Two precision problems were producing visibly wrong hatches on UTM-scale
(and similarly-offset) drawings even after world_offset was correct:
1. **Boundary tessellation** — `hatch_model_from_dxf`'s polyline-bulge
arc tessellation ran the whole arc-center computation in f32. With
vertices at WCS magnitude ~1e5 the f32 ULP is ~1 cm, which propagated
into the sampled arc points and showed up as visibly wavy / jagged
hatch boundaries on edges that crossed the long axis of the drawing.
Re-run the full bulge → arc math in f64; only cast at the to_xy
call that feeds the (also f64) boundary buffer.
2. **Pattern phase** — the per-fragment `perp / perp_step` modulo blew
precision when `xz` was at offset-rel WCS magnitude and perp_step
was sub-cm. Move the precision-critical bits out of f32 by keeping
each hatch's anchor in f64 on the model side: `HatchModel` now
carries a `world_origin: [f64; 2]` and the `boundary: [f32; 2]`
vertices are stored as small offsets from it. The GPU vertex shader
adds `origin` back inside the view_proj multiply for clip position
(sub-pixel-lossy at large hatches but invisible); the `xz` varying
that the fragment shader uses for in_polygon + pattern math stays
in hatch-local space at full f32 precision.
For pattern alignment across adjacent hatches with the same family,
the GPU pipeline snaps the anchor to the family's perp/along grid
in f64 before sending it down. Snap uses the same QCAD-PAT
convention (`fam.dy = perp_step`, `fam.dx = along_step`) the
shader's check_family uses — an earlier attempt used the world-frame
`(-dx*sin + dy*cos)` formula, which produced a meaningless grid and
no alignment.
Trade-off: pattern phase is "global modulo per-family-grid" rather
than tied to WCS (0,0). Same hatches still align with each other;
the absolute phase relative to the WCS origin may differ from the
pre-fix behaviour on small drawings.
Consumers that read `model.boundary` directly (hit_test, paper_canvas,
add_hatch round-trip) reconstruct WCS via `boundary[i] + world_origin`.
The hatch wgsl bind-group entry for binding 0 was bumped to
`VERTEX | FRAGMENT` because the vertex shader now reads `h.origin`.
The fragment-shader hatch LOD substitute + sub-pixel skip from e7675be
and the scissor plumbing from fbbac42 still work — those touched
different fields of the same struct.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Wire pass already uses `WireModel.vp_scissor` to clip viewport-projected
geometry to its paper-space viewport rect; hatch / image / wipeout
passes drew across the whole canvas. When a future paper-space path
projects fills/images into a viewport, they would bleed past the
viewport frame.
Add `vp_scissor: Option<[f32; 4]>` to HatchModel and ImageModel
(mirroring WireModel), forward it through HatchGpu / ImageGpu, and add
per-pass `compute_*_scissors` / `set_scissor_rect` machinery. Factored
the wire-side scissor-projection math into a shared `project_scissor`
helper. Field defaults to None at every constructor site — no producer
sets it yet, so this is plumbing for the paper-viewport fill projection
that Phase 1.4 unblocks without changing current visual output.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
