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PPTX_FONT_RESOLUTION.md

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PPTX Font Resolution and Text Style Inheritance

Summary

This note documents a PPTX fidelity issue that initially looked like a layout bug but was actually caused by incomplete font and text-style resolution.

The visible symptom was that text blocks on several slides, especially the slide 4 SKILLS area, did not match PowerPoint:

  • narrow labels wrapped differently
  • title badges had different sizing and weight
  • some text appeared visually shifted even when box coordinates were correct

The root cause was not hardcoded slide geometry. The real issue was that office2pdf did not resolve PPT text styling the way PowerPoint does.

Problem

PowerPoint text formatting is not determined only by direct run properties. For a text box, effective styling can come from multiple OOXML layers:

  1. direct run properties in a:rPr
  2. paragraph properties in a:pPr
  3. text body defaults in a:lstStyle
  4. default paragraph properties in a:defPPr
  5. level-specific paragraph properties in a:lvlNpPr
  6. default run properties in a:defRPr
  7. theme font references such as +mj-lt / +mn-lt

Before the fix, the PPTX parser mostly used direct a:rPr and direct a:latin values on runs. That was incomplete.

As a result:

  • runs with missing direct font settings lost their inherited family, size, color, and weight
  • text boxes using lstStyle defaults rendered with weaker styling than PowerPoint
  • line wrapping diverged because the effective font metrics were wrong

There was a second issue in fallback selection:

  • fallback ordering preferred "font source rank" too aggressively
  • this caused Office-managed fonts such as Malgun Gothic to outrank the metric-preferred substitute list for Pretendard
  • on this machine, that produced worse wrapping than using Apple SD Gothic Neo

Why PowerPoint Looked Correct

The affected deck does not rely only on theme fonts. It contains explicit typeface information such as Pretendard, Pretendard SemiBold, and related variants in slide XML.

PowerPoint applies:

  • direct run formatting
  • inherited lstStyle defaults
  • script-aware typeface resolution across latin, ea, and cs
  • local fallback behavior when the exact face is unavailable

office2pdf was missing part of that chain, so the generated PDF diverged even though the original PPTX rendered correctly in PowerPoint.

Fix

1. Parse text body style inheritance

Added a text-body default model in the PPTX parser so a text box can inherit style from a:lstStyle.

Implementation:

  • introduced PptxTextBodyStyleDefaults
  • introduced PptxTextLevelStyle
  • parse a:lstStyle, a:defPPr, a:lvlNpPr, and nested a:defRPr
  • merge inherited paragraph and run styles before applying direct paragraph/run overrides
  • apply typeface from a:latin, a:ea, and a:cs, not only direct a:latin

This makes text boxes behave much closer to PowerPoint's effective style model.

2. Preserve metric fallback order

Fallback selection for known families such as Pretendard now preserves the substitution table's metric-compatible order first and only uses source rank as a secondary tiebreaker.

That means:

  • preferred substitute family order is stable
  • Office-managed fonts no longer automatically jump ahead of better metric matches just because they were discovered from Office paths
  • the real deck now falls back to Apple SD Gothic Neo instead of Malgun Gothic

Files Involved

  • crates/office2pdf/src/parser/pptx.rs
  • crates/office2pdf/src/render/font_subst.rs
  • crates/office2pdf/src/render/typst_gen.rs

Verification

The fix was verified in three ways:

  1. unit tests for lstStyle default run inheritance
  2. unit tests for fallback ordering under a mixed Office/system font context
  3. reconversion of the real PPTX deck and manual visual inspection

Observed result after the fix:

  • slide 4 SKILLS labels no longer wrap as before
  • badge/title styling is closer to PowerPoint
  • regenerated PDF embeds Apple SD Gothic Neo variants instead of Malgun Gothic for Pretendard fallback

The remaining known gap is unrelated to text inheritance:

  • slide 17 still warns for unsupported hdphoto1.wdp

Key Lesson

When a PPTX rendering mismatch looks like a textbox position problem, do not assume the coordinates are wrong first. In PowerPoint, incorrect font resolution often becomes a layout problem:

  • different font family
  • different weight
  • different inherited size
  • different line spacing

Those differences change wrapping and visual alignment even if the text box geometry is correct.