If you just mean how to plot a circle in a figure, then this function will draw (an approximation to) a circle with a specified radius, at a specified (x,y) location. You can also specify the number of line segments that make up the circle.
function h = circle(x,y,r,nsegments)
if nargin<4
nsegments=50;
end
hold on
th = 0:2*pi/nsegments:2*pi;
xunit = r * cos(th) + x;
yunit = r * sin(th) + y;
h = plot(xunit, yunit);
hold off
end
I can't remember if I wrote this, or grabbed it off the File Exchange years ago, but there's no attribution in the file, so I can't give credit. You might search the FEX for other functions that may be better than this.
If you are talking more about image-processing, then I can't help you, except that I searched the FEX and found this:
