How to plot a circle on top of my plotted image

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Ruben Alfaro
Ruben Alfaro on 9 May 2014
Answered: Hugo Diaz on 5 Feb 2022
I have an image plotted and I wanted to plot a circle on top of it with the same centroid of the image.
I want the radius to be 500 and I want it done with a red color, of somebody could help me with this.
thank you very much

Answers (4)

lvn
lvn on 9 May 2014
To plot on top of an existing figure, start with
hold on

Ruben Alfaro
Ruben Alfaro on 9 May 2014
Thank you,
But it doesnt seem to work for me, maybe im doing something wrong. can somebody help me figure it out or recommend another way of doing this?
  1 Comment
Roberto
Roberto on 9 May 2014
post the error you're having, or explain why it doesn't work for you?

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Image Analyst
Image Analyst on 9 May 2014
Try this:
grayImage = imread('concordorthophoto.png');
imshow(grayImage);
axis on;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
[rows, columns, numberOfColorChannels] = size(grayImage);
hold on; % Don't let plot blow away the image.
xCenter = columns / 2;
yCenter = rows / 2;
theta = 0 : 0.01 : 2*pi;
radius = 500;
x = radius * cos(theta) + xCenter;
y = radius * sin(theta) + yCenter;
plot(x, y, 'r-', 'LineWidth', 3);
grid on;

Hugo Diaz
Hugo Diaz on 5 Feb 2022
%The trick is to do it 3D, and a 2D plot is like a view from above.
% your 2D plot. If your function f(x,y) < m, then
hold on
r = 1;
teta = -pi:0.01:pi;
x = r * cos(teta);
y = r * sin(teta);
plot3(x, y, m * ones(1, numel(x)), 'Color','k')
hold off

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