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Tight Boundary around a Set of Points
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Jason Nicholson
am 16 Aug. 2016
Bearbeitet: Jason Nicholson
am 4 Jan. 2018
I want to draw a tight boundary around a set of points. The code is used below to draw the black boundary around the blue points. The red areas show areas that are not tight enough for my application. Does anyone have a suggestion on how to get a tighter boundary? The data is attached.
clc; clear; close all;
load('xy.mat')
xy = [x y];
plot(xy(:,1), xy(:,2),'.')
grid on;
hold all;
xyNormalized = xy;
for iColumn = 1:2
minValue = min(xy(:,iColumn));
maxValue = max(xy(:,iColumn));
xyNormalized(:,iColumn) = (xy(:,iColumn)-minValue)/(maxValue - minValue);
end
k = boundary(xyNormalized,1);
xyBoundary = xy(k,:);
h = plot(xyBoundary(:,1), xyBoundary(:,2),'k');
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/155798/image.jpeg)
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FirefoxMetzger
am 16 Aug. 2016
This draws a boundary along the highest elements of each class, the lowest elements of each class and has the left and right outer classes as boundary:
load('xy.mat')
xy = [x y];
plot(xy(:,1), xy(:,2),'.')
grid on;
hold all;
%split data into classes and find max/min for each class
class_label = unique(x);
upper_boundary = zeros(size(class_label));
lower_boundary = zeros(size(class_label));
for idx = 1:numel(class_label)
class = y(x == class_label(idx));
upper_boundary(idx) = max(class);
lower_boundary(idx) = min(class);
end
% plot a nice boundary in red
left_boundary = y(x == class_label(1));
right_boundary = y(x == class_label(end));
plot(class_label(1)*ones(size(left_boundary)),left_boundary,'r')
plot(class_label,upper_boundary,'r')
plot(class_label(end)*ones(size(right_boundary)),right_boundary,'r')
plot(class_label,lower_boundary,'r')
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/174890/image.png)
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Sinvaldo Moreno
am 24 Dez. 2017
I believe the fast and easy way is get the boundaries:
% code
load('xy.mat');
xy = [x y]
[k, v] = boundary(xy(:,1),xy(:,2));
plot(xy(:,1), xy(:,2),'.');
hold on;
plot(xy(k,1),xy(k,2));
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