Finding concave hull around abstract shape

Question

Nicolas Kaiser 2024-10-10

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https://ww2.mathworks.cn/matlabcentral/answers/2158865-finding-concave-hull-around-abstract-shape

评论： Nicolas Kaiser 2024-10-28，7:24

matlab_answer_example.mat

Hallo together,

I have quite some abstract shape and try to found a more rectangular concave hull around it. I attached the polyshape data.

Now I wanna achieve a boundary faced like this and generate a patch object out of it:

I tried already a lot. Using the boundary function or alphaShape, but I could never reach a similar results. Either I had hole in my polyshape object at the end, or there was to much area close to the circle attached to the new polyshape object. Of course the with red indicated areas to be added to the final polyshape object dont have to be exactly like in the picture but similar I wanna achieve.

Also similar shapes should be solvelable.

Does somebody have a idear how to solve this? I would be really glad.

Thanks a lot!

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Answer 1

Star Strider 2024-10-10

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https://ww2.mathworks.cn/matlabcentral/answers/2158865-finding-concave-hull-around-abstract-shape#answer_1530395

在 MATLAB Online 中打开

matlab_answer_example.mat

I am not certain what you want. The information creates a 2D polyshape.

The boundary function is likely most appropriate here, although you will have to experiment with various values of the shrink factor to get the result you want.

Try this —

LD = load('matlab_answer_example.mat')

LD = struct with fields:

new_polyShape1: [1x1 polyshape]

ps = LD.new_polyShape1

ps =

polyshape with properties: Vertices: [722x2 double] NumRegions: 1 NumHoles: 0

xv = ps.Vertices(:,1);

yv = ps.Vertices(:,2);

[ixb1,v1] = boundary(xv, yv, 0.1)

ixb1 = 211×1

1 722 721 720 719 718 717 716 715 714

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v1 = 2.5577e+03

[ixb2,v1] = boundary(xv, yv, 0.8)

ixb2 = 240×1

1 722 721 720 719 718 717 716 715 714

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v1 = 821.0291

figure

plot(ps, 'DisplayName','Original Polyshape')

hold on

plot(xv(ixb1), yv(ixb1), '-c', 'DisplayName','Shrink = 0.1')

plot(xv(ixb2), yv(ixb2), '-m', 'DisplayName','Shrink = 0.8')

hold off

axis('equal')

legend('Location','best')

.

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Answer 2

Matt J 2024-10-10

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https://ww2.mathworks.cn/matlabcentral/answers/2158865-finding-concave-hull-around-abstract-shape#answer_1530485

编辑：Matt J 2024-10-11

在 MATLAB Online 中打开

matlab_answer_example.mat

ps = load('matlab_answer_example.mat').new_polyShape1;

tic;

V=cell2mat(arrayfun(@(i) upsamp(polybuffer(ps,-i)),[0,1],'UniformOutput',0)');

shp=alphaShape(V,6,'HoleThreshold',2*area(ps));

toc

Elapsed time is 1.110689 seconds.

plot(shp,'EdgeColor','none');hold on

plot(ps, 'FaceColor','none','EdgeColor','red');hold off

axis equal

function V=upsamp(ps)

V=ps.Vertices;

V(any(~isfinite(V),2),:)=[];

V(end+1,:)=V(1,:);

N=height(V);

V=interp1(V,linspace(1,N,100*N));

V(end,:)=[];

end

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Nicolas Kaiser 2024-10-11

Thanky Matt! That looks for this case already like what I wanted to achieve. I will check if I can adjust it also to different case. Maybe also to find in general a better approach.

Matt J 2024-10-11

I've modified it slightly to make it faster.

