Getting Started

This tutorial will walk you through the basics of using the IPC Toolkit.

Collision Mesh

First, we need to create a collision mesh. The CollisionMesh data structure represents the surface geometry used for collision processing.

We will start by creating a collision mesh from a bunny.obj mesh file (you can find the mesh here):

#include <ipc/ipc.hpp>

#include <Eigen/Core>
#include <Eigen/Sparse>
#include <igl/readOBJ.h>
#include <igl/edges.h>

// ...

Eigen::MatrixXd rest_positions;
Eigen::MatrixXi edges, faces;
igl::readOBJ("bunny.obj", rest_positions, faces);
igl::edges(faces, edges);

ipc::CollisionMesh collision_mesh(rest_positions, edges, faces);
import ipctk

import meshio

mesh = meshio.read("bunny.obj")
rest_positions = mesh.points
faces = mesh.cells_dict["triangle"]
edges = ipctk.edges(faces)

collision_mesh = ipctk.CollisionMesh(rest_positions, edges, faces)

The matrix rest_positions contains the undeformed positions of the vertices. It should be \(|V| \times d\) where \(|V|\) is the number of vertices and \(d \in \{2, 3\}\) is the dimension. Each row of the edges and faces matrices contains the vertex IDs (row number in rest_positions) of the edge or face’s vertices. The sizes of edges and faces are #E x 2 and #F x 3 respectively (#E and #F are the number of edges and faces).

Note

Only linear triangular faces are supported. If your mesh has nonlinear or non-triangular faces, you will need to triangulate them.

Note

In 2D only the edges matrix is required. In 3D both edges and faces are required.

Collisions

Now that we have a collision mesh, we can compute the collision barrier potential. To do this we first need to build the set of active collisions (Collisions).

To start we need the current positions of the vertices. For this tutorial, let us use squash the bunny has to 1% of its original height.

Eigen::MatrixXd vertices = collision_mesh.rest_positions();
vertices.col(1) *= 0.01; // Squash the bunny in the y-direction
vertices = collision_mesh.rest_positions.copy()
vertices[:, 1] *= 0.01  # Squash the bunny in the y-direction

Using these deformed positions, we can build the set of active collisions. For this, we need to specify \(\hat{d}\) (dhat), the IPC barrier activation distance. We will use a value of \(\hat{d} = 10^{-3}\).

const double dhat = 1e-3;

ipc::Collisions collisions;
collisions.build(collision_mesh, vertices, dhat);
dhat = 1e-3

collisions = ipctk.Collisions()
collisions.build(collision_mesh, vertices, dhat)

This will automatically use a spatial data structure to perform a broad-phase culling and then perform a narrow-phase culling by computing distances (discarding any collision candidates with a distance \(> \hat{d}\)).

Barrier Potential

Now we can compute the barrier potential using the BarrierPotential class.

const ipc::BarrierPotential B(dhat);
double barrier_potential = B(collisions, collision_mesh, vertices);
B = ipctk.BarrierPotential(dhat)
barrier_potential = B(collisions, collision_mesh, vertices)

This returns a scalar value barrier_potential which is the sum of the barrier potentials for each active collision.

Mathematically this is defined as

\[B(x) = \sum_{k \in C} b(d_k(x), \hat{d}),\]

where \(x\) is our deformed vertex positions, \(C\) is the active collisions, \(d_k\) is the distance (squared) of the \(k\)-th active collision, and \(b\) is IPC’s C2-clamped log-barrier function.

Note

This is not premultiplied by the barrier stiffness \(\kappa\).

Barrier Potential Derivatives

We can also compute the first and second derivatives of the barrier potential with respect to the vertex positions.

Eigen::VectorXd barrier_potential_grad =
    B.gradient(collisions, collision_mesh, vertices);

Eigen::SparseMatrix<double> barrier_potential_hess =
    B.hessian(collisions, collision_mesh, vertices);
barrier_potential_grad = B.gradient(collisions, collision_mesh, vertices)

barrier_potential_hess = B.hessian(collisions, collision_mesh, vertices)

These return the gradient and Hessian of the barrier potential as a dense vector and sparse matrix, respectively.

