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Triangulate 2D connects 2D points in the input geometry to create a 2D triangle mesh which is as "well-shaped" as possible (avoiding acute angles as much as possible).
Since it works on points in a plane, Triangulate 2D needs to assign a 2D position to each 3D input point. This can be done in three ways:
The points are projected into a plane that best fits the 3D point set,
The user chooses a plane into which the points are projected, or
The user provides a point attribute consisting of the 2D coordinates for each point.
Triangulate 2D can ensure that certain pairs of points appear as edges (or subdivisions of edges when refinement is performed) in the output mesh. These edges are called "constraints". You can specify constraints by providing either an edge group or a primitive group (or both) under the Constraints tab.
To make sure certain boundary edges are present in the final triangulation, enter the primitive number(s) of the edge(s) in the Constraint primitives parameter or list the edges you want as an edge group in the Constraint edges parameter.
Triangulate 2D can also remove triangles deemed as belong to the outside of the intended area in various ways which are specified under the Outside Removal tab.
You can also improve how well shaped the triangles are by allowing Triangulate 2D to add additional points under Refine tab.
To triangulate the input without adding new points, turn off Refine and leave Constraint primitives empty.
To increase the number of points to allow better triangles, turn on Refine. You can control the triangle quality with the Maximum Area (controls triangle size) and Minimum Angle (larger values prevent long skinny triangles with acute angles) parameters. You can limit refinement to large triangles by setting Minimum Edge Length parameter.
To introduce randomness into the triangulation, pipe the original geometry through a Scatter SOP to add extra points at random locations.
Technically, Triangulate 2D produces a constrained Delaunay triangulation, which has the largest minimum angle among all possible triangulations for the same set of points, and the refinement process is Delaunay refinement.
Subset of points to use for triangulation.
Determines the method for determining the 2D positions of the points used in triangulation.
Obtains 2D positions of the points by projecting them into a plane that best fits the 3D point set.
Select Projection Plane
Allows the user to specify a projection plane, into which the 3D points are projected.
When Select Projection Plane is used, the projection plane is determined relative to this point which is treated as "origin".
When Select Projection Plane is used, this parameter determines the distance between the projection plane and the point specified by Origin.
When Select Projection Plane is used, this vector determines the direction along which the projection plane is shifted away from the point specified by Origin.
Reads the 2D position of each point from a supplied point attribute (the attribute must be a floating point vector with at least two components).
When Use Attribute is used,, the 2D position of the points are read from the attribute with this name.
Restore Original Point Positions
If selected, triangles of the generated triangulation are transformed to use the original 3D positions of the input points. Newly generated points (for refinement or constraint conflict resolution) are accordingly assigned interpolated positions based on the 3D positions of the input points.
Keep all input primitives (except the constraint primitives) in the output in addition to new primitives created during triangulation. When this option is off, the input primitives do not appear in the output.
Recompute Point Normals
Recomputes point normals (if they exist).
Remove Unused Points
If selected, points outside the input point group are deleted from the output.
Remove Duplicate Points from Triangulation
For any given 2D position only one point can be inserted into the triangulation. If multiple points have the same 2D position only one of them is inserted into the triangulation and the others are ignored. If this option is selected, these duplicate points are deleted from the output, otherwise, they are left in as isolated points.
The triangulation and refinement algorithm implemented by this node is "randomized" in nature, meaning that its peek performance is expected with high probability only if the input points are processed in "random" order. Since Houdini node outputs must be deterministic, a fixed random-like reordering of the input points, determined by this parameter, is used. In the majority of cases, this fixed reordering should perform close to optimal but it is theoretically possible for any fixed reordering to perform poorly for a specific point set. Changing the seed value in such case may have tangible effects on the speed of the triangulation although it might affect the generated output, particularly in terms of the point and primitive numbers.
