diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..dc20f0c
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,34 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: pip
+          cache-dependency-path: requirements-dev.txt
+
+      # The tested modules only need NumPy and OpenCV, so PyTorch and
+      # Ultralytics are not installed to keep this job fast.
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install numpy opencv-python-headless pillow -r requirements-dev.txt
+
+      - name: Lint (syntax errors and undefined names only)
+        run: |
+          pip install ruff
+          ruff check --select E9,F63,F7,F82 .
+
+      - name: Run tests
+        run: pytest tests -q
diff --git a/README.md b/README.md
index f127304..4c41e2b 100644
--- a/README.md
+++ b/README.md
@@ -76,4 +76,13 @@ python src/app.py --model "Rigged Figure"
 python src/app.py --camera 1 --pose-size 448 --no-mirror
 ```
 
+## Tests
+
+Unit tests cover the smoothing filters, rotation/skinning math, glTF node matrices and mesh decimation. They need only NumPy and OpenCV (no PyTorch, weights or camera).
+
+```bash
+pip install pytest
+pytest tests
+```
+
 by Salimli Ayzek (Салимли Айзек): https://mathematiclove.github.io
diff --git a/requirements-dev.txt b/requirements-dev.txt
new file mode 100644
index 0000000..e079f8a
--- /dev/null
+++ b/requirements-dev.txt
@@ -0,0 +1 @@
+pytest
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..4af617c
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,5 @@
+import sys
+from pathlib import Path
+
+# The modules in src/ import each other by bare name (`import gltf`).
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "src"))
diff --git a/tests/test_decimate.py b/tests/test_decimate.py
new file mode 100644
index 0000000..d95c294
--- /dev/null
+++ b/tests/test_decimate.py
@@ -0,0 +1,76 @@
+import numpy as np
+
+import decimate
+import gltf
+
+
+def grid_primitive(n=30, with_uv=False):
+    """A flat n x n grid of quads (2 triangles each) bound to a single bone."""
+    xs, ys = np.meshgrid(np.linspace(0, 1, n), np.linspace(0, 1, n))
+    pos = np.column_stack([xs.ravel(), ys.ravel(), np.zeros(n * n)])
+    idx = np.arange(n * n).reshape(n, n)
+    tris = []
+    for r in range(n - 1):
+        for c in range(n - 1):
+            a, b, d, e = idx[r, c], idx[r, c + 1], idx[r + 1, c], idx[r + 1, c + 1]
+            tris += [[a, b, d], [b, e, d]]
+    count = n * n
+    return gltf.Primitive(
+        positions=pos,
+        normals=np.tile([0.0, 0.0, 1.0], (count, 1)),
+        joints=np.zeros((count, 4), dtype=np.int64),
+        weights=np.tile([1.0, 0.0, 0.0, 0.0], (count, 1)),
+        triangles=np.array(tris, dtype=np.int64),
+        uv=pos[:, :2].copy() if with_uv else None,
+    )
+
+
+def total_triangles(prims):
+    return sum(len(p.triangles) for p in prims)
+
+
+def test_under_budget_is_returned_untouched():
+    prims = [grid_primitive(10)]
+    assert decimate.to_budget(prims, budget=10_000) is prims
+
+
+def test_reduces_triangles_toward_budget():
+    prim = grid_primitive(40)
+    before = len(prim.triangles)
+    out = decimate.to_budget([prim], budget=before // 4)
+    assert total_triangles(out) < before
+
+
+def test_attributes_stay_consistent():
+    out = decimate.to_budget([grid_primitive(40)], budget=300)[0]
+    n = len(out.positions)
+    assert len(out.normals) == len(out.joints) == len(out.weights) == n
+    assert out.triangles.max() < n
+    assert out.triangles.min() >= 0
+
+
+def test_no_degenerate_triangles_remain():
+    out = decimate.to_budget([grid_primitive(40)], budget=300)[0]
+    t = out.triangles
+    assert np.all(t[:, 0] != t[:, 1])
+    assert np.all(t[:, 1] != t[:, 2])
+    assert np.all(t[:, 0] != t[:, 2])
+
+
+def test_uv_is_carried_along():
+    out = decimate.to_budget([grid_primitive(40, with_uv=True)], budget=300)[0]
+    assert out.uv is not None and len(out.uv) == len(out.positions)
+
+
+def test_different_bones_are_not_welded_together():
+    prim = grid_primitive(20)
+    # Alternate the dominant bone so neighbouring vertices belong to different bones.
