sophus-rs

2d and 3d geometry for Computer Vision and Robotics

Overview

sophus-rs is a Rust library for 2d and 3d geometry for Computer Vision and Robotics applications. It is a spin-off of the Sophus C++ library which focuses on Lie groups (e.g. rotations and transformations in 2d and 3d).

In addition to Lie groups, sophus-rs also includes other geometric/maths concepts.

Automatic differentiation

sophus-rs provides an automatic differentiation using dual numbers such as autodiff::dual::DualScalar and autodiff::dual::DualVector.

use sophus::prelude::*;
use sophus::autodiff::dual::{DualScalar, DualVector};
use sophus::autodiff::linalg::VecF64;
use sophus::autodiff::maps::VectorValuedVectorMap;

//       [[ x ]]   [[ x / z ]]
//  proj [[ y ]] = [[       ]]
//       [[ z ]]   [[ y / z ]]
fn proj_fn<S: IsSingleScalar<DM, DN>, const DM: usize, const DN: usize>(
    v: S::Vector<3>,
) -> S::Vector<2> {
    let x = v.elem(0);
    let y = v.elem(1);
    let z = v.elem(2);
    S::Vector::<2>::from_array([x / z, y / z])
}

let a = VecF64::<3>::new(1.0, 2.0, 3.0);
// Finite difference Jacobian
let finite_diff = VectorValuedVectorMap::<f64, 1>::sym_diff_quotient_jacobian(
    proj_fn::<f64, 0, 0>,
    a,
    0.0001,
);
// Automatic differentiation Jacobian
let auto_diff =
  proj_fn::<DualScalar<3, 1>, 3, 1>(DualVector::var(a)).jacobian();

approx::assert_abs_diff_eq!(finite_diff, auto_diff, epsilon = 0.0001);

Note that proj_fn is a function that takes a 3D vector and returns a 2D vector. The Jacobian of proj_fn is 2x3 matrix. When a (three dimensional) dual vector is passed to proj_fn, then a 2d dual vector is returned. Since we are expecting a 2x3 Jacobian, each element of the 2d dual vector must represent 3x1 Jacobian. This is why we use DualScalar<3, 1> as the scalar type.

Lie Groups

sophus-rs provides a number of Lie groups, including:

• The group of 2D rotations, lie::Rotation2, also known as the Special Orthogonal group SO(2),
• the group of 3D rotations, lie::Rotation3, also known as the Special Orthogonal group SO(3),
• the group of 2d isometries, lie::Isometry2, also known as the Special Euclidean group SE(2), and
• the group of 3d isometries, lie::Isometry3, also known as the Spevial Euclidean group SE(3).

use sophus::autodiff::linalg::VecF64;
use sophus::lie::{Rotation3F64, Isometry3F64};
use std::f64::consts::FRAC_PI_4;

// Create a rotation around the z-axis by 45 degrees.
let world_from_foo_rotation = Rotation3F64::rot_z(FRAC_PI_4);

// Create a translation in 3D.
let foo_in_world = VecF64::<3>::new(1.0, 2.0, 3.0);

// Combine them into an SE(3) transform.
let world_from_foo_isometry
    = Isometry3F64::from_translation_and_rotation(
        foo_in_world,
        world_from_foo_rotation);

// Apply world_from_foo_isometry to a 3D point in the foo reference frame.
let point_in_foo = VecF64::<3>::new(10.0, 0.0, 0.0);
let point_in_world = world_from_foo_isometry.transform(&point_in_foo);

// Manually compute the expected transformation:
//  - rotate (10, 0, 0) around z by 45°
//  - then translate by (1, 2, 3)
let angle = FRAC_PI_4;
let cos = angle.cos();
let sin = angle.sin();
let expected_point_in_world
    = VecF64::<3>::new(1.0 + 10.0 * cos, 2.0 + 10.0 * sin, 3.0);

approx::assert_abs_diff_eq!(
    point_in_world, expected_point_in_world, epsilon = 1e-9);

// Map isometry to 6-dimensional tangent space.
let omega = world_from_foo_isometry.log();
// Map tangent space element back to the manifold.
let roundtrip_world_from_foo_isometry = Isometry3F64::exp(&omega);
approx::assert_abs_diff_eq!(roundtrip_world_from_foo_isometry.matrix(),
                            world_from_foo_isometry.matrix(),
                            epsilon = 1e-9);

// Compose with another isometry.
let world_from_bar_isometry = Isometry3F64::rot_y(std::f64::consts::FRAC_PI_6);
let bar_from_foo_isometry
    = world_from_bar_isometry.inverse() * world_from_foo_isometry;

Sparse non-linear least squares optimization

sophus-rs also provides a sparse non-linear least squares optimization through crate::opt.

use sophus::prelude::*;
use sophus::autodiff::linalg::{MatF64, VecF64};
use sophus::autodiff::dual::DualVector;
use sophus::lie::{Isometry2, Isometry2F64, Rotation2F64};
use sophus::opt::nlls::{CostFn, CostTerms, EvaluatedCostTerm, optimize_nlls, OptParams};
use sophus::opt::robust_kernel;
use sophus::opt::variables::{VarBuilder, VarFamily, VarKind};

// We want to fit the isometry `T ∈ SE(2)` to a prior distribution
// `N(E(T), W⁻¹)`, where `E(T)` is the prior mean and `W⁻¹` is the prior
// covariance matrix.

