Saber (.sba / .sbn2) format

Status: read support for ink + typed text. Verified against a Mac Saber export (format v19, 2026-07-09; fixture core/tests/fixtures/saber/saber-mac-pens-text.sba). Saber (saber-notes/saber) is open source (GPL-3.0) with an intentionally open format — our reader is an independent implementation from the BSON spec and observed files; long-term the better lane is contributing IR export upstream.

Container [verified]

• .sba — zip: main.sbn2 (+ asset files for images).
• .sbn2 — one BSON document (bsonspec.org):

Key	Meaning	Confidence
v	format version (19 observed)	[verified]
ni	? (0 observed)	[unknown]
b	background? (null observed)	[unknown]
z	pages array	[verified]
z[].w, z[].h	page size in canvas units (1000x1400 observed)	[verified]
z[].s	strokes	[verified]
z[].q	Quill delta rich text (insert runs)	[verified]

Stroke [verified]

Key	Meaning
ty	tool name string: fountainPen, Pencil, Highlighter, …
pe	pressure-enabled bool
c	ARGB uint32 (alpha 0x65 on highlighter → translucent)
s	base size (5 pens, 50 highlighter observed)
sm	smoothing factor? [inferred]
i	page index
p	point array: binary structs, little-endian float32 — (x, y) when pe=0, (x, y, pressure 0-1) when pe=1

IR mapping

Tool names → families (TOOL_FAMILY in formats/saber/reader.py); ARGB alpha → appearance opacity; raw PRESSURE channel preserved; appearance is constant-width at the base size — Saber's own pressure→width curve is GPL code we don't reimplement, so exact fidelity is approximate [inferred] while raw/native are faithful. point_scale = 595/1000 (canvas → points) [inferred] from page proportions.

Writer (2026-07-09)

formats/saber/writer.py emits .sbn2/.sba (zip with main.sbn2) as the exact inverse of the reader: observed v19 top-level constants (v/ni/b/p/l/lt/z/c) and per-stroke fields (shape/p/i/ty/pe/c/s/sm/sp) [verified against the fixture]; ARGB packed as signed int32 (Dart BSON). NativeTool round-trips ty/s/sm/pe verbatim; foreign strokes map families via FAMILY_TOOL and take size from appearance/WIDTH median. Texts are re-emitted as Quill {"insert": ...} runs (trailing \n appended per Quill's invariant). RAW fidelity accepted — pressure is the one raw channel Saber stores; speed/tilt drop. validated=False pending the Saber Mac app-open check (docs/validated-writes.md).

Open questions

1. Pressure→rendered-width curve (ask upstream or measure from PDF exports; overlay-diff once corpus case 16 exists on iPad).
2. b background / templates; images (.sba assets) — corpus case 11.
3. Eraser & shape-pen encodings — cases 08/09.
4. Exact canvas-unit→point scale; Saber's PDF export page size.

Changelog

• 2026-07-09: initial spec + reader from a controlled Mac export (v19).
• 2026-07-09: writer (inverse mapping, BSON encoder), round-trip tested; ships behind --experimental.