Topology#

Connectivity, edges, paths and other structural queries and edits.

12 nodes.

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Contents#

• Connect Adjacent Pieces (PCG) — Connects nearby points or pieces with two-point polylines, for constraint or road-network skeletons.
• Connectivity (PCG) — Numbers the connected components of the geometry and writes the number to an integer attribute.
• Edge Cusp (PCG) — Splits geometry along edges so the faces on either side no longer share points, optionally filtered by dihedral angle.
• Edge Flip (PCG) — Flips edges shared by two faces, either by rotating them or by Delaunay-style flipping of triangle pairs.
• Find Shortest Path (PCG) — Finds least-cost paths along the edges of a mesh or curve network using Dijkstra's algorithm.
• Match Topology (PCG) — Renumbers a mesh so that its point, primitive and corner order matches a reference mesh.
• PolyCut (PCG) — Cuts polygons and polylines along their edges into open polylines, by point group, point attribute threshold, or edge rules.
• PolySplit (PCG) — Splits polygons along a path of points and edge positions.
• PolyStitch (PCG) — Welds and stitches the seams between patches of different resolution so both sides share the same points.
• Rewire Vertices (PCG) — Reconnects vertices to different points according to an integer attribute.
• Tri Divide (PCG) — Triangulates the selected faces and subdivides them until edges and areas are below limits, without creating T-points.
• Unsubdivide (PCG) — Reverses Catmull-Clark subdivision by a given number of levels.

Nodes#

Connect Adjacent Pieces (PCG)#

PCG_ConnectAdjacentPieces · category PCG/topology · Houdini: Connect Adjacent Pieces (SOP)

Connects nearby points or pieces with two-point polylines, for constraint or road-network skeletons.

Builds line primitives between neighbouring things. In points mode each point is linked to its nearest points within the search radius, up to Max Connections. In piece_centroids mode two pieces are linked between their centres when any pair of their points is within the radius. In piece_points mode the line joins the closest pair of points of the two pieces.

Each new line carries a restlength attribute and the piece numbers or point numbers of its two ends. Pieces are defined by the piece attribute; if that attribute cannot be found, pieces are the connected components of the geometry.

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Connectivity (PCG)#

PCG_Connectivity · category PCG/topology · Houdini: Connectivity (SOP)

Numbers the connected components of the geometry and writes the number to an integer attribute.

Computes connected components and writes an integer component index to the chosen attribute, on points or on primitives. Elements that belong to the same connected piece share the same number.

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Compared with Houdini

Corresponds to the Connectivity SOP. The Assemble SOP is only partly covered.

Edge Cusp (PCG)#

PCG_EdgeCusp · category PCG/topology · Houdini: Edge Cusp (SOP)

Splits geometry along edges so the faces on either side no longer share points, optionally filtered by dihedral angle.

Cuts along the edges of the edge group (all edges if empty), so the faces on both sides stop sharing endpoints. When Cusp Angle is above 0, only edges whose dihedral angle is at least that value are cut. Points that touch no cut edge are left alone.

With Compute Normals the node writes area-weighted point normals, which produce hard edges at the cuts.

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Edge Flip (PCG)#

PCG_EdgeFlip · category PCG/topology · Houdini: Edge Flip (SOP)

Flips edges shared by two faces, either by rotating them or by Delaunay-style flipping of triangle pairs.

In rotate mode the two faces sharing an edge are merged into one loop and the edge is rotated along that loop by Cycles steps. For two triangles this flips the diagonal; for two quads it rotates the edge. The number of sides of each face does not change.

In delaunay mode only triangle pairs are flipped, and only when the sum of the two opposite angles exceeds 180 degrees. Flipping repeats until no more flips are possible, and edges whose dihedral angle exceeds Max Dihedral Angle are not flipped.

An empty edge group means all edges. Flipped edges are replaced by the new edges in the edge group of the same name.

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Find Shortest Path (PCG)#

PCG_FindShortestPath · category PCG/topology · Houdini: Find Shortest Path (SOP)

Finds least-cost paths along the edges of a mesh or curve network using Dijkstra's algorithm.

The cost of an edge is its length (or 1 per step), multiplied by 1 + Cost Weight * cost and by 1 + Slope Weight * |slope|. On terrain, the slope weight makes roads follow contour lines, and a cost attribute steers them around water or forest. The cost attribute is averaged over the two points of an edge. The slope is the absolute cosine between the edge direction and the up vector. Houdini-style additive costs (point and primitive cost attributes), surface constraints and directed primitives are applied on top of that.

The output paths mode decides which paths are built; see the Output Paths menu. Paths that cannot be reached (disconnected parts, or above the max cost) are skipped and counted in the report.

Output chooses what is produced. Paths outputs only the path polylines (point copies carrying srcpt, and primitive attributes cost, length, startpt, endpt). Merge outputs the source geometry together with the paths. Group outputs no new geometry and writes an edge group and a point group of the same name onto the source.

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Cost

Surface Constraints

Output

Tips

• Costs must be positive; a negative cost attribute or weight is rejected with an error.
• For roads over terrain, combine a small Slope Weight with a cost attribute that is high on water.

