constrain surf() plot region

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!

Answer 1

[Kelly Kearney]

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);

[Pete]

Thanks Kelly, this did the job. Another question which I will post later after some browsing through help is to convert the Z scale colour to RGB, but only have the red channel used (so output would be scaled from 0 to 1 in red only (did try

colorplot([Z 0 0])

but ineffective.

Any advice?

Pete

[Kelly Kearney]

You mean you want a colorbar that goes from white to red over the range of 0-1?

cmap = flipud([ones(20,1) linspace(0,1,20)'*ones(1,2)]); 
colormap(cmap);
set(gca, 'clim', [0 1]);
colorbar;

[Pete]

Hi Kelly

Many thanks for this answer. This is almost exactly what I want, just that I want it going from black to red. Can you explain the ones function as used above - I'm trying to figure it out in terms of the [RGB] format, but I don't think I'm seeing the syntax properly (I am very new to matlab!)

Many thanks,

Pete

[Pete]

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]

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)];

Answer 2

[Pete]

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?