Thick-stroke isolation. The drawing is leveled, thresholded to ink, then morphologically opened so thin dimensions and leaders disappear. The remaining mask is the thick outline; its centreline is the profile boundary. No half-stroke offset is applied afterward.
Vectors. Zhang–Suen thinning plus spur pruning yields a skeleton. Closed loops are traced on that graph, simplified, and optionally snapped toward orthogonal. Even–odd fill of those loops is the material region.
Mesh. A regular lattice (0° or 45°) is classified against the compensated boundary. Intersecting triangles are subdivided; auto mode keeps the orientation with the lower boundary-error score. Section properties themselves are integrated on a pixel grid of the same compensated mask, not from the triangle mesh.
Torsion. Saint-Venant Prandtl stress function on a regular finite-volume grid. Exterior void is grounded at zero; each enclosed cell gets its own constant so closed-cell torsion is included approximately. Reduce the grid size and re-run to check convergence. This is an engineering estimate, not a certified section table.
Outline correction. A positive inset moves every boundary into the material: outer contours shrink and internal voids expand. Use about half the rendered outline width when the drawing stroke lies outside the metal.