Compensated tuning candidates

Compensated tuning is a deterministic two-master scaling workflow. It compares compatible outlines in a base and reference master, derives interpolation factors from their geometric difference, and scales while approximately preserving stem thickness. It is not a generic “make this master lighter” command.

Glyphs MCP 1.8 exposes this workflow only through candidate sessions:

1. preview_compensated_tuning_candidate
2. review_outline_candidate_session
3. accept_outline_candidate_session with dry_run=true
4. explicit designer approval
5. accept_outline_candidate_session with confirm=true

The old measurement, direct review, and direct batch-apply commands are removed. Their deterministic mathematics remains internal to the candidate adapter.

Preconditions

• Use different base and reference masters with compatible path and node topology.
• Target explicit glyphs and masters. Components are blocked because this operation promotes node coordinates, smooth flags, and width only.
• Use finite scale and compensation values. keep_stroke matters only when a real two-master geometric delta exists.
• Never expect base_master_id == ref_master_id to create a weight change.
• The tool never saves the font.

Important parameters

• base_master_id is the source master being tuned.
• ref_master_id supplies the compatible comparison geometry.
• output_master_id identifies the destination source layer.
• sx and sy are horizontal and vertical scale factors.
• keep_stroke controls compensation when factors are derived automatically.
• Explicit q_x and q_y override automatic compensation.
• extrapolation is clamp, allow, or error when a computed factor falls outside [0, 1].

Safe workflow

First resolve the font and master IDs with list_open_fonts and get_font_masters. Preview a small explicit glyph set:

{
  "font_index": 0,
  "glyph_names": ["A", "B", "C"],
  "base_master_id": "REGULAR_MASTER_ID",
  "ref_master_id": "BOLD_MASTER_ID",
  "output_master_id": "REGULAR_MASTER_ID",
  "sx": 0.97,
  "sy": 1.0,
  "keep_stroke": 0.9,
  "extrapolation": "clamp"
}

Inspect the golden-yellow difference-only Candidate Reporter. The normal Glyphs outline remains visible; unchanged geometry receives no overlay drawing. If manual edits are needed, call materialize_outline_candidate_session, edit the native candidate layer, and review it again.

Next call review_outline_candidate_session. Confirm that topology is intact, components are absent, the source is not stale, and only allowed fields differ. The review returns a short-lived token bound to source and candidate fingerprints.

Call accept_outline_candidate_session first with dry_run=true and the exact token. Show the designer the target count, skipped/error reasons, largest deltas, width changes, and rollback plan. Only after explicit approval, call it again with confirm=true. Acceptance consumes the token, rechecks both fingerprints on the main thread, promotes only allowed fields, verifies the read-back, removes session layers on complete success, and never saves.

Common mistakes

• Using one master as both base and reference leaves no second shape to interpolate against.
• keep_stroke is not an in-place thinning control.
• Explicit q_x or q_y supersede the corresponding automatic factor.
• Incompatible topology or components are safety failures, not candidates for automatic repair.
• A stale source or candidate requires a new review and token.

See the safety model and command set for the common result envelope and candidate lifecycle contracts.