Extract border points from surface points (compatible with autogeneration in C)

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Alessio
Alessio 2024-11-26
评论： Alessio 2024-11-27
Hello everyone! I am working in MatLab r2017a and I am doing some geometrical computations. I got all the points of a surface with which I am working in x and y coordinates. I am trying to extract only the points of the border of this surface, but I can't use MatLab functions that are not compatible with autogeneration in C and I also have to use only fixed-size variables. I already tried to extract them using a local search for extremes in x and y but it doesn't work properly. Can you help me finding a way to implement this?
This is an example of the kind of surface points I get. There are some regions with denser points because this is the result of the projection of a 3D surface in a 2D plane that should represents my FOV.
I hope someone will help me find a solution! Thanks a lot in advance!

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Image Analyst
Image Analyst 2024-11-26
Ran in:
Try alphaShape
help alphaShape
alphaShape - Polygons and polyhedra from points in 2-D and 3-D An alphaShape creates a bounding area or volume that envelops a set of 2-D or 3-D points. Creation Syntax shp = alphaShape(x,y) shp = alphaShape(x,y,z) shp = alphaShape(P) shp = alphaShape(___,a) shp = alphaShape(___,Name,Value) Input Arguments x - x-coordinates column vector y - y-coordinates column vector z - z-coordinates column vector P - Point coordinates matrix with two columns | matrix with three columns a - Alpha radius nonnegative scalar Name-Value Arguments HoleThreshold - Maximum interior holes 0 (default) | finite, nonnegative scalar RegionThreshold - Maximum regions 0 (default) | finite, nonnegative scalar Properties Points - Coordinates of points matrix Alpha - Alpha radius nonnegative scalar HoleThreshold - Maximum interior holes 0 (default) | finite nonnegative scalar RegionThreshold - Maximum regions 0 (default) | finite nonnegative scalar Object Functions numRegions - Number of regions in alpha shape area - Area of 2-D alpha shape perimeter - Perimeter of 2-D alpha shape surfaceArea - Surface area of 3-D alpha shape volume - Volume of 3-D alpha shape boundaryFacets - Boundary facets of alpha shape alphaTriangulation - Triangulation that fills alpha shape inShape - Determine if point is inside alpha shape alphaSpectrum - Alpha values giving distinct alpha shapes criticalAlpha - Alpha radius defining critical transition in shape nearestNeighbor - Determine nearest alpha shape boundary point plot - Plot alpha shape Examples openExample('matlab/AlphaShapeFrom2DPointCloudExample') openExample('matlab/AlphaShapeFrom3DPointCloudExample') openExample('matlab/FillHolesIn2DAlphaShapeExample') openExample('matlab/DiscardSmallRegionsOf3DAlphaShapeExample') openExample('matlab/ModifyPointsOf2DAlphaShapeExample') See also convhull, boundary, delaunayTriangulation, triangulation, trisurf, criticalAlpha Introduced in MATLAB in R2014b Documentation for alphaShape doc alphaShape
th = (pi/12:pi/12:2*pi)';
x1 = [reshape(cos(th)*(1:5), numel(cos(th)*(1:5)),1); 0];
y1 = [reshape(sin(th)*(1:5), numel(sin(th)*(1:5)),1); 0];
x = [x1; x1+15];
y = [y1; y1];
plot(x,y,'.')
axis equal
shp = alphaShape(x,y);
plot(shp)
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  1 个评论
Alessio
Alessio 2024-11-27
Ouch, sorry if I didn't follow the rules for writing questions. I haven't attached my data and code because it consists only of a surface generation and projection in the new reference frame in the present state.
Thanks a lot for the answer, it would work normally but unluckily alphaShape is not compatible with autogeneration in C.
I will keep looking for alternative solutions!

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