Crate soft_edge[−][src]

Expand description

Soft Edge: efficient bithackery for making 3D collision meshes out of grids and stacked tile maps

indecipherable wailing guitar noises and tape echo sounds - Wata (Boris - Soft Edge)

soft-edge is a crate which provides utilities for dealing with 3D grids where the cells are colliders defined as convex hulls of subsets of the unit cube.

Things it gives you:

  • Vertex, representing a vertex of the unit cube as an index in 0..8
  • VertexSet, representing a subset of the vertices of the unit cube as a 1-byte bitset
  • Atom, representing a valid convex non-coplanar subset of the vertices of the unit cube
  • CompoundHull, representing a clippable convex hull of an Atom
  • HullFacet, representing a polygon of a potentially clipped compound hull of an atom. Facets calculated from a CompoundHull will always be in CCW winding order.

Why?

If you’ve ever tried building a game physics system with a tile-based map before, you’ve probably run into the “crack problem”. This is where if you have two adjacent tiles, things which should smoothly move over the “crack” between them (which mathematically does not exist because the vertices of these two tiles are shared) instead get caught on the crack, because during collision detection the object is moved into a tile by some force and ends up spuriously colliding with an unexposed edge of one tile.

soft-edge’s CompoundHull type represents a sort of encoded tile collider with a join_exteriors method that erases any of these unexposed edges between two tiles, leaving you with the surface polygons that can’t cause the crack problem. In addition, these clipping operations are extremely fast, requiring only three bitwise operations (maybe some more because they’re done on only a subset of the bits of a thing, but still, it’s fast.)

Does it work?

Uh, I think so. All the tests pass, at least?

Many things in this crate are specialized/brute forced/handwritten as lookup tables. I’ve fixed all the bugs I could find, but I’m sure there are more hiding somewhere, probably in mistakes in bit-significance or something. Though most bitwise ops are done through the bitvec crate exactly to avoid this as much as possible.

Vertex numbering

Vertices are numbered as follows:

  v3      v7
    *----*
v2 /| v6/|
  *----* |   +Y
  | *--|-*   ^ ^ +Z
  |/v1 |/v5  |/
  *----*     +--> +X
 v0    v4

For more convenience, a conversion between vertex index and unit cube coordinates:

// v0 is the origin.
v0 <=> {0, 0, 0} <=> 0b000
v1 <=> {0, 0, 1} <=> 0b001
v2 <=> {0, 1, 0} <=> 0b010
v3 <=> {0, 1, 1} <=> 0b011
v4 <=> {1, 0, 0} <=> 0b100
v5 <=> {1, 0, 1} <=> 0b101
v6 <=> {1, 1, 0} <=> 0b110
v7 <=> {1, 1, 1} <=> 0b111

This is derived as a bitwise representation 0bXYZ where the X bit represents whether or not the X axis is 1, Y bit is whether or not the Y axis is 1, etc.

“Exact” coordinates for vertex index deduplication

In order to facilitate vertex deduplication (for an indexed triangle mesh, for example) most of the output coordinates for things like the CompoundHull’s facets (HullFacet) are presented using the Exact type. This wraps nalgebra’s Point3<i32>, and is essentially just the same coordinates you would expect but scaled by a factor of 2 on every axis. This allows us to represent centroids of faces with no possibility of error, and also provides a hashable type which can be used to build a set mapping vertices to indices.

Generally speaking, how do you expect me to use this?

  1. Encode your collision map as a grid of bytes, which correspond to valid Atoms. Deserialize these bytes into VertexSets using VertexSet::from_u8, and try to construct atoms from them with Atom::try_new.
  2. Construct a three-dimensional grid consisting of the CompoundHulls of the atom grid elements, and then for every hull in the grid, join it with its neighbors on the proper axes.
  3. Extract facets of these joined compound hulls as HullFacets, and then construct your collision geometry with them. This will end up as a very much non-convex mesh, which you may want to decompose into convex sub-meshes (but don’t have to, if you do collisions directly on polygons and only allow contact normals which penetrate through their “surface” side.)

Step 3 has some caveats. soft-edge attempts to produce hull facets with the correct winding order, which should allow you to easily calculate their normals. If it does not, this is a bug, and must be fixed. The caveat here is that soft-edge does not yet have a full test suite.

Re-exports

pub use bitvec;

Macros

bitvec

Constructs a new BitVec from a bit-pattern description.

const_vertex_set

Construct a vertex set in such a way that it can be used in a const expression. This macro requires that the bitarr! macro and Lsb0 type from the bitvec crate are in scope, for technical reasons related to the limitations of bitvec’s bitarr macro itself.

vertex_set

Construct a vertex set.

Structs

Atom

An Atom is the smallest individual unit of our collision mesh. It is a convex polyhedron represented as a subset of the vertices of a cube. For an Atom to be valid, it must have at least four non-coplanar vertices (it must have nonzero volume.)

CompoundHull

The “compound hull” of an atom is its convex hull, possibly with pieces missing from its exterior.

DegeneracyError

The error type returned on trying to construct an atom using a degenerate vertex set.

Exact

A point in “exact” cube coordinates; integer coordinates scaled by a factor of two.

ExteriorHull

Facets which are on an atom’s convex hull and which exist on the six “face planes” of the cube, and which may be clipped during join operations.

Face

A Face represents a combination of four triangles around a central vertex:

InteriorHull

Triangles which lie on the planes of the faces of an atom are taken care of by Faces. However, we still need to complete the picture by adding back in any missing facets which lie “inside” that cube (and not on its faces.) There are 8 ways to choose 3 triangles from eight vertices. Of these 56 possible triangles, there are 4 times 24 which exist on the faces of the cube, and which are therefore superseded by the Face calculations. We now have 32 triangles remaining. Of these, we have six “symmetry planes” of the cube; these are the planes that run through an edge each, and both contain four vertices. Like the face quads, these contain 24 possible triangles, which leaves us with a total of 8 anomalous configurations. To sum up (pun intended):

Plane

A plane, represented as a point on the plane and a normal vector (not necessarily guaranteed to be normalized).

Vertex

A vertex of a cube, represented as a byte index encoded w/ three bits, each representing an axis as a boolean (true = 1, false = 0.) See the constants [X_AXIS], [Y_AXIS], [Z_AXIS] for a reference of which bits are which.

VertexSet

A set of vertices represented as Vertexs.

Enums

Axis

Represents a single signed axis: +X, +Y, +Z, -X, -Y, or -Z.

HullFacet

A facet of a hull is either a rectangle or a triangle.

Constants

ANOMALOUS_CONFIGURATIONS

The eight “anomalous configurations” of subsets of the unit cube; these are triangular interior faces which can be calculated as part of finding the interior convex hull of an atom, and which are always triangular (within an atom, can never be joined with another triangle into a quad.) Also, “anomalous configuration” sounds badass. Admit it.

AXIS_VECTORS

Axis unit vectors in i32 coordinates corresponding to the same index order as faces and the axis to_index() and u8 representation.

FACES

The six faces of the unit cube, each corresponding to an axis.

POINTS

The eight possible points of a Vertex, in “exact coordinates” (see Exact.)

SYMMETRIES

Six quadrilaterals which happen to match planes of symmetry of the cube. These are the planes which exist between pairs of edges of the cube, and represent six possible ways we might end up with a quad as an interior face of an atom, plus a total of 24 possible ways we could end up with triangles as interior faces of an atom (by cutting those quads in half, four ways each.)