Unexpected alphaShape output - missing points?

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Andrew 2025-3-31

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https://ww2.mathworks.cn/matlabcentral/answers/2175817-unexpected-alphashape-output-missing-points

评论： Umeshraja 2025-6-16

Using alphaShape to generate a shape from a point cloud. The general principle is that in a point cloud, there is a shape with a certain thickness. The goal is to extract the points which lie on the inside and outside of the shape, and generate shapes with those. I have code that does this via the genus of the shape. It returns the point clouds of both the inner and outer shell just fine, but when passed through alphaShape, it produces results like below.

Below is the alphaShape for the "minimum shell", the alphaShape with seperated interior and exterior shells, and a portion of the original point cloud.

The alpha value used has been played with, but no matter what the points which are absent are never present in the alphaShape. Below is the exact code which is used to produce the above result. "innerShellPoints" is the first point cloud displayed.

innerShell = alphaShape(innerShellPoints, minShell.Alpha * 1.2);

Both inner and outer point clouds look exactly as expected, as does the minimum shell. What could be causing this issue? It seems almost like alphaShape is skipping entirely over some of the provided points.