Match Topology (PCG)#

PCG_MatchTopology · category PCG/topology · Houdini: Match Topology (SOP)

Renumbers a mesh so that its point, primitive and corner order matches a reference mesh.

Use it when geo and reference are the same mesh but their point numbers, primitive numbers or starting corners have been scrambled. Topology mode starts from a pair of seed corners and walks both meshes in step across opposite edges. It ignores positions, so it works however much the mesh has been deformed. Seeds left at -1 are found automatically. Position mode matches by nearest point instead.

The output is geo renumbered to follow reference.

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PolyCut (PCG)#

PCG_PolyCut · category PCG/topology · Houdini: PolyCut (SOP)

Cuts polygons and polylines along their edges into open polylines, by point group, point attribute threshold, or edge rules.

Faces and polylines are turned into polylines by cutting along edges. Three strategies are specific to this plugin: Cut Points breaks the polylines at the points of the point group, Keep Points only keeps edges whose two ends are both in the point group, and Threshold inserts a point where a point attribute crosses the threshold (shared by neighbouring faces). Keep selects all (only break), above or below. A closed face that is not cut is kept unchanged.

Remove and Cut are the semantics of Houdini's PolyCut. They look for edges that fail the test, which can be edges whose attribute crosses Cut Value or changes by more than Cut Threshold (with an empty attribute, all edges fail). With Type set to points, the failing points' ends are deleted or the line is broken there. With Type set to edges, the failing edge is deleted or broken in its middle. In these two strategies Cut Points is Houdini's Cut Points (empty means all points) and Cut Value is the cut value; Keep is not used.

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PolySplit (PCG)#

PCG_PolySplit · category PCG/topology · Houdini: PolySplit (SOP)

Splits polygons along a path of points and edge positions.

Locations are separated by spaces. 12 (or p12) is a point, 3-7@0.25 is the position t = 0.25 on the edge from point 3 to point 7, and omitting @ means the midpoint. Houdini's notation (p3-7:0.25, 0v2, 0e1:0.25) is accepted too; the in-face form 0f:u,v is not supported.

Two consecutive locations must lie on the same face, and that face is cut in two along the line between them. New points on edges are inserted into all faces on both sides, so no T-junctions are left. Close Path connects the last location back to the first. If an output edge group is named, the cut edges are written to it.

The Edge Loop path type inserts a ring of edges through quad loops starting from each edge location; with more than one loop the rings are spaced evenly.

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Compared with Houdini

The Shortest Path option is named to match Houdini, but no path is searched: consecutive locations still have to lie on the same face.

PolyStitch (PCG)#

PCG_PolyStitch · category PCG/topology · Houdini: PolyStitch (SOP)

Welds and stitches the seams between patches of different resolution so both sides share the same points.

Boundary points closer than the tolerance are welded. With snapping on, the remaining boundary points are snapped onto the boundary edges of other patches, and points that land on an edge are inserted into the face that owns that edge. T-junctions at the seam disappear and both sides use the same sequence of points. The report includes the number of boundary edges that remain.

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Rewire Vertices (PCG)#

PCG_RewireVertices · category PCG/topology · Houdini: Rewire Vertices (SOP)

Reconnects vertices to different points according to an integer attribute.

An integer attribute gives, for each vertex, the point number that the vertex should reference. A vertex attribute is preferred; if there is none, the point attribute of the same name is used. Values of -1 or out of range leave the vertex unchanged.

Optionally consecutive duplicate corners and primitives that can no longer stand as a face are removed, and points that nobody uses can be deleted.

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Tri Divide (PCG)#

PCG_TriDivide · category PCG/topology · Houdini: Tri Divide (SOP)

Triangulates the selected faces and subdivides them until edges and areas are below limits, without creating T-points.

Selected faces are triangulated and then bisected along their longest edge (longest-edge propagation) until each edge is no longer than Max Edge Length and each area no larger than Max Area. A value of 0 disables that limit. Both triangles sharing the split edge are bisected together, so there are no T-points, the shape stays the same and the smallest angle does not keep getting worse. Neighbouring faces that are not selected only receive a midpoint on the shared edge.

With Restore Delaunay on, a final Lawson edge-flip pass is done. Only edges whose dihedral angle is at most the flip angle are flipped.

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Unsubdivide (PCG)#

PCG_Unsubdivide · category PCG/topology · Houdini: Unsubdivide (SOP)

Reverses Catmull-Clark subdivision by a given number of levels.

Recognises the original vertices, edge points and face points that Catmull-Clark subdivision leaves behind, and rewinds the topology by Levels levels. With Fit positions are recovered by inverting the subdivision stencil in the least-squares sense, so a mesh that really came from subdivision is restored exactly. With Keep, the subdivided positions of the original vertices are simply kept, as Houdini does, which makes the shape shrink a little.

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Compared with Houdini

The Keep option is Houdini's behaviour; Fit is an addition that reconstructs the original positions exactly when the mesh came from Catmull-Clark subdivision.