# data pick by given geometry

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Yu Li 2019 年 4 月 4 日
コメント済み: Image Analyst 2019 年 4 月 8 日
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:
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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### 回答 (2 件)

Image Analyst 2019 年 4 月 5 日
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 年 4 月 8 日
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 年 4 月 8 日
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*');

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Walter Roberson 2019 年 4 月 5 日

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