ISOLINES show a curved solid from any angle, but its outline still cut across
whichever fixed longitudinal lines happened to face the eye. DISPSILH draws the
true silhouette — the two lines where a cylinder (or cone) turns edge-on to the
view — and it moves with the camera, so it cannot be a baked edge.
Tessellation stores a world-space generator per cone/cylinder face on the mesh
set (axis, radial frame, radius, cone taper, extent), sharing the face-extent
extraction with the ISOLINES collector. Each frame `upload_silhouettes` walks
those generators, solves `du·cosθ + dv·sinθ = -tanα·da` for the two edge-on
angles (`φ ± acos(...)`, `φ ± π/2` for a cylinder), and builds a line list in
the mesh vertex format — double-single, so it stays precise at UTM. It draws
through the existing wireframe / edge pipelines in the wireframe and
shaded-with-edges passes.
Gated on the document's DISPSILH header flag, which defaults off — so the
per-frame pass is a no-op unless the drawing asks for it. Scoped to top-level
solids: a block instance's generators aren't transformed by the INSTANCE pose
yet, so they are cleared on the block path (its isolines and feature edges
still draw).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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