The derivatives are taken with respect to the row-wise flattened vertices. That is, for vertices

\[\begin{split}\begin{bmatrix} x_1 & y_1 & z_1 \\ & \vdots & \\ x_n & y_n & z_n \\ \end{bmatrix},\end{split}\]

you will get the gradient of size \(|V|d \times 1\) with the order

\[\nabla B = \begin{bmatrix} \frac{\partial B}{\partial x_1} & \frac{\partial B}{\partial y_1} & \frac{\partial B}{\partial z_1} & \cdots & \frac{\partial B}{\partial x_n} & \frac{\partial B}{\partial y_n} & \frac{\partial B}{\partial z_n} \end{bmatrix}^T,\]

and the Hessian of size \(|V|d \times |V|d\) with the order

\[\begin{split}\nabla^2 B = \begin{bmatrix} \frac{\partial^2 B}{\partial x_1^2} & \frac{\partial^2 B}{\partial x_1 \partial y_1} & \frac{\partial^2 B}{\partial x_1 \partial z_1} & \cdots & \frac{\partial^2 B}{\partial x_1 \partial x_n} & \frac{\partial^2 B}{\partial x_1 \partial y_n} & \frac{\partial^2 B}{\partial x_1 \partial z_n} \\ % \frac{\partial^2 B}{\partial y_1 \partial x_1} & \frac{\partial^2 B}{\partial y_1^2} & \frac{\partial^2 B}{\partial y_1 \partial z_1} & \cdots & \frac{\partial^2 B}{\partial y_1 \partial x_n} & \frac{\partial^2 B}{\partial y_1 \partial y_n} & \frac{\partial^2 B}{\partial y_1 \partial z_n} \\ % \frac{\partial^2 B}{\partial z_1 \partial x_1} & \frac{\partial^2 B}{\partial z_1 \partial y_1} & \frac{\partial^2 B}{\partial z_1^2} & \cdots & \frac{\partial^2 B}{\partial z_1 \partial x_n} & \frac{\partial^2 B}{\partial z_1 \partial y_n} & \frac{\partial^2 B}{\partial z_1 \partial z_n} \\ % \vdots & \vdots & \vdots & \ddots & \vdots & \vdots & \vdots \\ % \frac{\partial^2 B}{\partial x_n \partial x_1} & \frac{\partial^2 B}{\partial x_n \partial y_1} & \frac{\partial^2 B}{\partial x_n \partial z_1} & \cdots & \frac{\partial^2 B}{\partial x_n^2} & \frac{\partial^2 B}{\partial x_n \partial y_n} & \frac{\partial^2 B}{\partial x_n \partial z_n} \\ % \frac{\partial^2 B}{\partial y_n \partial x_1} & \frac{\partial^2 B}{\partial y_n \partial y_1} & \frac{\partial^2 B}{\partial y_n \partial z_1} & \cdots & \frac{\partial^2 B}{\partial y_n \partial x_n} & \frac{\partial^2 B}{\partial y_n^2} & \frac{\partial^2 B}{\partial y_n \partial z_n} \\ % \frac{\partial^2 B}{\partial z_n \partial x_1} & \frac{\partial^2 B}{\partial z_n \partial y_1} & \frac{\partial^2 B}{\partial z_n \partial z_1} & \cdots & \frac{\partial^2 B}{\partial z_n \partial x_n} & \frac{\partial^2 B}{\partial z_n \partial y_n} & \frac{\partial^2 B}{\partial z_n^2} \end{bmatrix}.\end{split}\]

Adaptive Barrier Stiffness

The last piece of the barrier potential is the barrier stiffness. This is a weight that is multiplied by the barrier potential to better scale it relative to the energy potential. This can be a fixed value or adaptive.

To compute the adaptive barrier stiffness, we can use two functions: initial_barrier_stiffness and update_barrier_stiffness. The function initial_barrier_stiffness computes the initial value from the current energy and barrier potential gradients. This function also provides a minimum and maximum value for the barrier stiffness. The function update_barrier_stiffness updates the barrier stiffness if the minimum distance has become too small.

// (beginning of nonlinear solve)

Eigen::VectorXd grad_energy = ...; // gradient of elastic energy potential
Eigen::VectorXd grad_barrier = B.gradient(collisions, collision_mesh, vertices);

double bbox_diagonal = ipc::world_bbox_diagonal_length(vertices);

double max_barrier_stiffness; // output of initial_barrier_stiffness
double barrier_stiffness = ipc::initial_barrier_stiffness(
    bbox_diagonal, B.barrier(), dhat, avg_mass, grad_energy,
    grad_barrier, max_barrier_stiffness);

double prev_distance = collisions.compute_minimum_distance(
    collision_mesh, vertices);

// ...

