MATLAB: Cirle with color changing gradually.

plot

How can I plot a circle like this in matlab, please give me an example. Thanks in advance.

Best Answer

[Edited to add colorbar and image.]

As far as I know, you can't do it directly, but pol2cart is your friend. Look at

http://www.mathworks.com/help/matlab/creating_plots/contour-plot-in-polar-coordinates.html

and maybe

http://blogs.mathworks.com/videos/2013/05/17/polar-surface-plot-in-matlab/

% construct grid of polar coords
th = (0:5:360)*pi/180;
r = 0:.05:1;
[TH, R] = meshgrid(th, r);
% convert polar to cartesian
[X, Y] = pol2cart(TH, R);
% dummy function plotted over X Y data
Z = X + 1i*Y;
f = abs((Z.^4 - 1).^(1/4));
% use overhead view of surf plot for figure
figure; 
surf(X, Y, f); 
view(2);
axis equal;
axis square;
colorbar('SouthOutside');
colormap('Jet');

Note that axis(2) gives an overhead view, so it looks like a flat circle, and axis equal ensures it looks like a circle and not a distorted ellipse.

Please accept this answer if it helps, or let me know what's wrong in the comments.

Related Solutions

MATLAB: Need help implementing a 2D circular gaussian

If you want to plot a surface, you need to use matrix arguments.

Try this:

x = linspace(-3, 3);
y = x;
[X,Y] = ndgrid(x,y);
R = {3, 2, 1};
for i=1:length(R)
    f = exp(-((X/R{i}).^2 + (Y/R{i}).^2));
    hold on;
    grid on;
    mesh(X, Y, f);
end
view(-30,30)

If you first define your function in polar coordinates, you can then use the pol2cart function to convert them to Cartesian coordinates. The plot should have a circular shape.

EDIT —

For example:

r = linspace(-3, 3);
th = linspace(0, 2*pi, 90);
[R,T] = ndgrid(r,th);
Z = exp(-R.^2);
[X,Y,Z] = pol2cart(T, R, Z);
figure
surf(X, Y, Z)
shading('interp')
grid on

produces:

Need help implementing a 2D circular gaussian - 2019 10 19.png

Note that I defined the original matrices as ‘r’ (radius) and ‘th’ (angle), then converted them to Cartesian and plotted them. This loses the angle information in the plot, so you need to create them yourself:

r = linspace(-3, 3);
th = linspace(0, 2*pi, 90);
[R,T] = ndgrid(r,th);
Z = exp(-R.^2);
[X,Y,Z] = pol2cart(T, R, Z);
thg = linspace(0, 2*pi, 12);
polgrid = [5*cos(thg); 5*sin(thg)];
figure
surf(X, Y, Z)
hold on
plot3([zeros(size(thg)); polgrid(1,:)], [zeros(size(thg)); polgrid(2,:)], zeros(2, size(thg,2))-0.1, '-k')
hold off
shading('interp')
Ax = gca;
Ax.XAxis.Color = 'none';
Ax.YAxis.Color = 'none';
Ax.XGrid = 'off';
Ax.YGrid = 'off';
grid on

MATLAB: Can I create 3-D polar plots in MATLAB

There is no single command that will produce a 3-D polar plot in MATLAB.

As a workaround, you can use the SURF command in combination with the POL2CART command to create a three-dimensional polar plot. For example:

[th,r] = meshgrid((0:5:360)*pi/180,0:.05:1);
[x,y] = pol2cart(th,r);
z=x+i*y;
f=(z.^4-1).^(1/4);     %Use your function here
surf(x,y,abs(f))

For an example of code that can be used to create radial lines and ticks, you can examine the code in the POLAR function, in $MATLAB\toolbox\matlab\graph2d\polar.m.

Best Answer

Related Solutions

MATLAB: Need help implementing a 2D circular gaussian

MATLAB: Can I create 3-D polar plots in MATLAB

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