feat(schema): text autofit via slot metrics (options: ["autofit"]) - #100
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Closes #92 (phase 1: scale factor emission). Schema FieldSpec gains slot_width / slot_height / font_size / line_height. When a text field opts in via options ["autofit"] and declares slot metrics, the context gets <field>_font_scale (0.1..1.0): the factor to multiply the template's font-size so the wrapped text fits the slot. Width estimate = longest wrapped line x font-size x 0.52 (Inter-like average advance); height = lines x line_height. line_height defaults to font-size x 1.3. Templates opt in per field by multiplying their static font-size with {{ field_font_scale }} (rounding to int). Without the autofit option or slot metrics, nothing changes (backward compatible: default renders md5-identical, full suite passes). Autofit validation for social-quote (slot 920x616, font 52/68): a 280-char quote scales to 9 lines x reduced size and stays above the author line without overlap.
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What
Closes #92 — text autofit, phase 1 (scale factor emission).
FieldSpecgains optional slot metrics (slot_width,slot_height,font_size,line_height). A text field that opts in viaoptions: ["autofit"]and declares those metrics gets a<field>_font_scalecontext variable (0.1–1.0): the factor thetemplate multiplies its static font-size with so the wrapped text fits
the slot.
average advance width) vs
slot_widthslot_heightline_heightdefaults to font-size × 1.3; scale clamped to 0.1–1.0font-size="{{ (52.0 * quote_font_scale) | round | int }}"— or keepthe static size and ignore the variable entirely
No schema change is required for existing templates: without the
autofitoption + slot metrics, nothing is emitted and renders areunchanged.
Why
Shrink-to-fit is the single most-requested feature across commercial
image-generation APIs (Bannerbear makes it a per-layer setting; Robolly
and Placid ship equivalents). Cosy's char-based wrap + max-length
validation can't guarantee visual fit because glyph widths differ.
This gives template authors a deterministic, dependency-free escape
hatch while keeping full backward compatibility.
Testing
cargo test11/11 suites pass; clippy-D warningscleanpre-change baseline
quote triggers a scale < 1.0, reflows to 9 lines and clears the
author line (no overlap, vision-verified); short default quote stays
at scale 1.0
the bundled templates that benefit most (social-quote, quote-stack),
and a deterministic quality-gate fixture asserting max-length content
never overflows