Mosaic fonts

Explanation: the measured constraints that shape outsized mosaic lettering, and why a font format of Sextile’s own is warranted. The recipes that apply this are Large lettering and Boxed banners; the catalogue of faces is Fonts. The API is sextile.viewdata.lettering and sextile.viewdata.font.

Constraints, measured

• A cell is 2 blocks across and 3 down; a frame is 80 × 72 blocks, 78 across in practice, because a graphics attribute takes a cell on every row a picture spans. See Graphics and Viewdata encoding.

• The three block rows are 3, 4 and 3 scanlines tall, so vertical spacing is inherently uneven and not worth correcting.

• There is no alpha-black attribute, so dark lettering is a lit field with letter-shaped holes — block_runs(..., inverted=True) — costing one attribute a row and no background attributes at all.

Proportional and kerned spacing are required, not merely nicer: the row is only 78 blocks wide and a block is a large fraction of a letter at this resolution, so a block recovered on each glyph, or each leaning pair, is worth having. Set kerned, AVATAR takes 16 cells against 18 proportional and 23 fixed, because its A/V and A/T pairs overlap.

A font format of its own

font.read_font/write_font read and write a human-readable, dependency-free format, because none of the importable bitmap formats carries the thing most needed — a per-glyph advance in blocks — and a vendored font must be reviewed like any other file. Glyphs are named by code point, not by the character, so a space, # and . need no quoting in a file whose picture rows are drawn in # and .. The advance belongs to the font, not the renderer: trimming at render time would re-decide it on every frame and give a space no width at all, so a font carries a fixed advance for the face and an advance for each glyph.