Envelope Extraction Method

The AEEHMA method (Automatic Envelope Extraction for Hair Modeling and Animation) allows to automatically create a 3D envelope starting from any mesh. The output envelope consists on a set of parametric splines whose topology is based on the spatial behavior of the input geometry.

The goal of this method is to automate the retopologizing of large 3D meshes: for instance, after the sculpting stage, it could be interesting to extract significant structures from great hair masses not only for rendering purposes, but also to perform dynamic simulations in a playground where the original geometries may be unsuitable.

The Method

The 3D input mesh can be regular, semi-regular or irregular, with any kind of topology. After a preliminary optimization, the mesh is converted into a bi-directed graph.
The two topologically-furthest nodes within the mesh are identified and the shortest path between these two is chosen.

Method applied to s-shaped mesh
Method applied to u-shaped mesh

The path is then converted into a 3D spline. Taking into account this spline and the normals of its points to the input mesh, cross ribs are created. These ribs are parametric splines, so it is possible to cross-bridge them in sequence, thus forming an envelope re-proposing the natural flow of the input mesh.


Single hair strands

The first part of the algorithm can be used to extract automatically a behavioural spline out of multiple sculpted hair strands. Each strand is considered separately.

Single hair strands extraction

The output splines can be used as guides for hair rendering in animation softwares.

Great hair masses

This method can be useful to reconstruct envelopes also from sculpted great hair masses in a short reasonable time.

Reconstructing envelopes for great hair masses

The transversal splines can be used for hair rendering or simulation.