// (end of nonlinear iteration)

double curr_distance =
    collisions.compute_minimum_distance(collision_mesh, vertices);

barrier_stiffness = ipc::update_barrier_stiffness(
    prev_distance, curr_distance, max_barrier_stiffness, barrier_stiffness,
    bbox_diagonal);

prev_distance = curr_distance;

// (next iteration)
# (beginning of nonlinear solve)

grad_energy = ...  # gradient of elastic energy potential
grad_barrier = B.gradient(collisions, collision_mesh, vertices)

bbox_diagonal = ipctk.world_bbox_diagonal_length(vertices)

barrier_stiffness, max_barrier_stiffness = ipctk.initial_barrier_stiffness(
    bbox_diagonal, B.barrier, dhat, avg_mass, grad_energy, grad_barrier,
    max_barrier_stiffness)

prev_distance = collisions.compute_minimum_distance(collision_mesh, vertices)

# ...

# (end of nonlinear iteration)

curr_distance = collisions.compute_minimum_distance(collision_mesh, vertices)

barrier_stiffness = ipctk.update_barrier_stiffness(
    prev_distance, curr_distance, max_barrier_stiffness, barrier_stiffness,
    bbox_diagonal)

prev_distance = curr_distance

# (next iteration)

Modeling Thickness

We implement the thickness model of Li et al. [2021] to apply an offset (referred to as \(\xi\) in [Li et al., 2021] or \(d_\min\) here) to the collisions. This is useful for modeling the thickness of a shell or cloth.

To add a collision offset, we need to set the dmin variable. For example, we can set the collision offset \(d_\min=10^{-3}\) and \(\hat{d}=10^{-4}\):

const double dhat = 1e-4;
const double dmin = 1e-3;

ipc::Collisions collisions;
collisions.build(collision_mesh, vertices, dhat, dmin);
dhat = 1e-4
dmin = 1e-3

collisions = ipctk.Collisions()
collisions.build(collision_mesh, vertices, dhat, dmin)

This will then set the dmin field in all of the Collision objects stored in the collisions.

Note

Currently, only a single thickness value is supported for the entire mesh.

It is also important to use the same \(d_\min\) when performing CCD (see Minimum Separation CCD).

Friction

Computing the friction dissipative potential is similar to the barrier potential, but because it is a lagged model, we need to build it from a fixed set of collisions.

ipc::FrictionCollisions friction_collisions;
friction_collisions.build(
    collision_mesh, vertices, collisions, dhat, barrier_stiffness, mu);
friction_collisions = ipctk.FrictionCollisions()
friction_collisions.build(
    collision_mesh, vertices, collisions, dhat, barrier_stiffness, mu)

Here mu (\(\mu\)) is the (global) coefficient of friction, and barrier_stiffness (\(\kappa\)) is the barrier stiffness.

Friction Dissipative Potential

Now we can compute the friction dissipative potential using the FrictionPotential class.

const FrictionPotential D(epsv);
double friction_potential = D(friction_collisions, collision_mesh, velocity);
D = FrictionPotential(epsv)
friction_potential = D(friction_collisions, collision_mesh, velocity)

Here epsv (\(\epsilon_v\)) is the static friction threshold (in units of velocity) used to smoothly transition from dynamic to static friction.

Important

The friction potential is a function of the velocities rather than the positions. We can compute the velocities directly from the current and previous position(s) based on our time-integration scheme. For example, if we are using backward Euler integration, then the velocity is

\[v = \frac{x - x^t}{h},\]

where \(x\) is the current position, \(x^t\) is the previous position, and \(h\) is the time step size.

This returns a scalar value friction_potential which is the sum of the individual friction potentials.

Mathematically this is defined as

\[D(x) = \sum_{k \in C} \mu\lambda_k^nf_0\left(\|T_k^Tv\|, \epsilon_v\right),\]

where \(C\) is the lagged collisions, \(\lambda_k^n\) is the normal force magnitude for the \(k\)-th collision, \(T_k\) is the tangential basis for the \(k\)-th collision, and \(f_0\) is the smooth friction function used to approximate the non-smooth transition from dynamic to static friction.

