Imposition: n-up, gang-up, booklets, creep, and sheet furniture

[mberrys]

Scope

Its own subsystem, built on a neutral model rather than page-content rewriting.

Model

PressSheet (width, height, gripper, margins, front, back); Placement (source_document, source_page, transform, trim_box, bleed_box, clipping, rotation); Signature (pages[], binding, creep, sheet_sequence).

Modes

• N-up — place source pages as Form XObjects onto sheet pages; never flatten or rewrite their original content. Grid counts, gutters, orientation, scale policy, bleed overlap, duplex orientation.
• Step-and-repeat / gang-up — multiple copies of one product or mixed products on a sheet; start with manually specified quantities and layouts (an automatic packing optimiser is a separate later issue).
• Booklets — signature planning implemented separately from geometric placement: blank insertion, page ordering, left/right placement, front/back orientation, selectable signature sizes.
• Creep / shingling — per nested sheet, offset pages toward/away from the spine from substrate thickness and sheet position, stored as an explicit creep_offset on the Placement rather than altering source artwork.
• Sheet furniture — call the printer-marks engine (Printer marks and production furniture) for crop marks, registration marks, sheet identifiers and colour bars.

Architecture

Product pages → Signature Planner → Sheet Layout → Furniture → imposed PDF — which maps naturally to press/MIS integration later.

Deliverables

• Sheet/Placement/Signature schemas + fixtures (n-up grids, duplex, booklet signatures, gang-up, creep).
• Placement preview with page counts, sheet count, scaling and gutter values shown before commit.
• Imposed output opens in an independent parser with correct boxes; source pages byte-preserved.
• Creep stored on placements (never baked into source content).
• Regression corpus: single-page, mixed page sizes, rotated pages, bleed/no-bleed.

Prerequisites / related

Shared mutation foundation; Printer marks and production furniture; interacts with PDF/VT record boundaries (PDF/VT variable-data production).

Batch context (2026-09-15 architecture review). These items are epics sharing five foundations, not isolated UI features: a production PDF object/mutation engine (parse page content/resources into an editable object model, mutate transactionally, regenerate a valid PDF, preserve untouched objects, run render/preflight/separation regression after every mutation), a separation/color engine, a sheet/layout engine, a production semantic/job model, and a revision/provenance model. Every edit is expected to produce a change record (changed_object_ids, operation, before, after, warnings, reversible). Recommended sequencing: Ink Manager + mutation foundation + printer furniture + basic hairline normalization first; then imposition and direct-object editing foundations; then ISO 19593 packaging; then PDF/VT and XJDF; customer portals and cloud collaboration much later.

Parked: no milestone, labeled future planned. Promote by assigning a milestone and removing the label when a release train picks this up.