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

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Eric . 2016년 5월 10일
댓글: Seung Jae Lee . 2023년 4월 5일
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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### 답변 (2개)

Mike Garrity 2016년 5월 10일
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 2018년 5월 10일
Hello Mike,
How would I approach this problem if I was trying to solve for the area of the 2d projection?
Naveen Pathak 2021년 6월 7일
Hi Mike,
I have a simliar problem. Please, could you help me to plot the following:
Thank you.

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Y.S. 2023년 3월 29일
2 methods I found that I want to place here for future reference:
 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; %
 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);
I am still looking for a fast method that works for all shapes, but havent found any
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Naveen Pathak 2023년 3월 30일
Thanks "Y.S." at least you showed a kind gesture.
Matlab people are worth-less to help.
However, I have already solved this problem few months ago :)
Seung Jae Lee 2023년 4월 5일
Hi, Naveen. How could you solve this problem? Can you please share your findings?

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