Derivatives

We can also compute the first and second derivatives of the friction dissipative potential with respect to the velocities.

Eigen::VectorXd friction_potential_grad =
    D.gradient(friction_collisions, collision_mesh, velocity);

Eigen::SparseMatrix<double> friction_potential_hess =
    D.hessian(friction_collisions, collision_mesh, velocity);
friction_potential_grad = D.gradient(
    friction_collisions, collision_mesh, velocity)

friction_potential_hess = D.hessian(
    friction_collisions, collision_mesh, velocity)

Continuous Collision Detection

The last high-level component of the IPC Toolkit library is continuous collision detection (CCD). This is a method for determining if and at what time two objects will collide. This can be incorporated in a simulation nonlinear solver’s line search to determine the maximum step size allowable before a collision occurs.

There are two main functions for doing this: is_step_collision_free and compute_collision_free_stepsize. The former returns a boolean value indicating if the step is collision-free, and the latter returns the maximum step size that is collision-free. Both functions take the same arguments, but compute_collision_free_stepsize is the more convenient function to use as it returns the maximum step size.

The following example determines the maximum step size allowable between the rest_positions and the squashed bunny.

Eigen::MatrixXd vertices_t0 = collision_mesh.rest_positions(); // vertices at t=0
Eigen::MatrixXd vertices_t1 = vertices_t0;                     // vertices at t=1
vertices_t1.col(1) *= 0.01; // squash the mesh in the y-direction

double max_step_size = ipc::compute_collision_free_stepsize(
        collision_mesh, vertices_t0, vertices_t1);

Eigen::MatrixXd collision_free_vertices =
    (vertices_t1 - vertices_t0) * max_step_size + vertices_t0;
assert(ipc::is_step_collision_free(mesh, vertices_t0, collision_free_vertices));
vertices_t0 = collision_mesh.rest_positions() # vertices at t=0
vertices_t1 = vertices_t0.copy()              # vertices at t=1
vertices_t1[:, 1] *= 0.01 # squash the mesh in the y-direction

max_step_size = ipctk.compute_collision_free_stepsize(
        collision_mesh, vertices_t0, vertices_t1)

collision_free_vertices =
    (vertices_t1 - vertices_t0) * max_step_size + vertices_t0
assert(ipctk.is_step_collision_free(mesh, vertices_t0, collision_free_vertices))

CCD is comprised of two parts (phases): broad-phase and narrow-phase.

Broad-Phase

The broad phase takes all possible pairings (quadratic in size) and eliminates (culls) pairs whose bounding boxes do not overlap. This is done using a spatial data structure (e.g., a hash grid or spatial hash).

The Candidates class represents the culled set of candidate pairs and is built by using a broad-phase method. The following example shows how to use the broad phase to determine the candidate pairs between the rest_positions and the squashed bunny.

#include <ipc/candidates/candidates.hpp>

ipc::Candidates candidates;
candidates.build(
    mesh, vertices_t0, vertices_t1,
    /*inflation_radius=*/0.0,
    /*broad_phase_method=*/ipc::BroadPhaseMethod::HASH_GRID);
candidates = ipctk.Candidates()
candidates.build(
    mesh, vertices_t0, vertices_t1,
    broad_phase_method=ipctk.BroadPhaseMethod.HASH_GRID)

Possible values for broad_phase_method are: BRUTE_FORCE (parallel brute force culling), HASH_GRID (default), SPATIAL_HASH (implementation from the original IPC codebase), BVH (SimpleBVH), SWEEP_AND_PRUNE (method of Belgrod et al. [2023]), or SWEEP_AND_TINIEST_QUEUE (requires CUDA).

Narrow-Phase

The narrow phase computes the time of impact between two primitives (e.g., a point and a triangle or two edges in 3D). To do this we utilize the Tight Inclusion CCD method of Wang et al. [2021] for the narrow phase as it is provably conservative (i.e., never misses collisions), accurate (i.e., rarely reports false positives), and efficient.

The following example shows how to use the narrow phase to determine if a point is colliding with a triangle (static in this case).

#include <ipc/ccd/ccd.hpp>

// ...

