How to plot a circle in Matlab?

I would like to know how can I graph circles in matlab knowing the center and radius? I have tried circles() which does not seem to work because my matlab version does not have it. I know I can use the Rectangle function to do so but it is a rather complex way of doing it as I would need to work out the leftmost point everytime. Isnt there a more simple way for me to draw a circle JUST knowing center and radius?

many thanks

回答1:

Don't laugh, but the easiest would be to use the rectangle function, indeed ;)

%// radius
r = 2;

%// center
c = [3 3];

pos = [c-r 2*r 2*r];
rectangle('Position',pos,'Curvature',[1 1])
axis equal

but set the curvature of the rectangle to 1!

The position vector defines the rectangle, the first two values x and y are the lower left corner of the rectangle. The last two values define width and height of the rectangle.

pos = [ [x y] width height ]

The lower left corner of your circle - yes, this circle has corners, imaginary ones though - is the center c = [3 3] minus the radius r = 2 which is [x y] = [1 1]. Width and height are equal to the diameter of the circle, so width = 2*r; height = width;

In case you don't like the smoothness of the above solution, there is no way around using the obvious way of drawing an actual circle by use of trigonometric functions.

%// number of points
n = 1000;

%// running variable
t = linspace(0,2*pi,n);

x = c(1) + r*sin(t);
y = c(2) + r*cos(t);

%// draw line
line(x,y)

%// or draw polygon if you want to fill it with color
%// fill(x,y,[1,1,1])
axis equal

回答2:

If you want a circle that is not rectangle you can use annotation

center = [3, 4];
r = 2;
pos = [center-r, 2*r 2*r];
annotation('ellipse', pos );