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| --- | ||
| layout: post | ||
| title: "New in CGAL: Improvements for 3D Alpha Wrapping" | ||
| description: "New features, and speed-up" | ||
| category: | ||
| tags: [""] | ||
| --- | ||
| {% include JB/setup %} | ||
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| <h3><a href="https://geometryfactory.com/who-we-are/">Mael Rouxel-Labbé</a></h3> | ||
| <h4><a href="http://www.geometryfactory.com" target="_blank">GeometryFactory</a></h4> | ||
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| <br> | ||
| <p>Introduced in CGAL 5.5, the package | ||
| <a href="https://doc.cgal.org/latest/Manual/packages.html#PkgAlphaWrap3" text="3D Alpha Wrapping">3D Alpha Wrapping</a> | ||
| can be used to generate watertight, manifold surface triangle meshes from any given inputs provided we can compute distances | ||
| and intersections to this input. This enables the method to guarantee output properties, | ||
| and genericity with respect to the input format (triangles, polylines, ...). | ||
| In addition, it is guaranteed to strictly encloses the input, which is a mandatory property | ||
| for applications needing conservative distance queries, such as collision avoidance, | ||
| or motion planning. Finally, it is fully robust to defects in the input such as self-intersections, | ||
| degeneracies, non-manifoldness...</p> | ||
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| <div style="text-align:center;"> | ||
| <a href="../../../../images/christmas_wrap.png"><img src="../../../../images/christmas_wrap.png" style="max-width:95%"/></a><br> | ||
| <br><small>Season's wrappings.</small> | ||
| </div> | ||
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| <br><br> | ||
| <p>In the upcoming CGAL release, CGAL 6.0, we introduce a few improvements to the 3D Alpha Wrapping package: | ||
| pause & resuming functionalities, fast "LOD" wrapping, generation of volumetric wraps, and some speed-ups.</p> | ||
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| <br> | ||
| <h3>Resume and Restart Functionalities</h3> | ||
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| <p>The main function of the package 3D Alpha Wrapping takes two input parameters: the size of the | ||
| the feature size (<i>alpha</i>), and the isosurface of the distance field on which new vertices are created | ||
| (offset, also named <i>delta</i>). The parameter alpha can be understood as the size of a spoon carving around the input. | ||
| A larger alpha will thus remove small features and fill large gaps, and a small alpha will preserve those features, | ||
| and enter the gaps and cavities (see also Section <a href="https://doc.cgal.org/latest/Alpha_wrap_3/index.html#title5">Choosing Parameters</a> | ||
| of the documentation). Although the physicality can be understood, it might be that one | ||
| is not exactly sure which value should be used; or, one might simply wish to create a wrap that is | ||
| as detailed as possible within a fixed amount of time.</p> | ||
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| <p>This is now possible thanks to the addition of a visitor to the wrapping algorithm, which gives | ||
| more information and control to the user during the wrapping process. An example showing how to use | ||
| the visitor to pause and resume a wrapping process has been added: | ||
| <a href="https://github.com/CGAL/cgal/blob/master/Alpha_wrap_3/examples/Alpha_wrap_3/pause_and_resume_wrapping.cpp">pause_and_resume_wrapping.cpp</a>.</p> | ||
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| <p>Another closely related feature being added is the possibility of generating multiple wraps for different | ||
| values of alpha for the same input, for example to generate different levels of detail (LODs). | ||
| One could of course simply launch the wrapping process from scratch as many times as needed, but | ||
| the resuming functionality improves greatly on this: by sorting the value of alpha in decreasing order | ||
| (coarse to fine), we can restart from a previous result to generate the next version. | ||
| By avoiding a recomputation from scratch, a lot of time is saved. Here is a new example | ||
| illustrating the process: <a href="https://github.com/CGAL/cgal/blob/master/Alpha_wrap_3/examples/Alpha_wrap_3/successive_wraps.cpp">successive_wraps.cpp</a>.</p> | ||
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| <div style="text-align:center;"> | ||
| <a href="../../../../images/alpha_wrap.png"><img src="../../../../images/alpha_wrap.png" style="max-width:95%"/></a><br> | ||
| <br><small>Wrapping of an engine piece with complex interior (left) for different values of alpha.</small> | ||
| </div> | ||
|
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| <br> | ||
| <h3>Volumetric Meshing</h3> | ||
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| <p>The wrapping algorithm constructs an underlying 3D triangulation of which the final wrap is a selected | ||
| set of faces. Thus, at the end of the algorithm, we have a tetrahedrization of the volume bounded | ||
| by the wrap. This tetrahedral mesh was totally unused so far, but a getter has been added. | ||
| By combining this with the recent package <a href="https://doc.cgal.org/latest/Manual/packages.html#PkgTetrahedralRemeshing" Text="Tetrahedral Remeshing">Tetrahedral Remeshing</a>, | ||
| one can generate high-quality volumetric tetrahedral meshes of the space enclosed by the wrap.</p> | ||
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| <div style="text-align:center;"> | ||
| <a href="../../../../images/volumetric_wrap.png"><img src="../../../../images/volumetric_wrap.png" style="max-width:95%"/></a><br> | ||
| <br><small>Volumetric mesh of the notoriously painful input <a href="https://ten-thousand-models.appspot.com/detail.html?file_id=996816">996816.stl</a> (Thingi10k). 380k cells in ~10s.</small> | ||
| </div> | ||
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| <br> | ||
| <p>The following example describes this process: <a href="https://github.com/CGAL/cgal/blob/master/Alpha_wrap_3/examples/Alpha_wrap_3/volumetric_wrap.cpp">volumetric_wrap.cpp</a>.</p> | ||
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| <br> | ||
| <h3>Speed-up</h3> | ||
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| <p>Finally, we always look for improvements while working on code. Various enhancements | ||
| to the wrapping algorithms have resulted in a 2x speed-up of the code since CGAL 5.6!</p> | ||
|
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| <h3>Status</h3> | ||
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| <p>All these new improvements are already integrated in CGAL's master branch on the | ||
| <a href="https://github.com/CGAL/cgal/">CGAL GitHub repository</a>, and will be officially released | ||
| in the upcoming version of CGAL, CGAL 6.0, scheduled for June 2024.</p> | ||
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| <i class="glyphicon glyphicon-book"></i> | ||
| <a href="https://doc.cgal.org/latest/Manual/packages.html#PkgAlphaWrap3">Documentation of the package Alpha_wrap_3</a> | ||
| <br> | ||
| <i class="glyphicon glyphicon-download"></i> | ||
| <a href="https://github.com/CGAL/cgal/tree/master">CGAL master branch on GitHub</a> |
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