Eigen::Vector3d p_t0(0.0, -1.0, 0.0); // point at t=0
Eigen::Vector3d p_t1(0.0,  1.0, 0.0); // point at t=1

Eigen::Vector3d t0_t0(-1.0, 0.0,  1.0); // triangle vertex 0 at t=0
Eigen::Vector3d t1_t0( 1.0, 0.0,  1.0); // triangle vertex 1 at t=0
Eigen::Vector3d t2_t0( 0.0, 0.0, -1.0); // triangle vertex 2 at t=0

// static triangle
Eigen::Vector3d t0_t1 = t0_t0; // triangle vertex 0 at t=1
Eigen::Vector3d t1_t1 = t1_t0; // triangle vertex 1 at t=1
Eigen::Vector3d t2_t1 = t2_t0; // triangle vertex 2 at t=1

double toi; // output time of impact
bool is_colliding = ipc::point_triangle_ccd(
    p_t0, t0_t0, t1_t0, t2_t0, p_t1, t0_t1, t1_t1, t2_t1, toi);
assert(is_colliding);
assert(abs(toi - 0.5) < 1e-8);
import numpy as np
import ipctk

p_t0 = np.array([0.0, -1.0, 0.0]) # point at t=0
p_t1 = np.array([0.0,  1.0, 0.0]) # point at t=1

t0_t0 = np.array([-1.0, 0.0,  1.0]) # triangle vertex 0 at t=0
t1_t0 = np.array([ 1.0, 0.0,  1.0]) # triangle vertex 1 at t=0
t2_t0 = np.array([ 0.0, 0.0, -1.0]) # triangle vertex 2 at t=0

# static triangle
t0_t1 = t0_t0 # triangle vertex 0 at t=1
t1_t1 = t1_t0 # triangle vertex 1 at t=1
t2_t1 = t2_t0 # triangle vertex 2 at t=1

# returns a boolean indicating if the point is colliding with the triangle
# and the time of impact (TOI)
is_colliding, toi = ipctk.point_triangle_ccd(
    p_t0, t0_t0, t1_t0, t2_t0, p_t1, t0_t1, t1_t1, t2_t1)
assert(is_colliding)
assert(abs(toi - 0.5) < 1e-8)

Alternatively, the FaceVertexCandidate class contains a ccd function that can be used to determine if the face-vertex pairing is colliding:

ipc::FaceVertexCandidate candidate = ...; // face-vertex candidate

double toi; // output time of impact
bool is_colliding = candidate.ccd(
    vertices_t0, vertices_t1, collision_mesh.edges(), collision_mesh.faces(), toi);
candidate = ... # face-vertex candidate

# returns a boolean indicating if the point is colliding with the triangle
# and the time of impact (TOI)
is_colliding, toi = candidate.ccd(
    vertices_t0, vertices_t1, collision_mesh.edges, collision_mesh.faces)

The same can be done for point-edge collisions using the point_edge_ccd function or EdgeVertexCandidate class and for edge-edge collisions using the edge_edge_ccd function or EdgeEdgeCandidate class.

Minimum Separation

We can also perform CCD with a minimum separation distance. This is useful when modeling thickness (see, e.g., Modeling Thickness).

To do this, we need to set the min_distance parameter when calling is_step_collision_free and compute_collision_free_stepsize. For example, we can set the minimum separation distance to \(d_\min=10^{-4}\):

double max_step_size = ipc::compute_collision_free_stepsize(
        collision_mesh, vertices_t0, vertices_t1,
        /*broad_phase_method=*/ipc::DEFAULT_BROAD_PHASE_METHOD,
        /*min_distance=*/1e-4);

Eigen::MatrixXd collision_free_vertices =
    (vertices_t1 - vertices_t0) * max_step_size + vertices_t0;
assert(ipc::is_step_collision_free(
    mesh, vertices_t0, collision_free_vertices,
    /*broad_phase_method=*/ipc::DEFAULT_BROAD_PHASE_METHOD,
    /*min_distance=*/1e-4
));
max_step_size = ipctk.compute_collision_free_stepsize(
        collision_mesh, vertices_t0, vertices_t1, min_distance=1e-4)

collision_free_vertices =
    (vertices_t1 - vertices_t0) * max_step_size + vertices_t0
assert(ipctk.is_step_collision_free(
    mesh, vertices_t0, collision_free_vertices, min_distance=1e-4))

Last update: Jul 15, 2024