sdf.Shell
sdf.Shell(sdf, thickness)Hollow shell of a shape’s surface: |f(p)| - thickness/2.
The result is a wall of the given total thickness centered on the child’s surface. Exact wherever the child field is exact.
Parameters
| Name | Type | Description | Default |
|---|---|---|---|
| sdf | Child shape (SDF node or plain callable). | required | |
| thickness | float | Scalar | Total wall thickness. | required |
Methods
| Name | Description |
|---|---|
| patch_fields | Both offsets of every child patch: outward first, then inward. |
| sdf | Pure function for the shell operation. |
patch_fields
sdf.Shell.patch_fields()Both offsets of every child patch: outward first, then inward.
sdf(p) = |f(p)| - t/2 is the max composition max(f - t/2, -(f + t/2)), so the wall’s two surfaces sit where the child field reads +t/2 and -t/2. Splitting that over the child’s own patches, child patch f_i contributes exactly two fields, kept adjacent and always in this sense:
f_i - t/2— the outward offset, patch index2*i, whose zero set is child patchipushed out by half the thickness;-(f_i + t/2)— the inward offset, patch index2*i + 1, child patchipulled in by half the thickness, negated so it too reads positive outside the wall.
Two caveats a reader needs, neither of which the consumer minds:
The offsets are only as exact as the child’s patch field is a true distance near its own surface — f_i(p) = ±t/2 has to mean “half a thickness off patch i”. Every primitive that declares patch fields satisfies this (they are plane, cylinder and revolved-profile distances, exact along their own normal), so the offset surfaces are exact for them; an approximate child field would offset by the wrong amount.
The trims between the offset patches are not the child’s trims. Offsetting outward pushes neighbouring patches apart and the node’s own |f| - t/2 rounds the gap over, while the max composition here continues both patches to a sharp intersection. The consumer re-derives every trim curve from surface-surface intersections rather than inheriting it, so the untrimmed surfaces are what matters — but do not read argmin_i |f_i| as exact ownership in the rounded sliver just outside a convex child edge.
sdf
sdf.Shell.sdf(child_sdf, p, thickness)Pure function for the shell operation.