# Problem using boundary and trisurf functions to create a mesh with three points

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Elena Martín 2019 年 3 月 25 日

Hi, I am having some difficulties using these functions. I have three points and I would like to create a mesh with them, if I have four points these work perfectly but when I only insert three points boundary returns and empty matrix. I would like to know whats wrong and if there is anyway I can fix it. What I want is to create a surface between those three points, maybe there is another way to do it but I cant figure it out. Thanks!
p(1,1) = -26,9030200000000;
p(1,2) = 74,5056900000000;
p(1,3) = 1,76957800000000
p(2,1) = -31,3498700000000;
p(2,2) = 71,4396500000000;
p(2,3)= 26,4499500000000;
p(3,1) = 26,9030200000000;
p(3,2) = 74,5056900000000
p(3,3) = 1,76957800000000;
k = boundary(p)
hold on
trisurf(k,p(:,1),p:,2),p(:,3),'Facecolor','red','FaceAlpha',1)
%'EdgeColor', 'none')
grid off

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### 採用された回答

John D'Errico 2019 年 3 月 25 日
You have THREE points, in a 3 dimensional space.
How many points determine a volume in 3-d? 4. Essentially, a tetrahedron in 3 dimensions is as simple as you can get. So if you have only 3 points, they determine a triangle. But they are not sufficient to determine a tetrahedron. And you certainly will not create a boundary surface in 3-d.
If your goal is as simple as creating a surface that spans those 3 points, then this is simple. Just use patch.
patch(p(:,1),p(:,2),p(:,3),'r')
box on
grid on

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