+    prim.joints[:, 0] = np.arange(len(prim.joints)) % 2
+    welded = decimate._weld(prim, cells=4)
+    plain = decimate._weld(grid_primitive(20), cells=4)
+    assert len(welded.positions) > len(plain.positions)
+
+
+def test_weld_returns_input_if_everything_collapses():
+    prim = grid_primitive(5)
+    assert decimate._weld(prim, cells=1) is prim
diff --git a/tests/test_geometry.py b/tests/test_geometry.py
new file mode 100644
index 0000000..67f71ba
--- /dev/null
+++ b/tests/test_geometry.py
@@ -0,0 +1,126 @@
+import numpy as np
+import pytest
+
+from character import (
+    _axis_rotation,
+    _orthonormal_inverse,
+    _rotation_between,
+    _scale_rotation,
+)
+from gltf import _node_matrix
+from mesh3d import _rotation
+
+
+def is_rotation(m):
+    return np.allclose(m @ m.T, np.eye(3), atol=1e-9) and np.isclose(np.linalg.det(m), 1.0)
+
+
+class TestRotationBetween:
+    def test_maps_a_onto_b(self):
+        a = np.array([1.0, 0.0, 0.0])
+        b = np.array([0.0, 1.0, 1.0])
+        r = _rotation_between(a, b)
+        assert is_rotation(r)
+        assert np.allclose(r @ a, b / np.linalg.norm(b))
+
+    def test_same_vector_is_identity(self):
+        v = np.array([0.0, 2.0, 0.0])
+        assert np.allclose(_rotation_between(v, v), np.eye(3))
+
+    def test_opposite_vectors_flip(self):
+        a = np.array([0.0, 0.0, 1.0])
+        r = _rotation_between(a, -a)
+        assert np.allclose(r @ a, -a)
+        assert np.allclose(r @ r.T, np.eye(3))
+
+    def test_opposite_along_x_uses_other_axis(self):
+        a = np.array([1.0, 0.0, 0.0])
+        assert np.allclose(_rotation_between(a, -a) @ a, -a)
+
+
+class TestOrthonormalInverse:
+    def test_rotation_inverse_is_transpose(self):
+        r = _rotation(0.4, -0.7, 0.2)
+        assert np.allclose(_orthonormal_inverse(r) @ r, np.eye(3))
+
+    def test_uniform_scale_is_removed(self):
+        r = 2.5 * _rotation(0.3, 0.2)
+        assert np.allclose(_orthonormal_inverse(r) @ r, np.eye(3))
+
+    def test_degenerate_matrix_returns_identity(self):
+        assert np.allclose(_orthonormal_inverse(np.zeros((3, 3))), np.eye(3))
+
+
+class TestAxisRotation:
+    def test_quarter_turn_about_z(self):
+        r = _axis_rotation(np.array([0.0, 0.0, 1.0]), np.pi / 2)
+        assert np.allclose(r @ [1.0, 0.0, 0.0], [0.0, 1.0, 0.0])
+
+    def test_axis_is_normalized(self):
+        a = _axis_rotation(np.array([0.0, 0.0, 5.0]), 0.8)
+        b = _axis_rotation(np.array([0.0, 0.0, 1.0]), 0.8)
+        assert np.allclose(a, b)
+
+    def test_zero_axis_is_identity(self):
+        assert np.allclose(_axis_rotation(np.zeros(3), 1.0), np.eye(3))
+
+    def test_result_is_a_rotation(self):
+        assert is_rotation(_axis_rotation(np.array([1.0, 2.0, 3.0]), 1.1))
+
+
+class TestScaleRotation:
+    def test_gain_two_doubles_angle(self):
+        axis = np.array([0.0, 1.0, 0.0])
+        r = _axis_rotation(axis, 0.4)
+        assert np.allclose(_scale_rotation(r, 2.0), _axis_rotation(axis, 0.8))
+
+    def test_gain_zero_gives_identity(self):
+        r = _axis_rotation(np.array([1.0, 0.0, 0.0]), 0.5)
+        assert np.allclose(_scale_rotation(r, 0.0), np.eye(3))
+
+    def test_identity_is_returned_unchanged(self):
+        assert np.allclose(_scale_rotation(np.eye(3), 3.0), np.eye(3))
+
+    def test_angle_is_clamped(self):
+        r = _axis_rotation(np.array([0.0, 0.0, 1.0]), 2.0)
+        assert is_rotation(_scale_rotation(r, 10.0))
+
+
+class TestMeshRotation:
+    def test_zero_angles_is_identity(self):
+        assert np.allclose(_rotation(0.0, 0.0, 0.0), np.eye(3))
+
+    @pytest.mark.parametrize("angles", [(0.3, 0.1, 0.0), (1.2, -0.8, 0.5), (3.0, 2.0, -1.0)])
+    def test_is_a_rotation(self, angles):
+        assert is_rotation(_rotation(*angles))
+
+    def test_yaw_turns_around_y(self):
+        r = _rotation(np.pi / 2, 0.0)
+        assert np.allclose(r @ [0.0, 1.0, 0.0], [0.0, 1.0, 0.0])
+        assert np.allclose(r @ [0.0, 0.0, 1.0], [1.0, 0.0, 0.0])
+
+
+class TestNodeMatrix:
+    def test_empty_node_is_identity(self):
+        assert np.allclose(_node_matrix({}), np.eye(4))
+
