Patch with holes

Question

Ronen 2011-7-25

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https://ww2.mathworks.cn/matlabcentral/answers/12335-patch-with-holes

回答： Chad Greene 2018-5-24

Your quick assistance please :

I got a group of vertices that actually define a square with a squared hole inside. I want the matlab to patch the vertices so it displays the square with the hole inside. As you may know, patch treats the vertices as if they define a closed shape, that is, without the hole inside. can I display that holed shape after all ? Thanks fellows

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Answer 1

Chad Greene 2018-5-24

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在 MATLAB Online 中打开

There's now a way to do this with standard Matlab functions. Starting with R2017b, the new polyshape function makes it easy. Just put NaNs between the outer shape and the holes.

x = [1 5 5 1 nan 2 4 3]; 
y = [1 1 6 6 nan 3 2 5]; 
pgon = polyshape(x,y); 
plot(pgon)

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Answer 2

the cyclist 2011-7-25

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Can you identify the vertices of the hole? If so, you could create a patch for the square, then overlay a patch with the hole.

You should post some code with what you have so far, to help us better understand what you've got.

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Answer 3

Kelly Kearney 2011-7-25

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If you have the Mapping Toolbox, look at poly2fv, then plot using patch's face/vertex syntax.

This function uses a mex-ed version of the General Polygon Clipper (GPC) library. If you don't have the Mapping Toolbox, and your polygons are complicated enough that triangulating by hand would not be possible, there's at least one FEX entry that seems also mex this library: Polygon Clipper... might be helpful.

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Answer 4

Ronen 2011-7-26

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Thanks, it goes very well so far. Now for one more small question : suppose I define a face (completely filled or with holes) using triangular composition (with that poly2fv etc.). would it be possible to extract the normal vector to the face ? In fact, how much information can I extract from my face (normal, which edges belong to outer-line and which aren't..)

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Kelly Kearney 2011-7-27

Assuming your polygon lies within a plane, then the surface normal should be easy to calculate (cross product of any two edges). As far as distinguishing outer and inner edges, Matlab uses clockwise vertices to define the outer face and counterclockwise vertices to define any inner faces. Once the polygon has been triangularized, then this wouldn't be so obvious to determine, but I assume you still have the original set of vertices.

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