MATLAB: What’s wrong with the 3d plot

3d plotsMATLABmeshplotsurf

Hello everyone, I want to plot a 3d plot of the function z=2*X.^2-3*(X.*Y)+4*Y.^2 My code is the following: [X,Y]=meshgrid(-2:0.1:2); z=2*X.^2-3*(X.*Y)+4*Y.^2 surf(X,Y,z)

However when the plot appears the z axis is going up to 35 which shouldn't be correct since maximum value of z whould be for X=-2 and Y=2 which gives Z=12. What am I doing wrong?

Best Answer

If you add this max call to your code (after you calculate ‘z’), you arrive at a different result:

max_z = max(z(:))
max_z =
    36

So there is nothing wrong with your plot.

You can find the indices of the maximum with:

[r,c] = find(z == max_z)

that returns [41,1] and [1,41] for the two maxima.

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MATLAB: Add contours to scatter3 plot

Contours can be drawn on a 2D slice in a 3D figure using contourslice.

I'm not sure what the contours are that you want to plot. I just specify some random contour in variable V.

Note that you are not projecting points at the wall, but at the location of the maximum. I fixed this by actually plotting at the limits.

x = mvnrnd(1, 3, 100);
y = mvnrnd(2, 3, 100);
z = mvnrnd(3, 3, 100);
scatter3 (x, y, z, 'filled')
hold on
xl = xlim;
yl = ylim;
zl = zlim;
xlim(xl); ylim(yl); zlim(zl); % fix the limits for if we rotate
scatter3 (0*x+xl(2), y, z, '.')
scatter3 (x, 0*y+yl(2), z, '.')
scatter3 (x, y, 0*z+zl(1), '.')
rotate3d on
[X,Y,Z]=meshgrid(round(xl(1)):round(xl(2)),round(yl(1)):round(yl(2)),round(zl(1)):round(zl(2)));
V = X.^2+Y.^2+Z.^2;
contourslice(X,Y,Z,V,xl(2),yl(2),[zl(1) zl(2)])
hold off

MATLAB: How can i plot this 3D function

Paraboloid is Z = X.^2 + Y.^2;

[X,Y]= meshgrid(-10:10);
Z = X.^2 + Y.^2;
surf(X,Y,Z)

Due to exp() term in your equation, you won't necessarily get a paraboloid.

Best Answer

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MATLAB: Add contours to scatter3 plot

MATLAB: How can i plot this 3D function

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