Fonts

kou does not bundle fonts. Instead, like ort for ONNX Runtime, it fetches a curated family into a shared cache (once, then cached across runs) and locates it transparently. A [FontSet] pairs a primary monospace face with an optional CJK fallback; the renderer loads them with ab_glyph and, for each codepoint, picks the first face that covers it — so a single render mixes Latin and CJK without tofu.

Families

Slot	`KOU_FONT_*` value	Family
primary (default)	`fira-code`	Fira Code
primary	`jetbrains-mono`	JetBrains Mono
CJK (default)	`sarasa`	Sarasa Mono SC (更纱黑体)
CJK	`sourcehansans`	Source Han Sans SC (思源黑体)
CJK	`smileysans`	Smiley Sans (得意黑)
CJK	`none`	disable the fallback

Resolution order

Explicit file — KOU_FONT_PATH (primary) / KOU_FONT_CJK_PATH.
Shared cache — <cache>/kou/fonts/<family>.ttf|otf.
Runtime download — the font-fetch feature fetches the family from a public source (GitHub / jsDelivr) into the cache.

Knobs

Env	Purpose
`KOU_FONT_PRIMARY`	Override the primary family.
`KOU_FONT_CJK`	Override / disable the CJK family.
`KOU_FONT_MIRROR`	Replace the canonical GitHub / jsDelivr host with a mirror (e.g. for use behind a restrictive network).
`KOU_DOWNLOAD_PROXY`	Route font downloads through `http://` / `https://` / `socks5://` proxy.
`KOU_DOWNLOAD_TIMEOUT_SECS`	Per-request timeout (default 120).
`KOU_SKIP_FONT_FETCH`	Disable runtime fetching (use cache / explicit paths only).

No fonts at all

If no face can be resolved, FontCache is empty and the renderer degrades to drawing a solid block per non-empty cell — so kou still produces something without fonts, and never panics on a missing file.