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dellaert merged 3 commits into from
Jan 19, 2026
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switch Hat/Vee to orthonormal sl(4) basis and update adjoint/tests #2360

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aa5e42b
switch Hat/Vee to orthonormal sl(4) basis and update adjoint/tests
yluo5820 Jan 17, 2026
80ac1ff
refactor Hat/Vee to use SO4 helpers and symmetric split
yluo5820 Jan 19, 2026
a32dfc1
add more documentation
yluo5820 Jan 19, 2026
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166 changes: 108 additions & 58 deletions gtsam/geometry/SL4.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15,59 +15,68 @@
using namespace std;

namespace {
Eigen::Matrix<double, 15, 15> I_15x15 =
Eigen::Matrix<double, 15, 15>::Identity();
constexpr double kInvSqrt2 = 0.7071067811865475244;
constexpr double kInvSqrt6 = 0.4082482904638630164;
constexpr double kInvSqrt12 = 0.2886751345948128823;

Eigen::Matrix<double, 16, 15> setVecToAlgMatrix() {
Eigen::Matrix<double, 16, 15> alg = Eigen::Matrix<double, 16, 15>::Zero();

// 12 Off-diagonal E_ij generators
int k = 0;
for (int i = 0; i < 4; ++i) {
for (int j = 0; j < 4; ++j) {
if (i != j) {
alg(i * 4 + j, k++) = 1.0;
}
}
}

// For Diagonal generators B1 = diag(1, -1, 0, 0)
alg(0, 12) = 1.0;
alg(5, 12) = -1.0;

// For B2 = diag(0, 1, -1, 0)
alg(5, 13) = 1.0;
alg(10, 13) = -1.0;

// For B3 = diag(0, 0, 1, -1)
alg(10, 14) = 1.0;
alg(15, 14) = -1.0;
auto set_skew = [&](int i, int j) {
alg(i * 4 + j, k) = kInvSqrt2;
alg(j * 4 + i, k) = -kInvSqrt2;
++k;
};
auto set_sym = [&](int i, int j) {
alg(i * 4 + j, k) = kInvSqrt2;
alg(j * 4 + i, k) = kInvSqrt2;
++k;
};

// Rotations (skew-symmetric).
set_skew(0, 1);
set_skew(0, 2);
set_skew(0, 3);
set_skew(1, 2);
set_skew(1, 3);
set_skew(2, 3);

// Symmetric off-diagonal shears.
set_sym(0, 1);
set_sym(0, 2);
set_sym(0, 3);
set_sym(1, 2);
set_sym(1, 3);
set_sym(2, 3);

// Traceless diagonal scalings.
alg(0, k) = kInvSqrt2;
alg(5, k) = -kInvSqrt2;
++k;

alg(0, k) = kInvSqrt6;
alg(5, k) = kInvSqrt6;
alg(10, k) = -2.0 * kInvSqrt6;
++k;

alg(0, k) = kInvSqrt12;
alg(5, k) = kInvSqrt12;
alg(10, k) = kInvSqrt12;
alg(15, k) = -3.0 * kInvSqrt12;
++k;

return alg;
}

Eigen::Matrix<double, 15, 16> setAlgtoVecMatrix() {
Eigen::Matrix<double, 15, 16> mat;
mat << 0., 1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 1., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 0., 0., 0., 0., 1., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., -1.;
return mat;
Eigen::Matrix<double, 15, 16> setAlgtoVecMatrix(
const Eigen::Matrix<double, 16, 15>& vec_to_alg) {
return vec_to_alg.transpose();
}

// ALG_TO_VEC * VEC_TO_ALG is equals to I_15x15
// For the orthonormal basis, ALG_TO_VEC * VEC_TO_ALG is the identity.
const Eigen::Matrix<double, 16, 15> VEC_TO_ALG = setVecToAlgMatrix();
const Eigen::Matrix<double, 15, 16> ALG_TO_VEC = setAlgtoVecMatrix();
const Eigen::Matrix<double, 15, 16> ALG_TO_VEC = setAlgtoVecMatrix(VEC_TO_ALG);

} // namespace
namespace gtsam {
Expand Down Expand Up @@ -185,28 +194,69 @@ Matrix44 SL4::Hat(const Vector& xi) {
"SL4::Hat: xi must be a vector of size 15. Got size " +
std::to_string(xi.size()));
}
Matrix44 A;
const double d11 = xi(12);
const double d22 = -xi(12) + xi(13);
const double d33 = -xi(13) + xi(14);
const double d44 = -xi(14);

A << d11, xi(0), xi(1), xi(2), xi(3), d22, xi(4), xi(5), xi(6), xi(7), d33,
xi(8), xi(9), xi(10), xi(11), d44;
Matrix44 A = Matrix44::Zero();

