Plot 3D Graph and make level curves

Hi

I have 3 vectors, where:

X is a vector of numbers;

Y is a vector of numbers;

Z is a function that depends on X and Y (i.e)

Z = 4.6597*x.^2 + 2.6897*x.*y - 0.023578*x + 8.1189*y.^2 - 1.9553*y - 1.2304;

I want to plot a 3D curve and I am not succeding. Can someone help me please? Thanks

Answer 1

[Star Strider]

Use the plot3 function.

If you then want it as a 2D plot, use the view function to rotate it.

EDIT — (23 Apr 2021 at 03:17)

I was intending that to use any of the q3D plotting functions, the vectors need to be transformed into matrices first, and then transformed back into vector to use plot3:

X = ...;
Y = ...;
[Xm,Ym] = ndgrid(X,Y);
Zfcn = @(x,y) 4.6597*x.^2 + 2.6897*x.*y - 0.023578*x + 8.1189*y.^2 - 1.9553*y - 1.2304;
Zm = Zfcn(Xm,Ym);

figure
plot3(Xm(:), Ym(:), Zm(:))
grid on

.

[Bryan Ambrósio]

Great man, thank you so much.

[Star Strider]

My pleasure!

[Bryan Ambrósio]

I am also trying to use

contour(Xm, Ym, Zm), it worked just fine

But i want to be sure if the Z axis is beeing cutted in points 1, 2, 3, ..., 8.

How can i make sure of that?

And i think it will be better if in figure of contour the ordened pair appears only as a 'x' or 'o', and not the entire line

[Star Strider]

If you want to specify those contours:

contour(Xm, Ym, Zm, (1:8))

The extra parentheses are not strictly necessay, I added them for clarification.

X = ...;
Y = ...;
[Xm,Ym] = ndgrid(X,Y);
Zfcn = @(x,y) 4.6597*x.^2 + 2.6897*x.*y - 0.023578*x + 8.1189*y.^2 - 1.9553*y - 1.2304;
Zm = Zfcn(Xm,Ym);

figure
contour(Xm, Ym, Zm, (1:8))
axis('equal')

If you want to label the contours:

contour(Xm, Ym, Zm, (1:8), 'ShowText','on')

That worked when I tried it with my own set of vectors.

[Bryan Ambrósio]

Hi, im sorry to bother you again, but now i need to make the same plots, but:

X, Y and Z are vector (not function anymore), how do i do that?

I have tried:

X = ...;

Y = ...;

Z = ...;

[Xm,Ym, Zm] = ndgrid(X,Y,Z);

figure

contour(Xm, Ym, Zm, (1:8))

axis('equal')

but its not working

[Star Strider]

No worries!

Do this instead —

X = ...;
Y = ...;
Z = ...;
xv = linspace(min(X),max(X),numel(X));
yv = linspace(min(Y),max(Y),numel(Y));
[Xm,Ym] = ndgrid(xv,yv);
Zm = griddata(X,Y,Z,Xm,Ym);

figure
contour(Xm, Ym, Zm, (1:8))
axis('equal')

That should work.

.

[Bryan Ambrósio]

Thank you so much!

[Star Strider]

As always, my pleasure!

Answer 2

[Bryan Ambrósio]

I have tried plot3, but it doesnt fill my needs entirely

Answer 3

[Bryan Ambrósio]

in plot3 z cannot be a function