gvar table

Stores per-glyph variation deltas for interpolating TrueType outlines across the design space.

Scope

• Format family: TTF-specific
• Table tag in JSON: gvar

Specs

• https://learn.microsoft.com/en-us/typography/opentype/spec/gvar
• 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": {
    "gvar": {
      "majorVersion": 0,
      "minorVersion": 0,
      "axisCount": 0,
      "glyphVariationData": null,
      "sharedTuples": null,
      "flags": 0,
      "_checksum": 0
    }
  }
}

Top-level Fields

• majorVersion - number (0..65535)
• minorVersion - number (0..65535)
• axisCount - number (0..65535)
• glyphVariationData - implementation-defined
• sharedTuples - implementation-defined
• flags - number (0..65535)

Nested JSON Structure

Parsed gvar container shape:

{
	"majorVersion": 1,
	"minorVersion": 0,
	"axisCount": 2,
	"flags": 0,
	"sharedTuples": [
		[1.0, 0.0],
		[0.5, -0.25]
	],
	"glyphVariationData": [
		[0, 16, 32],
		[]
	]
}

Notes:

• This implementation parses/writes gvar container metadata and stores per-glyph variation tuple data as raw bytes.
• glyphVariationData length defines glyph count for write.
• Short vs long internal offsets are selected automatically on write based on payload size/alignment.

Validation Constraints

• gvar is meaningful with variable-font axis definitions from fvar (validator warns when missing).
• axisCount should match the number of variation axes.
• glyphVariationData.length defines glyph count for writing offsets.
• Per-glyph tuple payloads are raw bytes in this implementation; preserve exact bytes when editing.

Authoring Example

{
	"tables": {
		"gvar": {
			"majorVersion": 1,
			"minorVersion": 0,
			"axisCount": 2,
			"flags": 0,
			"sharedTuples": [[1.0, 0.0]],
			"glyphVariationData": [[], []],
			"_checksum": 0
		}
	}
}

Additional Nested Keys Seen In Implementation

• None inferred from source.

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.