// Rotations (skew-symmetric), normalized by 1/sqrt(2).
A(0, 1) += kInvSqrt2 * xi(0);
A(1, 0) -= kInvSqrt2 * xi(0);
A(0, 2) += kInvSqrt2 * xi(1);
A(2, 0) -= kInvSqrt2 * xi(1);
A(0, 3) += kInvSqrt2 * xi(2);
A(3, 0) -= kInvSqrt2 * xi(2);
A(1, 2) += kInvSqrt2 * xi(3);
A(2, 1) -= kInvSqrt2 * xi(3);
A(1, 3) += kInvSqrt2 * xi(4);
A(3, 1) -= kInvSqrt2 * xi(4);
A(2, 3) += kInvSqrt2 * xi(5);
A(3, 2) -= kInvSqrt2 * xi(5);

// Symmetric off-diagonal shears, normalized by 1/sqrt(2).
A(0, 1) += kInvSqrt2 * xi(6);
A(1, 0) += kInvSqrt2 * xi(6);
A(0, 2) += kInvSqrt2 * xi(7);
A(2, 0) += kInvSqrt2 * xi(7);
A(0, 3) += kInvSqrt2 * xi(8);
A(3, 0) += kInvSqrt2 * xi(8);
A(1, 2) += kInvSqrt2 * xi(9);
A(2, 1) += kInvSqrt2 * xi(9);
A(1, 3) += kInvSqrt2 * xi(10);
A(3, 1) += kInvSqrt2 * xi(10);
A(2, 3) += kInvSqrt2 * xi(11);
A(3, 2) += kInvSqrt2 * xi(11);

// Traceless diagonal scalings.
const double a = kInvSqrt2 * xi(12);
const double b = kInvSqrt6 * xi(13);
const double c = kInvSqrt12 * xi(14);

A(0, 0) += a + b + c;
A(1, 1) += -a + b + c;
A(2, 2) += -2.0 * b + c;
A(3, 3) += -3.0 * c;

return A;
}

/* ************************************************************************* */
// NOTE(hlim): Why 'X'? - I just follow the convention of GTSAM
Vector SL4::Vee(const Matrix44& X) {
Vector vec(15);
const double x12 = X(0, 0);
const double x13 = X(1, 1) + x12;
const double x14 = -X(3, 3);
vec << X(0, 1), X(0, 2), X(0, 3), X(1, 0), X(1, 2), X(1, 3), X(2, 0), X(2, 1),
X(2, 3), X(3, 0), X(3, 1), X(3, 2), x12, x13, x14;
return vec;
// used consistent notation with Hat()
Vector SL4::Vee(const Matrix44& A) {
Vector xi(15);
xi << kInvSqrt2 * (A(0, 1) - A(1, 0)),
kInvSqrt2 * (A(0, 2) - A(2, 0)),
kInvSqrt2 * (A(0, 3) - A(3, 0)),
kInvSqrt2 * (A(1, 2) - A(2, 1)),
kInvSqrt2 * (A(1, 3) - A(3, 1)),
kInvSqrt2 * (A(2, 3) - A(3, 2)),
kInvSqrt2 * (A(0, 1) + A(1, 0)),
kInvSqrt2 * (A(0, 2) + A(2, 0)),
kInvSqrt2 * (A(0, 3) + A(3, 0)),
kInvSqrt2 * (A(1, 2) + A(2, 1)),
kInvSqrt2 * (A(1, 3) + A(3, 1)),
kInvSqrt2 * (A(2, 3) + A(3, 2)),
kInvSqrt2 * (A(0, 0) - A(1, 1)),
kInvSqrt6 * (A(0, 0) + A(1, 1) - 2.0 * A(2, 2)),
kInvSqrt12 * (A(0, 0) + A(1, 1) + A(2, 2) - 3.0 * A(3, 3));
return xi;
}

} // namespace gtsam
14 changes: 14 additions & 0 deletions gtsam/geometry/SL4.h
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,20 @@ class GTSAM_EXPORT SL4 : public MatrixLieGroup<SL4, 15, 4> {
/// @{

using LieAlgebra = Matrix44;
/**
* Lie algebra coordinates for sl(4) using an orthonormal basis.
*
* Ordering of xi (15x1):
* [r12 r13 r14 r23 r24 r34 s12 s13 s14 s23 s24 s34 h1 h2 h3]
*
* Basis:
* - r_ij scale (E_ij - E_ji)/sqrt(2): skew-symmetric rotations.
* - s_ij scale (E_ij + E_ji)/sqrt(2): symmetric off-diagonal shears.
* - h1,h2,h3 scale orthonormal traceless diagonals:
* H1 = (1/sqrt(2)) diag( 1, -1, 0, 0)
* H2 = (1/sqrt(6)) diag( 1, 1, -2, 0)
* H3 = (1/sqrt(12)) diag( 1, 1, 1, -3)
*/
static Matrix44 Hat(const Vector& xi);
static Vector Vee(const Matrix44& X);

