How can I create 2D projections from a 3D object?

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Eric 2016-5-10

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https://ww2.mathworks.cn/matlabcentral/answers/283474-how-can-i-create-2d-projections-from-a-3d-object

评论： Seung Jae Lee 2023-4-5

I know I can use "radon" for creating 1D projections from 2D, but how can I get 2D projections from 3D objects (such as from a 3D file like an STL file).

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Answer 1

Mike Garrity 2016-5-10

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One approach is the technique I showed in answers to this question, and this question.

The basic idea is that you take keep the faces of your geometry, but project the vertices onto the plane. That does mean that you've got multiple conincident polys, so it's not good for things like area computation. But if you just want the visual result, it works well and it's simple.

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Thomas Cornew 2018-5-10

Hello Mike,

How would I approach this problem if I was trying to solve for the area of the 2d projection?

Naveen Pathak 2021-6-7

Hi Mike,

I have a simliar problem. Please, could you help me to plot the following:

https://www.mathworks.com/matlabcentral/answers/815765-2d-projection-of-a-3d-f-x-y-z-plot?s_tid=srchtitle

Thank you.

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Answer 2

Y.S. 2023-3-29

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2 methods I found that I want to place here for future reference:

[1] fast, but only works for convex shapes. Assuming you have a struct (fvIn) with vertices & faces

fvIn.vertices(:,3)=0; % squash all Z coords
verts = fvIn.vertices;
faces = fvIn.faces;
a = verts(faces(:, 2), :) - verts(faces(:, 1), :); % compute area of all triangles
b = verts(faces(:, 3), :) - verts(faces(:, 1), :);
c = cross(a, b, 2);
area2 = 1/2 * sum(sqrt(sum(c.^2, 2))); % Calculate total area, but this gives double the area because the shape is squashed
Ap = area2/2; %

[2] slow, but works for all shapes

Loop over all triangles, project them on the Z=0 plane, create a polyshape and combine(union) it with the other projected triangles

P = fvIn.vertices(fvIn.faces(1,:),1:2);
psSurfTot = polyshape(P);
for N = 2:size(fvIn.faces,1)
    P = fvIn.vertices(fvIn.faces(N,:),1:2);
    psSurfTMP = polyshape(P);
    psSurfTot = union(psSurfTot,psSurfTMP);
end
% calculate projected area
Ap = area(psSurfTot);