42 lines
1.3 KiB
JavaScript
42 lines
1.3 KiB
JavaScript
import { toBytesInt32 } from './util';
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export default function readMetrics(arrayBuffer) {
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const font = new DataView(arrayBuffer);
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const tableCount = font.getUint16(4);
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const ppem = 1;
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let ascent = 0;
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let descent = 0;
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let lineGap = 0;
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let unitsPerEm;
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for (let i = 0; i < tableCount; i++) {
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const tag = font.getUint32(12 + i * 16);
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const tagStr = toBytesInt32(tag);
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if (tagStr === 'hhea') {
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const offset = font.getUint32(12 + i * 16 + 8);
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const length = font.getUint32(12 + i * 16 + 12);
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const hhea = new DataView(arrayBuffer, offset, length);
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ascent = hhea.getInt16(4);
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descent = hhea.getInt16(6);
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lineGap = hhea.getInt16(8);
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}
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else if (tagStr === 'head') {
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const offset = font.getUint32(12 + i * 16 + 8);
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const length = font.getUint32(12 + i * 16 + 12);
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const head = new DataView(arrayBuffer, offset, length);
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unitsPerEm = head.getUint16(18);
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}
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}
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if (!ascent || !descent || !unitsPerEm) {
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return undefined;
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}
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return {
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ascent: ascent * ppem / unitsPerEm,
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descent: descent * ppem / unitsPerEm,
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lineGap: lineGap * ppem / unitsPerEm,
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unitsPerEm,
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};
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}
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