Expand Down
92 changes: 91 additions & 1 deletion gtsam/geometry/tests/testSL4.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,9 @@
#include <gtsam/base/testLie.h>
#include <gtsam/geometry/SL4.h>

#include <cmath>
#include <vector>

using namespace std;
using namespace gtsam;

Expand Down Expand Up @@ -171,9 +174,96 @@ TEST(SL4, HatVeeAreInverses) {
EXPECT(assert_equal(xi0, xi_recovered, 1e-8));
}

/* ************************************************************************* */
TEST(SL4, HatTraceIsZero) {
Vector15 eta =
(Vector15() << 0.31, -0.22, 0.14, -0.09, 0.27, -0.18, 0.05, -0.12, 0.08,
0.11, -0.06, 0.02, 0.07, -0.04, 0.13)
.finished();
Matrix4 A = SL4::Hat(eta);
EXPECT_DOUBLES_EQUAL(0.0, A.trace(), 1e-12);
}

/* ************************************************************************* */
TEST(SL4, HatVeeRoundTrip) {
Vector15 eta =
(Vector15() << -0.25, 0.19, -0.11, 0.07, -0.03, 0.29, -0.17, 0.09, 0.21,
-0.13, 0.04, -0.06, 0.15, -0.08, 0.02)
.finished();
Vector eta_recovered = SL4::Vee(SL4::Hat(eta));
EXPECT(assert_equal(eta, eta_recovered, 1e-12));
}

/* ************************************************************************* */
TEST(SL4, HatBasisIsOrthonormal) {
std::vector<Matrix4> generators;
generators.reserve(15);

for (int k = 0; k < 15; ++k) {
Vector15 e = Vector15::Zero();
e(k) = 1.0;
generators.push_back(SL4::Hat(e));
}

for (int i = 0; i < 15; ++i) {
for (int j = 0; j < 15; ++j) {
const double inner =
(generators[i].array() * generators[j].array()).sum();
const double expected = (i == j) ? 1.0 : 0.0;
EXPECT_DOUBLES_EQUAL(expected, inner, 1e-12);
}
}
}

/* ************************************************************************* */
TEST(SL4, HatMatchesOrthonormalDefinition) {
const double inv_sqrt2 = 1.0 / std::sqrt(2.0);
const double inv_sqrt6 = 1.0 / std::sqrt(6.0);
const double inv_sqrt12 = 1.0 / std::sqrt(12.0);

const Vector15 eta = xi0;
Matrix4 expected = Matrix4::Zero();

expected(0, 1) += inv_sqrt2 * eta(0);
expected(1, 0) -= inv_sqrt2 * eta(0);
expected(0, 2) += inv_sqrt2 * eta(1);
expected(2, 0) -= inv_sqrt2 * eta(1);
expected(0, 3) += inv_sqrt2 * eta(2);
expected(3, 0) -= inv_sqrt2 * eta(2);
expected(1, 2) += inv_sqrt2 * eta(3);
expected(2, 1) -= inv_sqrt2 * eta(3);
expected(1, 3) += inv_sqrt2 * eta(4);
expected(3, 1) -= inv_sqrt2 * eta(4);
expected(2, 3) += inv_sqrt2 * eta(5);
expected(3, 2) -= inv_sqrt2 * eta(5);

expected(0, 1) += inv_sqrt2 * eta(6);
expected(1, 0) += inv_sqrt2 * eta(6);
expected(0, 2) += inv_sqrt2 * eta(7);
expected(2, 0) += inv_sqrt2 * eta(7);
expected(0, 3) += inv_sqrt2 * eta(8);
expected(3, 0) += inv_sqrt2 * eta(8);
expected(1, 2) += inv_sqrt2 * eta(9);
expected(2, 1) += inv_sqrt2 * eta(9);
expected(1, 3) += inv_sqrt2 * eta(10);
expected(3, 1) += inv_sqrt2 * eta(10);
expected(2, 3) += inv_sqrt2 * eta(11);
expected(3, 2) += inv_sqrt2 * eta(11);

const double a = inv_sqrt2 * eta(12);
const double b = inv_sqrt6 * eta(13);
const double c = inv_sqrt12 * eta(14);
expected(0, 0) = a + b + c;
expected(1, 1) = -a + b + c;
expected(2, 2) = -2.0 * b + c;
expected(3, 3) = -3.0 * c;

EXPECT(assert_equal(expected, SL4::Hat(eta), 1e-12));
}

/* ************************************************************************* */
int main() {
TestResult tr;
return TestRegistry::runAllTests(tr);
}
/* ************************************************************************* */
/* ************************************************************************* */
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