tableVmtx

Table vmtx — Vertical Metrics (OT §6.4.11).

Module tableVmtx | Source packages/front/office/fonts/src/table/vmtx.js | Deps fontErrors, fontReader, fontWriter | Worker-safe yes

Mirror of hmtx for vertical layout. Layout:

vMetrics[numOfLongVerMetrics]                      (advanceHeight uint16 + tsb int16)
topSideBearings[numGlyphs - numOfLongVerMetrics]   (int16)

The tsb-only tail reuses the last advanceHeight from vMetrics.

Exports

Usage

import fw from '@awacloud/fw';
import { fw_require, modules } from '@awacloud/fonts';

for (const m of [...fw_require, ...modules]) fw.runtime.register(m);
const { parseSfnt } = fw.runtime.resolve('fontSfnt');
const { parseVhea } = fw.runtime.resolve('tableVhea');
const { parseMaxp } = fw.runtime.resolve('tableMaxp');
const { parseVmtx, encodeVmtx } = fw.runtime.resolve('tableVmtx');

const sfnt = parseSfnt(bytes);   // bytes: Uint8Array of a font with vhea + vmtx
const vhea = parseVhea(sfnt.tables.vhea.bytes);
const { numGlyphs } = parseMaxp(sfnt.tables.maxp.bytes);
const { metrics } = parseVmtx(sfnt.tables.vmtx.bytes, vhea.numOfLongVerMetrics, numGlyphs);
console.log(metrics[0].advanceHeight, metrics[0].tsb);

// Re-encode: the trailing run of equal advances is folded into the tsb-only tail.
const { bytes: vmtxBytes, numOfLongVerMetrics } = encodeVmtx({ metrics });

Notes

• The caller supplies the two counts: numOfLongVerMetrics comes from vhea and numGlyphs from maxp.
• Glyphs beyond numOfLongVerMetrics get an advanceHeight equal to the last long metric's advance.
• advanceHeight is read as uint16 and tsb as int16. On encode, advanceHeight is masked to 16 bits and tsb is truncated to an integer.
• encodeVmtx compacts: it keeps, as the last long metric, the first glyph of the trailing run of equal advances (at least one long metric is always kept), and returns that count as numOfLongVerMetrics so the caller can store it in vhea.
• Errors (all ParseError): fonts/vmtx-bad-count (numOfLongVerMetrics outside 1..numGlyphs), fonts/vmtx-short (fewer bytes than numOfLongVerMetrics * 4 + (numGlyphs - numOfLongVerMetrics) * 2), fonts/vmtx-empty (encodeVmtx given an empty metrics array).

See also

• vhea — supplies numOfLongVerMetrics
• vorg — vertical origins
• hmtx — horizontal counterpart
• maxp — supplies numGlyphs