Extracting Bold, Italic, and Underline from PDFs Without Guessing

TLDR

Regex matching PostScript font names (e.g. /bold/i.test(fontName)) fails when PDFs contain subset prefixes (ABCDEF+Font12) or synthetic styling. Authoritative font styling requires querying page.commonObjs for true .bold and .italic boolean properties, checking matrix transform shear components (transform[2] > 0.01) for synthetic italic rendering, and pairing horizontal vector segments (~0.35 * fontSize) to detect underlines.

Typographic Style	Primary Metadata Source	Fallback / Synthetic Check	HTML Output Tag
Bold	page.commonObjs.get(fontName).bold	Cleaned PostScript name regex	<strong>
Italic	page.commonObjs.get(fontName).italic	Matrix Shear Math.abs(transform[2]) > 0.01	<em>
Underline	Vector Line Segment Pairing	Vertical proximity math ($\le 0.35 \times \text{fontSize}$)	<u>

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Technical implementation

// Extract authoritative font styles using page.commonObjs and transform shear matrix
export function buildFontStyleMap(page, textContentItems) {
  const fontStyleMap = {};
  const uniqueFontNames = [...new Set(textContentItems.map(item => item.fontName).filter(Boolean))];
for (const fontName of uniqueFontNames) {     const fontObj = page.commonObjs.get(fontName);     const cleanedName = (fontObj?.name || fontName).replace(/^[A-Z]{6}\+/, '');
fontStyleMap[fontName] = {       bold: !!fontObj?.bold || /bold|heavy|black/i.test(cleanedName),       italic: !!fontObj?.italic || /italic|oblique|slanted/i.test(cleanedName)     };   }
return fontStyleMap; }
// Evaluate synthetic italic slanting and format HTML inline wrappers export function wrapInlineStyles(textString, textItemStyle, transformMatrix) {   let html = escapeHtml(textString);      // Synthetic Italic check via horizontal shear matrix component (transform[2])   const isSyntheticItalic = Math.abs(transformMatrix[2]) > 0.01;   const isItalic = textItemStyle.italic || isSyntheticItalic;
if (textItemStyle.underlined) html = <u>${html}</u>;   if (isItalic)                 html = <em>${html}</em>;   if (textItemStyle.bold)       html = <strong>${html}</strong>;
return html; }

Rule of thumb: Read .bold and .italic properties from page.commonObjs, using matrix shear components (transform[2]) to detect synthetic slant rendering.