Add scaffolding generation for overhangs

README.md

@@ -55,6 +55,21 @@ unioned = combine(boxes)
 result = safe_difference(unioned, Cylinder(0.5, 1).mesh())
 ```
 
+## Overhang Scaffolding
+
+`generate_scaffolding` creates simple cylindrical supports beneath
+downward facing surfaces that exceed a chosen overhang angle.  The supports
+are meant to be easy to remove after printing.
+
+```python
+from parametric_cad import generate_scaffolding, Box, combine
+
+base = Box(20, 20, 10)
+ledge = Box(10, 10, 5).at(15, 5, 10)
+model = combine([base, ledge])
+supports = generate_scaffolding(model)
+```
+
 ## License
 
 This project is licensed under the [MIT License](LICENSE).

parametric_cad/__init__.py

@@ -11,6 +11,7 @@ from .primitives.sphere import Sphere
 from .mechanisms.butthinge import ButtHinge
 from .export.stl import STLExporter
 from .printability import PrintabilityValidator
+from .scaffolding import generate_scaffolding
 
 __all__ = [
     "tm",
@@ -26,6 +27,7 @@ __all__ = [
     "ButtHinge",
     "STLExporter",
     "PrintabilityValidator",
+    "generate_scaffolding",
     "Polygon",
     "Point",
     "box",

parametric_cad/scaffolding.py

@@ -0,0 +1,68 @@
+from __future__ import annotations
+
+import numpy as np
+
+from .core import tm
+
+
+def generate_scaffolding(
+    mesh: tm.Trimesh,
+    *,
+    max_angle_deg: float = 45.0,
+    support_radius: float = 0.5,
+    grid_size: float = 5.0,
+    sections: int = 8,
+) -> tm.Trimesh:
+    """Return support scaffolding for downward overhangs of ``mesh``.
+
+    Parameters
+    ----------
+    mesh:
+        Mesh to analyze for overhangs.
+    max_angle_deg:
+        Angles from vertical greater than this require support.
+    support_radius:
+        Radius of the cylindrical support columns.
+    grid_size:
+        Grid spacing for clustering support columns. ``0`` disables clustering.
+    sections:
+        Number of cylinder sections used to generate the columns.
+    """
+    normals = mesh.face_normals
+    centers = mesh.triangles_center
+    min_z = float(mesh.bounds[0, 2])
+
+    angles = np.degrees(np.arccos(np.clip(normals[:, 2], -1.0, 1.0)))
+    overhang = (angles > 90.0 + max_angle_deg) & (centers[:, 2] > min_z + 1e-6)
+    if not np.any(overhang):
+        return tm.Trimesh()
+
+    pts = centers[overhang]
+    if grid_size > 0:
+        xy = np.round(pts[:, :2] / grid_size) * grid_size
+        unique_xy, inverse = np.unique(xy, axis=0, return_inverse=True)
+        top_z = np.zeros(len(unique_xy))
+        for i in range(len(unique_xy)):
+            top_z[i] = pts[inverse == i][:, 2].max()
+    else:
+        unique_xy = pts[:, :2]
+        top_z = pts[:, 2]
+
+    supports = []
+    for (x, y), z in zip(unique_xy, top_z):
+        height = z - min_z
+        if height <= 0:
+            continue
+        cyl = tm.creation.cylinder(
+            radius=support_radius, height=height, sections=sections
+        )
+        cyl.apply_translation([0, 0, height / 2])
+        cyl.apply_translation([x, y, min_z])
+        supports.append(cyl)
+
+    if not supports:
+        return tm.Trimesh()
+    return tm.util.concatenate(supports)
+
+
+__all__ = ["generate_scaffolding"]

tests/test_scaffolding.py

@@ -0,0 +1,14 @@
+import pytest
+from parametric_cad import Box, combine, generate_scaffolding
+
+
+def test_scaffolding_generation():
+    base = Box(2.0, 2.0, 1.0)
+    overhang = Box(1.0, 1.0, 0.5).at(1.5, 0.5, 1.0)
+    mesh = combine([base, overhang])
+    scaff = generate_scaffolding(mesh)
+    assert isinstance(scaff, type(mesh))
+    # Should extend from model base upwards
+    assert scaff.bounds[0, 2] == pytest.approx(mesh.bounds[0, 2])
+    assert scaff.bounds[1, 2] <= mesh.bounds[1, 2]
+    assert scaff.vertices.shape[0] > 0