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Convert 3D polygon into homogeneous matrix

Asked by João Viveiros on 13 Oct 2012

Hi. I have defined a 3D polygon creating a vertice and a face matrix to use the function Patch. Now i would like to know: how do i transform my 3D polygon in an homogeneous matrix.

Any clues ?

Thanks for the help.

2 Comments

Walter Roberson on 13 Oct 2012

What would the homogenous matrix consist of? For example are you looking to render it into a matrix? If so what value do you want at each location, taking into account that the polygon would in general be colored ?

João Viveiros on 13 Oct 2012

I need it homogeneous in order to aply geométric transformatrions to my polygon. I have this at the moment:

clc;clear all;close all;
vert=[2,0,0;2,1,0;1,1,0;1,0,0;1,0,1;2,0,1;2,1,1;1,1,1;1,2,1;0,2,1;0,1,1;
      0,1,0;1,2,0;0,2,0;0,0,0;1,1,2;0,1,2;0,0,2;1,0,2;0,0,1;];
faces=[1,2,3,4;2,3,8,7;1,4,5,6;5,6,7,8;1,2,7,6;
       3,8,9,13;3,12,14,13;9,13,14,10;11,12,14,10;8,9,10,11;
       19,18,17,16;20,11,17,18;16,8,11,17;5,8,16,19;18,19,5,20;
       20,11,12,15;5,20,15,4;3,4,15,12;];
P=patch('Faces',faces,'Vertices',vert,'FaceColor','r');
axis equal;
axis([-6 6 -6 6 -6 6])
xlabel('x');ylabel('y');zlabel('z');
hold on;grid on;
view(140,40);

'I need that P to be the homogeneous matrix so i can apply the geométric transformation to it by doing B=translation(3,0,0)*P (For example)

B is the new polygon transformed.

thanks

João Viveiros

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3 Answers

Answer by Walter Roberson on 13 Oct 2012
Accepted answer

The matrix "vert" in your code is your homogenous matrix for your purposes described in your comment.

1 Comment

João Viveiros on 13 Oct 2012

Thanks. I homegenised my "vert" then, did a transformation to that matrix then i just re-patched.

Walter Roberson
Answer by Matt J on 13 Oct 2012

Couldn't you just transform all of the vertices and recreate the patch? E.g.,

 vert_translated=bsxfun(@plus, vert, [3,0,0]);
 P=patch('Faces',faces,'Vertices',vert_translated,'FaceColor','r');

Note that invertible linear/affine transformations don't change the faces.

1 Comment

João Viveiros on 13 Oct 2012

I did that ;) thanks. its working

Matt J
Answer by Matt J on 13 Oct 2012

Instead of using PATCH, you could also consider using these 2 FEX files,

http://www.mathworks.com/matlabcentral/fileexchange/30892-representing-polyhedral-convex-hulls-by-vertices-or-inequalities

http://www.mathworks.com/matlabcentral/fileexchange/9261-plot-2d3d-region

So first, this willl let you obtain your polyhedron as a set of inequalities A*x<=b

 [A,b]=lcon2vert(vert);

Now you can transform the polyhedron as you like by appropriately transforming A and b. In the case of your translation, this would be

 Anew=A;
 bnew=b-A*[3;0;0];

Finally, you can plot using the plotregion() tool

 plotregion(-Anew,-bnew);

0 Comments

Matt J

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