data pick by given geometry

Question

Yu Li 2019-4-4

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454477-data-pick-by-given-geometry

评论： Image Analyst 2019-4-8

test.mat

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

use a given coordinate in the x-y system as input
use this coordinate as center, draw a circle in this figure with given radius
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

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Answer 1

Image Analyst 2019-4-5

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454477-data-pick-by-given-geometry#answer_369161

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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);

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Walter Roberson 2019-4-8

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 2019-4-8

在 MATLAB Online 中打开

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*');