data pick by given geometry

I have a set of scatter data, I want to do someting like below:

1. use a given coordinate in the x-y system as input
2. use this coordinate as center, draw a circle in this figure with given radius
3. find the points inside this circle

is there anyway to do this in Matlab?

I attached a set of test data, below is the test code to reproduce a similiar figure like before:

load test
f=figure(1);
f.Units='pixels';
scatter(test(:,1),test(:,2));
f.Position(4)=f.Position(4)*0.5;

Thanks!

Yu

Answer 1

[Image Analyst]

Get the indexes with distances less than R then extract the corresponding x and y coordinates.

x = test(:,1);
y = test(:,2);
plot(x, y, 'b.', 'MarkerSize', 18)
uiwait(msgbox('Locate circle center'));
[xCenter, yCenter] = ginput(1)
distances = sqrt((x-xCenter).^2 + (y-yCenter).^2);
indexes = distances <= R;
xInside = x(indexes);
yInside = y(indexes);

[Yu Li]

Thanks for your reply, I have think about your approach, but...

in the figure, the x-y coordinate is not a 'axis-equal' format, which means that, the circle you see in the figure is indeed a ellipse.. not a circle. so I think the first approach should be to derive the ellipse function, and I'm working on it.

[Image Analyst]

How are you having your users interactively select points? As far as I know, there is no equivalent to the Image Processing Toolbox's roipoly() or imrect() or imfreehand() for non-image data (that is, plots).

[Walter Roberson]

I am confused about which coordinate system the circle needs to be in ? Does the circle need to be in X Y (data) coordinates? Does the circle need to be in screen coordinates?

[Image Analyst]

If you have the Image Processing Toolbox, you can use drawfreehand() to lassoo points. The function works on graphs also (in addition to images of course).

% Draw data
x = rand(100,1);
y = rand(length(x), 1);
scatter(x, y);
grid on;

% Let user lassoo some points interactively.
uiwait(msgbox('Draw a region'));
hFH = drawfreehand() % Required Image Processing Toolbox
% Get x and y points of what user drew.
xfh = hFH.Position(:, 1)
yfh = hFH.Position(:, 2)

% Find points inside the shape.
itsInside = false(length(x), 1);
for k = 1 : length(x)
	itsInside(k) = inpolygon(x(k), y(k), xfh, yfh);
end
% Extract the points that are inside to new variables. (Optional)
xInside = x(itsInside);
yInside = y(itsInside);
% Plot a red asterisk over the points 
% that are inside the region they drew.
hold on;
plot(xInside, yInside, 'r*');

Answer 2

[Walter Roberson]

See https://www.mathworks.com/help/stats/exhaustivesearcher.rangesearch.html