Using constraints you can enforce certain pairs of points to be connected by edges, aka. constrained edge, in the output triangulation. You can specify constraints using a constraint primitives group (consisting of polygons or polygonal curves all whose edges are treated as constraints), or using an edge group, or both.
All edges of each polygon or polygonal curve in this primitive group will be treated as a constraint and are forced upon to the output triangulation.
All edges in the supplied edge group are enacted as constraints in the output triangulation. Note that edge groups edges need not exist as primitive edges in the input. As an example, using the expression "p10-20 p16-24" as the constraint edge group ensures that points 10 and 20 are connected with an edge in the output as are point 16 and 24.
No New Points on Constraints
It is not possible to simultaneously enforce two constraints if their corresponding 2D line segments intersect. Such constraints are said to be in "conflict" with one another. If this option is selected, one of each pair of conflicting constraints is dropped so that the other one can be enforced. Otherwise, conflicts are resolved by splitting constraints into smaller ones by introducing new points at intersections.
Create Group for New Points on Constraints
If selected, a point group is created for the points the are added in order to resolve conflicts in constraints.
When Create Group for New Constraint Points on Constraints is on, Triangulate 2D creates a group with this name to contain points created on the constrained edges.
Create Group for Output Constraint Edges
If selected, an edge group is created which will contain all constraint edge in the output triangulation.
When Create Group for Output Constraint Edges is on, an edge group is created with this name to contain the output constrained edges.
This node can optionally remove a subset of the generated triangles which can be interpreted as belonging to the exterior determined by the constraints. This is done through a series of flooding operations in which, starting from an initial set of exterior triangles, the flood successively extends to neighboring triangles that are not separated from the currently flooded set by constrained edges. In other words, the removal flood is only blocked by constrained edges.
Reachable From Convex Hull Boundary
If selected, a removal flood is started from the outer boundary of the triangulation (coinciding with the boundary of the convex hull of the point set) and is propagated through unconstrained edges of the triangulation.
For example, on the following constrained triangulation:
removing triangles Reachable From Convex Hull Boundary will produce this result:
Outward Reachable From Constraint Primitives
If selected, a removal flood is started right outside the boundary of each closed constraint primitive.
For example, removing triangles Outward Reachable From Constraint Primitives on the previous example will produce this result:
But Keep Triangles
Inward Reachable From Constraint Primitives
This option cancels removal of a triangle in a flooding initiated outside of a constraint primitive whenever the same triangle can be reached via flooding from inside the boundary of another constraint primitive.
Removing triangles Outward Reachable From Constraint Primitives while keeping triangles Inward Reachable From Constraint Primitives on the previous example will produce this result:
Improves the quality of the triangulation by adding new points. The triangulation will include some new points beyond the ones in the input, but the triangles will have better shapes.
If you start with the following points:
With Refine off, Triangulate 2D will create triangles using the available points, and may in the process introduce undesirable long skinny triangles:
With Refine on, Triangulate 2D will add points to create better triangles conforming to the Maximum Area, Minimum Angle, and Minimum Edge Length parameters.
The maximum area for the interior of each triangle. Decrease this value to get smaller and more similarly sized triangles. Very small values can make the triangulation take a long time.
The minimum angle, in degrees, for the interior of each triangle. Increasing this value to get more symmetrical triangles and eliminate long skinny triangles. Values larger than 30 degrees cannot be guaranteed and often force the refinement to go for ever, and only stopped by the reaching the maximum allowed number of points (Maximum New Points)
Minimum Edge Length
To avoid over-refinement in a local area, you can specify a minimum length for the longest edge of a triangle that is allowed to be further refined. Once all edges of a triangle become shorter than the specified length, no further attempts are made to improve the shape of the triangle.
Maximum New Points
Limits the number of new points that can be added during triangulation. New points might be added to satisfy constraints or during refinement, up to this limit. This is mostly useful to ensure that the triangulation doesn’t run forever, and should usually be set to a value much larger than the number of points you expect to generate.