Why do I get holes in my AlphaShape in MATLAB, even though the point spacing is smaller than the alpha value?

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Lars Urban
Lars Urban 2024-9-17
评论： Lars Urban 2024-9-19
I have a set of 3D points and want to reconstruct the shape of a car using AlphaShape. The problem I'm encountering is that I still get holes, even though the distance between the points is smaller than the alpha value. I have already tried adjusting the HoleThreshold with different values, but it has no effect on the result. I have included the code I am using and a picture (zoomed in on the door) showing the type of holes I am referring to.
%% Downsampling
gridstep = 0.008;
ptCloudDownSampled = pcdownsample(ptCloud,"gridAverage",gridstep);
pcshow(ptCloudDownSampled)
x = double(ptCloudDownSampled.Location(:,1));
y = double(ptCloudDownSampled.Location(:,2));
z = double(ptCloudDownSampled.Location(:,3));
%% AlphaShape
shp = alphaShape(x, y, z, 0.13);
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John D'Errico
John D'Errico 2024-9-17
The data might be of value. In fact, it will be terribly difficult to know for sure what happened otherwise. But my guess is it is a large data set, and that might make it problematic to upload the data.
I might make a random conjecture, but any guess will be certainly wrong, based on nothing more than a picture.
Lars Urban
Lars Urban 2024-9-19
@Matt J I attached the data i use here now.
@Walter Roberson See below my Code I use for plotting. But i am exporting the results as a stl file and i can see the holes in other softwares too. Like CloudCompare or Autodesk Fusion.
@John D'Errico Yes the data is large and i would say realitve noisy. But i could not find anything about problems reconstructing a noisy pointcloud with alphashape.
%% Plot
figure;
plot(shp, 'FaceColor', [0.2 0.6 0.8], 'EdgeColor', 'none');
hold on;
ax = gca;
ax.FontSize = 14;
ax.FontWeight = 'bold';
ax.XColor = 'k';
ax.YColor = 'k';
ax.ZColor = 'k';
%
title('AlphaShape 3D', 'FontSize', 16, 'FontWeight', 'bold');
xlabel('X-Achse', 'FontSize', 14, 'FontWeight', 'bold');
ylabel('Y-Achse', 'FontSize', 14, 'FontWeight', 'bold');
zlabel('Z-Achse', 'FontSize', 14, 'FontWeight', 'bold');
%
axis equal;
az = -90;
el = 0;
camlight(az,el);
set(gcf, 'Color', 'w');
view(az, el);
grid off;
hold off;

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