+    def test_translation(self):
+        m = _node_matrix({"translation": [1, 2, 3]})
+        assert np.allclose(m[:3, 3], [1, 2, 3])
+
+    def test_scale(self):
+        m = _node_matrix({"scale": [2, 3, 4]})
+        assert np.allclose(np.diag(m)[:3], [2, 3, 4])
+
+    def test_identity_quaternion(self):
+        assert np.allclose(_node_matrix({"rotation": [0, 0, 0, 1]}), np.eye(4))
+
+    def test_quaternion_quarter_turn_about_z(self):
+        s = np.sqrt(0.5)
+        m = _node_matrix({"rotation": [0, 0, s, s]})
+        assert np.allclose(m[:3, :3] @ [1, 0, 0], [0, 1, 0])
+
+    def test_explicit_matrix_is_column_major(self):
+        col_major = [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 5, 6, 7, 1]
+        m = _node_matrix({"matrix": col_major})
+        assert np.allclose(m[:3, 3], [5, 6, 7])
diff --git a/tests/test_smoothing.py b/tests/test_smoothing.py
new file mode 100644
index 0000000..546048d
--- /dev/null
+++ b/tests/test_smoothing.py
@@ -0,0 +1,101 @@
+import numpy as np
+import pytest
+
+from smoothing import LandmarkFilter, OneEuroFilter, ScalarEMA, _LowPass
+
+
+class TestLowPass:
+    def test_first_sample_passes_through(self):
+        f = _LowPass()
+        assert np.allclose(f(np.array([3.0, 4.0]), 0.5), [3.0, 4.0])
+
+    def test_alpha_one_follows_input(self):
+        f = _LowPass()
+        f(np.array([0.0]), 1.0)
+        assert f(np.array([10.0]), 1.0)[0] == 10.0
+
+    def test_blends_with_previous(self):
+        f = _LowPass()
+        f(np.array([0.0]), 0.5)
+        assert f(np.array([10.0]), 0.5)[0] == pytest.approx(5.0)
+
+    def test_reset_forgets_state(self):
+        f = _LowPass()
+        f(np.array([1.0]), 0.5)
+        f.reset()
+        assert f(np.array([9.0]), 0.1)[0] == 9.0
+
+
+class TestOneEuroFilter:
+    def test_alpha_is_between_zero_and_one_and_grows_with_cutoff(self):
+        low = OneEuroFilter._alpha(0.5, 30.0)
+        high = OneEuroFilter._alpha(5.0, 30.0)
+        assert 0.0 < low < high < 1.0
+
+    def test_constant_signal_stays_constant(self):
+        f = OneEuroFilter()
+        for _ in range(20):
+            out = f([100.0, 200.0])
+        assert np.allclose(out, [100.0, 200.0])
+
+    def test_first_point_is_unchanged(self):
+        assert np.allclose(OneEuroFilter()([5.0, 6.0]), [5.0, 6.0])
+
+    def test_jitter_is_reduced(self):
+        rng = np.random.default_rng(1)
+        f = OneEuroFilter()
+        noisy = 50 + rng.normal(0, 3, size=(200, 2))
+        out = np.array([f(p, dt=1 / 30) for p in noisy])
+        assert out[50:].std() < noisy[50:].std()
+
+    def test_dt_updates_frequency(self):
+        f = OneEuroFilter()
+        f([0.0, 0.0], dt=0.5)
+        assert f.freq == pytest.approx(2.0)
+
+    def test_reset(self):
+        f = OneEuroFilter()
+        f([1.0, 1.0])
+        f.reset()
+        assert np.allclose(f([8.0, 8.0]), [8.0, 8.0])
+
+
+class TestLandmarkFilter:
+    def test_first_frame_unchanged(self):
+        pts = np.arange(34, dtype=float).reshape(17, 2)
+        assert np.allclose(LandmarkFilter()(pts), pts)
+
+    def test_constant_pose_stays_constant(self):
+        pts = np.random.default_rng(2).random((17, 2)) * 100
+        f = LandmarkFilter()
+        for _ in range(15):
+            out = f(pts)
+        assert np.allclose(out, pts)
+
+    def test_shape_change_resets_state(self):
+        f = LandmarkFilter()
+        f(np.zeros((17, 2)))
+        new = np.ones((17, 3)) * 4
+        assert np.allclose(f(new), new)
+
+    def test_output_keeps_shape(self):
+        out = LandmarkFilter()(np.zeros((17, 3)))
+        assert out.shape == (17, 3)
+
+
+class TestScalarEMA:
+    def test_moves_toward_target(self):
+        ema = ScalarEMA(alpha=0.5, value=0.0)
+        assert ema(10.0) == pytest.approx(5.0)
+        assert ema(10.0) == pytest.approx(7.5)
+
+    def test_converges(self):
+        ema = ScalarEMA(alpha=0.35)
+        for _ in range(100):
+            ema(3.0)
+        assert ema.value == pytest.approx(3.0, abs=1e-6)
+
+    def test_set(self):
+        ema = ScalarEMA()
+        ema.set(4.0)
+        assert ema.value == 4.0