How to convert 2D triangulation to binary image?

Question

Geert 2013-8-21

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https://ww2.mathworks.cn/matlabcentral/answers/85261-how-to-convert-2d-triangulation-to-binary-image

评论： T. Dunker 2022-7-6

采纳的回答： Sven

在 MATLAB Online 中打开

Does anyone know of an easy way to convert a 2D triangulation to a binary image, which represents the inside?

Let my clarify my question with a picture: http://i41.tinypic.com/2gvvvnl.jpg The goal is to go from left to right...

My triangulation is given its Points and the ConnectivityList (i.e., it was created with the matlab function "triangulation").

It is important that no holes get filled and all features are preserved.

BEST ANSWER SO FAR:

% Make some data
pts = rand(50,2);
DT = delaunayTriangulation(pts);
% Find a central triangle to remove
[~,holeTrias] = min(sum(abs(DT.incenter - 0.5),2));
holeTrias = [holeTrias DT.neighbors(holeTrias)];
triasMask = ~ismember(1:DT.size(1), holeTrias);
T = triangulation(DT.ConnectivityList(triasMask,:), DT.Points);
% Define query points
X = 0:0.05:1;
Y = 0:0.05:1;
[xMat,yMat] = meshgrid(X,Y);
IN = false(size(xMat));
for t = 1:size(T,1)
    vertsXY = T.Points(T.ConnectivityList(t,:),:);
    IN = IN | inpolygon(xMat,yMat, vertsXY(:,1), vertsXY(:,2));
end
% Show the result
figure
patch('faces',T.ConnectivityList,'vertices',T.Points,'FaceColor','g')
hold on
plot(xMat(IN),yMat(IN),'b.',xMat(~IN),yMat(~IN),'r.')

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Answer 1

Sven 2013-8-21

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https://ww2.mathworks.cn/matlabcentral/answers/85261-how-to-convert-2d-triangulation-to-binary-image#answer_94764

编辑：Sven 2013-8-21

在 MATLAB Online 中打开

Hi Geert,

Here's something that does what you want. It's not optimised for speed as it simply iterates naively through triangles, but it's at least correct.

% Make some data
pts = rand(50,2);
DT = delaunayTriangulation(pts);
% Find a central triangle to remove
[~,holeTrias] = min(sum(abs(DT.incenter - 0.5),2));
holeTrias = [holeTrias DT.neighbors(holeTrias)];
triasMask = ~ismember(1:DT.size(1), holeTrias);
T = triangulation(DT.ConnectivityList(triasMask,:), DT.Points);
% Define query points
X = 0:0.05:1;
Y = 0:0.05:1;
[xMat,yMat] = meshgrid(X,Y);
% Query each point
IN = false(size(xMat));
for i = 1:numel(IN)
    for t = 1:size(T,1)
        vertsXY = T.Points(T.ConnectivityList(t,:),:);
        if inpolygon(xMat(i),yMat(i), vertsXY(:,1), vertsXY(:,2))
            IN(i) = true;
            break;
        end
    end
end
% Show the result
figure
patch('faces',T.ConnectivityList,'vertices',T.Points,'FaceColor','g')
hold on
plot(xMat(IN),yMat(IN),'b.',xMat(~IN),yMat(~IN),'r.')

How does that work for you?

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Geert 2013-8-22

在 MATLAB Online 中打开

Thanks you guys, nice work!

I was able to improve the speed even more (by a factor 10), by applying some vectorization:

% Query each point
IN = false(size(xMat));
for t = 1:size(T,1)
    vertsXY = T.Points(T.ConnectivityList(t,:),:);
    IN = IN | inpolygon(xMat,yMat, vertsXY(:,1), vertsXY(:,2));
end

Speeds can be compared with the following code:

% Make some data
pts = rand(50,2);
DT = delaunayTriangulation(pts);
% Find a central triangle to remove
[~,holeTrias] = min(sum(abs(DT.incenter - 0.5),2));
holeTrias = [holeTrias DT.neighbors(holeTrias)];
triasMask = ~ismember(1:DT.size(1), holeTrias);
T = triangulation(DT.ConnectivityList(triasMask,:), DT.Points);
% Define query points
X = 0:0.05:1;
Y = 0:0.05:1;
[xMat,yMat] = meshgrid(X,Y);
tic
% Query each point
IN1 = false(size(xMat));
k = convhull(T.Points);
for i = find(inpolygon(xMat,yMat, T.Points(k,1), T.Points(k,2)))'
    for t = 1:size(T,1)
        vertsXY = T.Points(T.ConnectivityList(t,:),:);
        if inpolygon(xMat(i),yMat(i), vertsXY(:,1), vertsXY(:,2))
            IN1(i) = true;
            break;
        end
    end
end
disp(['First method elapsed time  = ', num2str(toc)])
tic
IN2 = false(size(xMat));
for t = 1:size(T,1)
    vertsXY = T.Points(T.ConnectivityList(t,:),:);
    IN2 = IN2 | inpolygon(xMat,yMat, vertsXY(:,1), vertsXY(:,2));
end
disp(['Second method elapsed time = ', num2str(toc)])
% Show the result
figure
subplot(1,2,1)
patch('faces',T.ConnectivityList,'vertices',T.Points,'FaceColor','g')
hold on
plot(xMat(IN1),yMat(IN1),'b.',xMat(~IN1),yMat(~IN1),'r.')
title('first result: IN1')
subplot(1,2,2)
patch('faces',T.ConnectivityList,'vertices',T.Points,'FaceColor','g')
hold on
plot(xMat(IN2),yMat(IN2),'b.',xMat(~IN2),yMat(~IN2),'r.')
title('second result: IN2')

Once again: many thanks!

T. Dunker 2022-7-6

在 MATLAB Online 中打开

with R2018b and later this simplifies to

%% Make some data

pts = rand(50,2);

DT = delaunayTriangulation(pts);

% Find a central triangle to remove

[~,holeTrias] = min(sum(abs(DT.incenter - 0.5),2));

holeTrias = [holeTrias DT.neighbors(holeTrias)];

triasMask = ~ismember(1:DT.size(1), holeTrias);

T = triangulation(DT.ConnectivityList(triasMask,:), DT.Points);

% Define query points

X = 0:0.02:1;

Y = 0:0.02:1;

%% use polyshape

tic

[xMat,yMat] = meshgrid(X,Y);

bp = boundaryshape(T);

IN = reshape(isinterior(bp, xMat(:), yMat(:)), size(xMat));

toc

Elapsed time is 0.178962 seconds.

%%

colormap(gray(2))

imagesc(X, Y, IN)

hold on;

patch('faces', T.ConnectivityList, 'vertices', T.Points, ...

'FaceColor', 'r', 'FaceAlpha', .2)

hold off;

axis equal;

axis tight;

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How to convert 2D triangulation to binary image?

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