beautiful-mermaid

Render Mermaid diagrams to SVG, PNG, or terminal ASCII through the bm CLI from beautiful-mermaid-cli.

When to use

• The user wants to render a Mermaid diagram (graph, flowchart, sequenceDiagram, classDiagram, stateDiagram, erDiagram, gantt, …) to an image file.
• The user pastes Mermaid source and asks for a rendered visual.
• The user wants ASCII / Unicode box-drawing output for a diagram in the terminal.
• The user asks to render with a specific font, or to know which fonts are available — first call bm fonts --json to enumerate the system, then pass --font <family> (or --font-file <path>) to bm render.

Prerequisites

Confirm the bm binary, fonts, and PNG WASM runtime are usable before rendering programmatically:

bm doctor --json

If bm is missing: npm i -g beautiful-mermaid-cli (or brew install okooo5km/tap/bm).

Core commands

# SVG (default)
bm render -c '<mermaid source>' -o out.svg

# PNG with theme & 2x scale
bm render -c '<mermaid source>' -o out.png --theme dracula --scale 2

# Terminal ASCII / Unicode box-drawing
bm ascii -c '<mermaid source>'

# List the 15 built-in themes
bm themes

# List system fonts (use --filter cjk / mono / latin to narrow)
bm fonts --json --filter cjk

# Render with a custom font (CJK example) — affects both SVG & PNG
bm render -c '<mermaid source>' -o out.png --font 'Source Han Sans CN'

# Distinct mono font for code-class text (PNG only)
bm render -c '<mermaid source>' -o out.png --font 'Inter' --font-mono 'JetBrains Mono'

# Render using a font file directly (when --font name is ambiguous; PNG only)
bm render -c '<mermaid source>' -o out.png --font-file /path/to/MyFont.ttf

Font semantics differ between SVG and PNG:

• SVG: --font is baked into the markup as a font-family declaration. The

reader (browser, editor) needs that font installed to see it. --font-mono and --font-file are ignored for SVG. Emoji and CJK render as long as the reader has matching system fonts — this almost always works.

• PNG: bm resolves the font on the local system, embeds rasterized glyphs.

All three flags participate in resvg's per-glyph fallback, so mixed CJK/Latin diagrams render correctly even when the chosen font has only Latin coverage.

PNG emoji: glyphs missing, surrounding text intact

PNG output cannot render emoji glyphs — resvg-wasm does not implement COLRv1 (Noto Color Emoji, Twemoji, Segoe UI Emoji v2) or sbix (Apple Color Emoji). Emoji codepoints in the input render as blank space in the PNG.

The good news: surrounding Latin / CJK text renders correctly. bm loads the OS-bundled emoji font (Apple Color Emoji on macOS, Noto Color Emoji on Linux, Segoe UI Emoji on Windows) into fontBuffers purely as a shaping shim — its presence claims the emoji codepoints in cmap, which prevents them from corrupting the per-glyph fallback for nearby CJK characters. Without the shim, mixed [🚀 中文] text would render entirely as tofu boxes.

bm render -o foo.png emits a one-shot stderr warning when the input contains emoji.

SVG output preserves emoji — the viewer renders them via system fonts.

If a user asks to render emoji:

1. Strongly prefer bm render -o foo.svg.
2. If they need PNG specifically, warn them emoji glyphs will be missing

(surrounding text is fine), or suggest rendering to SVG first then rasterizing with a real-text-engine tool (Inkscape, librsvg, Chrome headless).

1. bm fonts --filter emoji lists emoji fonts the system has — useful

awareness but does not change PNG output.

Source may also come from a file (bm render diagram.mmd ...) or stdin (cat x.mmd | bm render -o out.svg).

For programmatic / agent use

Every subcommand accepts --json for a stable, machine-readable contract (schema_version: 1):

• Success payloads → stdout; error payloads → stderr. Never mixed.
• Exit codes: 0 success, 1 WASM/unclassified, 2 usage, 3 parse, 4 I/O.
• PNG cannot be inlined in JSON — always pass -o <file> for PNG output.

bm render --json -c 'graph LR\n  A-->B' -o out.svg
bm themes --json
bm ascii --json -c 'graph LR\n  A-->B'
bm fonts --json --filter cjk      # discover available CJK fonts

Deep reference

See reference.md for the full flag table, the 15-theme list, and the --json schema summary.