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Answer 3

John D'Errico 2024-10-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2158865-finding-concave-hull-around-abstract-shape#answer_1530505

编辑：John D'Errico 2024-10-10

在 MATLAB Online 中打开

matlab_answer_example.mat

I don't know the soure of this data. But, can you start with the two pieces as separate entities, form polyshapes from each, and then use the union tool? For example, here I'll separate the two into pieces artificially.

load matlab_answer_example.mat

I1 = 70:570;

I2 = [1:70,571:722];

xy = new_polyShape1.Vertices;

S1 = polyshape(xy(I1,:));

S2 = polyshape(xy(I2,:));

Warning: Polyshape has duplicate vertices, intersections, or other inconsistencies that may produce inaccurate or unexpected results. Input data has been modified to create a well-defined polyshape.

H1 = plot(S1);

H1.FaceColor = 'r';

hold on

H2 = plot(S2);

H2.FaceColor = 'b';

hold off

S3 = union(S1,S2);

plot(S3)

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Nicolas Kaiser 2024-10-11

MatlabAnswer_polyShapes.mat

Okay let me try to explain my aim and the origin of my question. I uploaded the corresponding data.

In general I have multiple circels in region 1 and in region 2. The assigment of the circels to this regions are deterministic. For region 1 and region 2 I calculated the convex hull around the corresponding circles, leading to polyShape_1 and polyShape_2. In same cases thats all what I have to do. Unfortunatly in same cases, like the case mentioned before the two polyShapes overlap.

I want now to adjust the polyshapes to resolve the overlapping area without excluding a circle from the assigned region and polyShape_1 or polyShape_2. Additionaly I want, that both polyshapes have a distance of x (x=1) towards each other. This should be always possible due to adjustment of the placement of the circles. But I also, as stated in the original question, do not want to just cut-out the overlapping part from each polyshape to gain a more rectangular or straight polyshape.

I hope I could explain it now better.

Steven Lord 2024-10-11

在 MATLAB Online 中打开

MatlabAnswer_polyShapes.mat

What specifically do you expect the two polyshapes to look like in this case after you've adjusted them?

clear all

load MatlabAnswer_polyShapes.mat

whos

Name Size Bytes Class Attributes polyShape_1 1x1 3529 polyshape polyShape_2 1x1 3497 polyshape polyShapes_1_CutOut 8x1 40136 polyshape polyShapes_2_CutOut 4x1 20068 polyshape

plot(polyShapes_1_CutOut, FaceColor='r')

hold on

plot(polyShapes_2_CutOut, FaceColor='c')

Do you want one of the polyshapes to contain only the cyan circles and the other only the red?

figure

plot(polyShapes_1_CutOut, FaceColor='r')

hold on

plot(polyShapes_2_CutOut, FaceColor='c')

plot(polyShape_1, FaceColor = 'none', EdgeColor = 'r')

plot(polyShape_2, FaceColor = 'none', EdgeColor = 'c')

polyShape_3 = subtract(polyShape_2,polyShape_1);

plot(polyShape_3, FaceColor = 'none', EdgeColor = 'k')

The subtract function got part of the way there, though the edge goes through one of the cyan circles. Is this on the right track for what you're hoping to create?

And just to confirm that you mean what I think you mean, when you say "Additionaly I want, that both polyshapes have a distance of x (x=1) towards each other." do you mean that you want the shortest distance between a point on the polyshape surrounding the cyan circles and a point on the polyshape surrounding the red circles to be 1 unit (exactly 1 unit or at least 1 unit?) So if we applied polybuffer with a distance of 1 to one of the polyshapes, it would not overlap (or not even touch) the other?

Nicolas Kaiser 2024-10-28，7:24

Sorry for the late response. You summarized it quite well. So one polyshapes should include the cyan coloured circles and the other one the red coloured circles. Exactly if we apply polybuffer to one of the polyshapes with a distance of 1 they should touch but should not overlapp (so exactly 1).

In the end I want to gain a polyshape that doesn't have the exact boundary the subtraction of the overlapping area from both polyshapes would have, to prevent a polyshape like in the post of John D'Errico but more like one in the post of Matt J. So to speak I want the resulting polyshape to be more straight and "smooth".

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Finding concave hull around abstract shape

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