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Tutorial</a></li><liclass="nav-list-item "><ahref="/meshedit/local/bevel/"class="nav-list-link">Bevelling</a></li></ul></li><liclass="nav-list-item "><ahref="#"class="nav-list-expander"><svgviewBox="0 0 24 24"><usexlink:href="#svg-arrow-right"></use></svg></a><ahref="/meshedit/global/"class="nav-list-link">Global Operations</a><ulclass="nav-list"><liclass="nav-list-item "><ahref="/meshedit/global/catmull/"class="nav-list-link">Catmull-Clark Subdivision</a></li><liclass="nav-list-item "><ahref="/meshedit/global/remesh/"class="nav-list-link">Isotropic Remeshing</a></li><liclass="nav-list-item "><ahref="/meshedit/global/linear/"class="nav-list-link">Linear Subdivision</a></li><liclass="nav-list-item "><ahref="/meshedit/global/loop/"class="nav-list-link">Loop Subdivision</a></li><liclass="nav-list-item "><ahref="/meshedit/global/simplify/"class="nav-list-link">Simplification</a></li><liclass="nav-list-item 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a documentation theme for Jekyll. </footer></div><divclass="main"id="top"><divid="main-header"class="main-header"><divclass="search"><divclass="search-input-wrap"><inputtype="text"id="search-input"class="search-input"tabindex="0"placeholder="Search "aria-label="Search "autocomplete="off"><labelfor="search-input"class="search-label"><svgviewBox="0 0 24 24"class="search-icon"><usexlink:href="#svg-search"></use></svg></label></div><divid="search-results"class="search-results"></div></div></div><divid="main-content-wrap"class="main-content-wrap"><navaria-label="Breadcrumb"class="breadcrumb-nav"><olclass="breadcrumb-nav-list"><liclass="breadcrumb-nav-list-item"><ahref="/meshedit/">A2: MeshEdit</a></li><liclass="breadcrumb-nav-list-item"><span>Local Operations</span></li></ol></nav><divid="main-content"class="main-content"role="main"><h1id="local-mesh-operations"><ahref="#local-mesh-operations"class="anchor-heading"aria-labelledby="local-mesh-operations"><svgviewBox="0 0 16 16"aria-hidden="true"><usexlink:href="#svg-link"></use></svg></a> Local Mesh Operations </h1><p>Many of the actions that need to be implemented in the MeshEdit mode are local mesh operations (like edge collapse, face bevel, etc.).</p><p>A good recipe for ensuring that all pointers are still valid after a local remeshing operation is:</p><ol><li>Draw a picture of all the elements (vertices, edges, faces, halfedges) that will be needed from the original mesh, and all the elements that should appear in the modified mesh.</li><li>Allocate any new elements that are needed in the modified mesh, but do not appear in the original mesh.</li><li>For every element in the “modified” picture, set <strong>all</strong> of its pointers – even if they didn’t change. For instance, for each halfedge, make sure to set <codeclass="language-plaintext highlighter-rouge">next</code>, <codeclass="language-plaintext highlighter-rouge">twin</code>, <codeclass="language-plaintext highlighter-rouge">vertex</code>, <codeclass="language-plaintext highlighter-rouge">edge</code>, and <codeclass="language-plaintext highlighter-rouge">face</code> to the correct values in the new (modified) picture. For each vertex, make sure to set its <codeclass="language-plaintext highlighter-rouge">halfedge</code> pointer. Etc. A convenience method <codeclass="language-plaintext highlighter-rouge">Halfedge::set_neighbors()</code> has been created for this purpose.</li><li>Deallocate any elements that are no longer used in the modified mesh, which can be done by calling <codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::erase()</code>.</li></ol><p>The reason for setting all the pointers (and not just the ones that changed) is that it is very easy to miss a pointer, causing your code to crash.</p><h3id="interface-with-global-mesh-operations"><ahref="#interface-with-global-mesh-operations"class="anchor-heading"aria-labelledby="interface-with-global-mesh-operations"><svgviewBox="0 0 16 16"aria-hidden="true"><usexlink:href="#svg-link"></use></svg></a> Interface with global mesh operations </h3><p>To facilitate user interaction, as well as global mesh processing operations (described below), local mesh operations should return the following values when possible. However, should it happen that the specified values are not available, or that the operation should not work on the given input, we need a way to signify the failure case. To do so, each local operation actually returns a <codeclass="language-plaintext highlighter-rouge">std::optional</code> value parameterized on the type of element it returns. For example, <codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::erase_vertex</code> returns a <codeclass="language-plaintext highlighter-rouge">std::optional<Halfedge_Mesh::Face></code>. An <codeclass="language-plaintext highlighter-rouge">optional</code> can hold a value of the specified type, or, similarly to a pointer, a null value (<codeclass="language-plaintext highlighter-rouge">std::nullopt</code>). See <codeclass="language-plaintext highlighter-rouge">student/meshedit.cpp</code> for specific examples.</p><p>Also, remember that in any case, <em>the program should not crash!</em> So for instance, you should never return a pointer to an element that was deleted.</p><p>See the <ahref="/Scotty3D/guide/model">User Guide</a> for demonstrations of each local operation.</p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::flip_edge</code> - should return the edge that was flipped</li></ul><p><imgsrc="flip_edge.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::split_edge</code> - should return the inserted vertex</li></ul><p><imgsrc="split_edge.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::collapse_edge</code> - should return the new vertex, corresponding to the collapsed edge</li></ul><p><imgsrc="collapse_edge.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::collapse_face</code> - should return the new vertex, corresponding to the collapsed face</li></ul><p><imgsrc="collapse_face.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::erase_vertex</code> - should return the new face, corresponding to the faces originally containing the vertex</li></ul><p><imgsrc="erase_vertex.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::erase_edge</code> - should return the new face, corresponding to the faces originally containing the edge</li></ul><p><imgsrc="erase_edge.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::bevel_vertex</code> - should return the new face, corresponding to the beveled vertex</li></ul><p><imgsrc="bevel_vertex.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::bevel_edge</code> - should return the new face, corresponding to the beveled edge</li></ul><p><imgsrc="bevel_edge.svg"alt=""/></p><ul><li><codeclass="language-plaintext highlighter-rouge">Halfedge_Mesh::bevel_face</code> - should return the new, inset face</li></ul><p><imgsrc="bevel_face.svg"alt=""/></p></div></div><divclass="search-overlay"></div></div></body></html>
