constrain surf() plot region

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Pete
Pete 2014-11-10
评论： Kelly Kearney 2014-11-11
Hi all, I'm working with the surf(X,Y,Z) function, and trying to force some values to be specific. I'm basically trying to plot a 3D sin-wave, rotationally symmetric about (0,0) and contain a specific number of waves in any direction, then outside this region, force the Z values to zero. Here's code attempts using an 'if' but the Z=0 isn't applied at larger radii than Xmax:
NumberWaves = 2;
Each_Division = 0.1;
X_max = NumberWaves * (2 * pi) ;
X_min = -X_max;
Y_min = X_min;
Y_max = X_max;
[X,Y] = meshgrid(X_min : Each_Division : X_max);
Rad = sqrt (X.^2 + Y.^2 );
if (Rad < X_max)
Z=0;
else
Z = (((sin(Rad)) + 1) / 2);
end
figure;
colormap hsv;
surf(X,Y,Z, 'EdgeColor' ,'None');
xlabel('X (**)');ylabel('Y (**)');zlabel('Red Scale');
Alternatively to the forcing Z to a value, I was trying to find references to only plot the surface out to a given radius, which again would be rotationally symmetric. I'm very new to Matlab, so any help appreciated! Thanks!

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Kelly Kearney
Kelly Kearney 2014-11-10
I would recommend setting up your grid in polar coordinates. The surf command (along with mesh, pcolor, etc.) doesn't require a rectilinear grid, so starting in terms of r and theta allows you to set the cutoff exactly without any setting-to-0 or NaN-masking:
NumberWaves = 2;
Each_Division = 0.1;
rmax = NumberWaves * (2 * pi) ;
r = linspace(0, rmax, 50);
theta = linspace(0, 2*pi, 100);
[r,theta] = meshgrid(r, theta);
z = (sin(r) + 1)./2;
x = r .* cos(theta);
y = r .* sin(theta);
surf(x,y,z);
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Pete
Pete 2014-11-11
OK, all sorted - changed the main line here to:
cmap = ([linspace(0,1,101)'*ones(1,1) linspace(0,0,101)'*zeros(1,2)])
and it's now all working. A handy little tool that will come in useful I'm sure! Many thanks Kelly for this.
Pete
Kelly Kearney
Kelly Kearney 2014-11-11
Just so you know, you've got some unnecessary calculations there. You can get the same with
cmap = [linspace(0,1,101)' zeros(101,2)];

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Pete
Pete 2014-11-10
Hi Star Strider,
Surely Xmax is my domain (square in XY), but if I look at +X+Y, the radius is > Xmax, so at this point I would expect the Z value to be forced to zero?
Is there a way to crop the surface? Looking at the figure, ideally I want to set my Z value to 0.5, but the value is arbitrary at this point - or can I crop the graph once 2 waves have been completed?

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