CFF table

Contains PostScript-flavored (Type 2) cubic Bézier glyph outlines for CFF-based OpenType fonts.

Scope

• Format family: OTF-specific
• Table tag in JSON: CFF

Specs

• (No explicit spec URL found in implementation source)
• OpenType table registry: https://learn.microsoft.com/en-us/typography/opentype/spec/otff#font-tables

JSON Skeleton

This skeleton reflects fields currently parsed/written by Font Flux JS for this table.

{
	"tables": {
		"CFF ": {
			"majorVersion": null,
			"minorVersion": null,
			"names": null,
			"strings": null,
			"globalSubrs": null,
			"fonts": null,
			"_checksum": 0
		}
	}
}

Top-level Fields

• majorVersion - implementation-defined
• minorVersion - implementation-defined
• names - implementation-defined
• strings - implementation-defined
• globalSubrs - implementation-defined
• fonts - implementation-defined

Nested JSON Structure

Parsed CFF v1 table:

{
	"majorVersion": 1,
	"minorVersion": 0,
	"names": ["FontName"],
	"strings": ["CustomString"],
	"globalSubrs": [[0, 1, 2]],
	"fonts": [
		{
			"topDict": { "FullName": 391, "Weight": 392 },
			"charset": { "format": 0, "glyphSIDs": [0, 1, 2] },
			"encoding": { "format": 0, "codes": [{ "code": 65, "glyph": 1 }] },
			"charStrings": [[139, 14]],
			"privateDict": { "BlueValues": [0, 10] },
			"localSubrs": [[11]],
			"isCIDFont": true,
			"fdArray": [
				{
					"fontDict": { "FontName": 393 },
					"privateDict": { "BlueScale": 0.039625 },
					"localSubrs": [[11]]
				}
			],
			"fdSelect": {
				"format": 3,
				"ranges": [{ "first": 0, "fd": 0 }],
				"sentinel": 500
			}
		}
	]
}

Notes:

• charStrings, globalSubrs, and localSubrs are byte arrays (Type 2 charstring programs) preserved as raw bytes.
• Offsets (for charset/encoding/Private/FDArray/FDSelect) are resolved during parse and re-derived during write.

Interpreting charStrings

The charStrings byte arrays are opaque Type 2 programs that encode glyph outlines. Font Flux provides two functions for working with them directly:

FontFlux.interpretCharString(bytes, localSubrs, globalSubrs)

Interprets a charstring byte array into an array of cubic Bézier contour commands:

import { FontFlux } from 'font-flux-js';

const contours = FontFlux.interpretCharString(
	font.charStrings[glyphIndex],
	font.localSubrs,
	font.globalSubrs,
);
// Returns: [[{ type: 'M', x, y }, { type: 'L', x, y }, { type: 'C', x1, y1, x2, y2, x, y }, ...], ...]

Command types:

• M — moveTo (start of contour)
• L — lineTo
• C — cubic curveTo with two control points (x1,y1, x2,y2) and endpoint (x,y)

FontFlux.disassembleCharString(bytes)

Converts a charstring byte array into human-readable disassembly text:

const text = FontFlux.disassembleCharString(font.charStrings[glyphIndex]);
// Returns: "100 700 rmoveto 300 0 rlineto 0 -200 -200 -200 -100 0 rrcurveto endchar"

This is useful for debugging and understanding what a charstring program does without running it.

Simplified glyph output

When using FontFlux.open(), CFF glyphs automatically include:

• contours — decoded cubic Bézier commands (same format as interpretCharString output)
• charString — the raw byte array (for lossless round-tripping)
• charStringDisassembly — human-readable disassembly text

Converting to/from SVG paths

Use FontFlux.contoursToSVG() and FontFlux.svgToContours() to convert between CFF contours and SVG path d strings:

import { FontFlux } from 'font-flux-js';

// CFF contours → SVG (produces C commands for cubic curves)
const d = FontFlux.contoursToSVG(glyph.contours);
// "M100 700 L400 700 C400 500 200 300 100 300 Z"

// SVG → CFF contours
const contours = FontFlux.svgToContours(d, 'cff');

This enables visual editing workflows: export contours to SVG, modify the path in an SVG editor, then import the result back as CFF contours.

See the main docs for full details on SVG path conversion.

Validation Constraints

• Use tag "CFF " (with trailing space) in table JSON.
• CFF outlines are an alternative to TrueType glyf/loca; avoid mixing outline models unless intentional.
• fonts[] entries should include consistent charStrings, charset, and encoding payloads.
• CID-keyed fonts require coherent fdArray and fdSelect structures.

Authoring Example

{
	"tables": {
		"CFF ": {
			"majorVersion": 1,
			"minorVersion": 0,
			"names": ["ExampleFont"],
			"strings": [],
			"globalSubrs": [],
			"fonts": [
				{
					"topDict": {},
					"charset": { "format": 0, "glyphSIDs": [0] },
					"encoding": { "format": 0, "codes": [] },
					"charStrings": [[14]],
					"privateDict": {},
					"localSubrs": []
				}
			],
			"_checksum": 0
		}
	}
}

Additional Nested Keys Seen In Implementation

• fontDict
• privateDict
• localSubrs
• operator
• operands

Notes

• Preserve _checksum for stable round-tripping.
• If a table is only partially understood, prefer keeping unknown bytes in _raw instead of dropping data.
• Validate with .validate() after edits.