// (1) First we define the residual cost term.
#[derive(Clone, Debug)]
pub struct Isometry2PriorCostTerm {
    // Prior mean, `E(T)` of type [Isometry2F64].
    pub isometry_prior_mean: Isometry2F64,
    // `W`, which is the inverse of the prior covariance matrix.
    pub isometry_prior_precision: MatF64<3, 3>,
    // We only have one variable, so this will be `[0]`.
    pub entity_indices: [usize; 1],
}

impl Isometry2PriorCostTerm {
    // (2) Then we define  residual function for the cost term:
    //
    // `g(T) = log[T * E(T)⁻¹]`
    pub fn residual<Scalar: IsSingleScalar<DM, DN>, const DM: usize, const DN: usize>(
        isometry: Isometry2<Scalar, 1, DM, DN>,
        isometry_prior_mean: Isometry2<Scalar, 1, DM, DN>,
    ) -> Scalar::Vector<3> {
        (isometry * isometry_prior_mean.inverse()).log()
    }
}

// (3) Implement the `HasResidualFn` trait for the cost term.
impl HasResidualFn<3, 1, (), Isometry2F64> for Isometry2PriorCostTerm {
    fn idx_ref(&self) -> &[usize; 1] {
        &self.entity_indices
    }

    fn eval(
        &self,
        _global_constants: &(),
        idx: [usize; 1],
        args: Isometry2F64,
        var_kinds: [VarKind; 1],
        robust_kernel: Option<robust_kernel::RobustKernel>,
    ) -> EvaluatedCostTerm<3, 1> {
        let isometry: Isometry2F64 = args;

        let residual = Self::residual(isometry, self.isometry_prior_mean);
        let dx_res_fn = |x: DualVector<3, 3, 1>| -> DualVector<3, 3, 1> {
            Self::residual(
                Isometry2::exp(x) * isometry.to_dual_c(),
                self.isometry_prior_mean.to_dual_c(),
            )
        };

        (|| dx_res_fn(DualVector::var(VecF64::<3>::zeros())).jacobian(),).make(
            idx,
            var_kinds,
            residual,
            robust_kernel,
            Some(self.isometry_prior_precision),
        )
    }
}

let prior_world_from_robot = Isometry2F64::from_translation(
    VecF64::<2>::new(1.0, 2.0),
);

// (4) Define the cost terms, by specifying the residual function
// `g(T) = Isometry2PriorCostTerm` as well as providing the prior distribution.
const POSE: &str = "poses";
let obs_pose_a_from_pose_b_poses = CostTerms::new(
    [POSE],
    vec![Isometry2PriorCostTerm {
        isometry_prior_mean: prior_world_from_robot,
        isometry_prior_precision: MatF64::<3, 3>::identity(),
        entity_indices: [0],
    }],
);

// (5) Define the decision variables. In this case, we only have one variable,
// and we initialize it with the identity transformation.
let est_world_from_robot = Isometry2F64::identity();
let variables = VarBuilder::new()
    .add_family(
        POSE,
        VarFamily::new(VarKind::Free, vec![est_world_from_robot]),
    )
    .build();

// (6) Perform the non-linear least squares optimization.
let solution = optimize_nlls(
    variables,
    vec![CostFn::new_boxed((), obs_pose_a_from_pose_b_poses.clone(),)],
    OptParams::default(),
)
.unwrap();

// (7) Retrieve the refined transformation and compare it with the prior one.
let refined_world_from_robot
    = solution.variables.get_members::<Isometry2F64>(POSE)[0];
approx::assert_abs_diff_eq!(
    prior_world_from_robot.compact(),
    refined_world_from_robot.compact(),
    epsilon = 1e-6,
);

And more...

such unit vector, splines, image classes, camera models, and some visualization tools. Check out the documentation for more information.

Building

sophus-rs builds on stable.

[dependencies]
sophus = "0.14.0"

To allow for batch types, such as BatchScalarF64, the 'simd' feature is required. This feature depends on portable-simd, which is currently only available on nightly. There are no plans to rely on any other nightly features.

[dependencies]
sophus = { version = "0.14.0", features = ["simd"] }

Crate Structure and Usage

sophus-rs is an umbrella crate that provides a single entry point to multiple sub-crates (modules) under the sophus:: namespace. For example, the automatic differentiation sub-crate can be accessed via use sophus::autodiff, and the lie group sub-crate via use sophus::lie, etc.

• If you want all of sophus’s functionalities at once (geometry, AD, manifolds, etc.), simply add sophus in your Cargo.toml, and then in your Rust code:

  use sophus::prelude::*;
  use sophus::autodiff::dual::DualScalar;
  // ...
  

• If you only need the autodiff functionalities in isolation, you can also depend on the standalone crate underlying sophus_autodiff.

  use sophus_autodiff::prelude::*;
  use sophus_autodiff::dual::DualScalar;
  // ...
  

Status

This library is in an early development stage - hence API is highly unstable. It is likely that existing features will be removed or changed in the future.