
title('Imaginary part');

Here below is the image I getenter image description here

I tried to rescale and resize the picture, the domain etc... but couldn't get the gradient to display (the arrows). What am I doing wrong here?

EDIT: I found out it displays blank perhaps because there are some Inf and -Inf values in the Dy and Dx variables, is there an option to ignore these values or set them to 0?

Upvotes: 2

Views: 809

Answers (2)

Benoit_11
Benoit_11

Reputation: 13945

It looks to me like it works, but the line width of your arrows is too small for you to see.

You can increase it by assigning a handle to the quiver plot like so:

hQuiver = quiver(x,y1,Dx,Dy);

And then, after the plot is created, change any of its many properties like so:

set(hQuiver,'LineWidth',4)

or do it all in the call to quiver:

hQuiver = quiver(x,y1,Dx,Dy,'LineWidth',4);

In this case it gives the following:

enter image description here

EDIT:

To answer your secondary question, you can set elements that are equal to +Inf or -Inf to any value you want using isinf:

Dx(isinf(Dx)) = 0;

and

Dy(isinf(Dy)) = 0;

Upvotes: 3

optimist
optimist

Reputation: 1018

It is not blank. You've plotted the quiver diagram (usually used in optical flow diagrams). It is giving an inward pointing arrow at each of the locations formed by the grid with points in x and y1.

Upvotes: 1

Related Questions