# how to get the contour/shape of a representation in 2D?

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Jessie Bessel am 16 Mär. 2022
Kommentiert: Star Strider am 24 Mär. 2022
Hello! I have two sets of x and y data that I plot. Following the representation, the result is the following figure.
When I use the plot function for representation, to connect the dots, I get the following result:
But I would be interested in the points being connected so as to create a closed surface, such as a contour or a shape, like in the following image:
Is there a matlab function that could help me in this regard?
I also attach the data in .mat.
Thank you very much!
!! Edit:
I just find out about the matlab function boundary and i use it and obtain the results from the figure. But I am not satisfied with the result because I would like all the points to be connected in one representation. And here, two point are left behind.
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### Akzeptierte Antwort

Star Strider am 16 Mär. 2022
Another approach —
x = LD1.x;
y = LD2.y;
cntrd = mean([x(:) y(:)]); % Centroid
angls = atan2(y(:)-cntrd(2), x(:)-cntrd(1)); % Angles Of Each Point W.R.T. Centroid
rads = hypot(y(:)-cntrd(2), x(:)-cntrd(1)); % Distances To Each Point From Centroid
figure
scatter(x, y, 10, 'k', 'filled') % Plot Points
hold on
plot(xc, yc, '-r') % Plot Connecting Lines
hold off
grid
axis equal
This should be reasonably robust, although if two points are essentially collinear with respect to their orientation from the centroid (have the same angle, so not all the angles are unique), it could have problems. The solution for that would be to ‘nudge’ the centroid so that all the angles are unique.
.
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Star Strider am 24 Mär. 2022
As always, my pleasure!

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### Weitere Antworten (2)

Torsten am 16 Mär. 2022
[k,~] = convhull(P);
plot(P(k,1),P(k,2))
with P being your points as an nx2 matrix.
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Torsten am 16 Mär. 2022
Bearbeitet: Torsten am 16 Mär. 2022
If you order the points in x,y arrays in the sequel they should be connected by lines, MATLAB will plot correctly, e.g.
x = [1 2 4 1 ]
y = [3 6 8 3 ]
plot(x,y)

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Simon Chan am 16 Mär. 2022
Point #8 is close to the previous one and you have to break it into 2 parts and combine them manually.
Of course, if the code is used for another combination of points, it does not work.
x1 = x(1:9); y1 = y(1:9); % Separate at point #8 & #9
x2 = x(8:end); y2 = y(8:end);
k1 = boundary(x1',y1');
k2 = boundary(x2',y2');
pgon1 = polyshape(x1(k1),y1(k1));
pgon2 = polyshape(x2(k2),y2(k2));
pgon = union(pgon1,pgon2); % Union 2 polygons
plot(pgon);
hold on
plot(x,y,'r*')
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