fvar table

Defines the variation axes (e.g. weight, width) and their ranges for a variable font.

Scope

• Format family: Shared SFNT
• Table tag in JSON: fvar

Specs

• https://learn.microsoft.com/en-us/typography/opentype/spec/fvar
• 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": {
    "fvar": {
      "majorVersion": 0,
      "minorVersion": 0,
      "reserved": 0,
      "axes": null,
      "instances": null,
      "_checksum": 0
    }
  }
}

Top-level Fields

• majorVersion - number (0..65535) [spec type: uint16]
• minorVersion - number (0..65535) [spec type: uint16]
• reserved - number (0..65535)
• axes - implementation-defined
• instances - implementation-defined

Validation Constraints

• axisSize is 20 for the current format; each axis needs axisTag, min/default/max values, and name/flags metadata.
• Instance coordinate count should match the number of axes.
• instanceSize grows by 2 bytes when any instance uses postScriptNameID.
• reserved is typically 2.

Authoring Example

{
	"tables": {
		"fvar": {
			"majorVersion": 1,
			"minorVersion": 0,
			"reserved": 2,
			"axes": [
				{ "axisTag": "wght", "minValue": 100, "defaultValue": 400, "maxValue": 900, "flags": 0, "axisNameID": 256 }
			],
			"instances": [
				{ "subfamilyNameID": 257, "flags": 0, "coordinates": [400], "postScriptNameID": 258 }
			],
			"_checksum": 0
		}
	}
}

Additional Nested Keys Seen In Implementation

• axisTag
• minValue
• defaultValue
• maxValue
• flags
• axisNameID
• subfamilyNameID
• coordinates

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.