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Mutool Glyph Index Error: Difference between revisions
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(Created page with "'''mutool glyph index error''' is an issue that occurs when using the mutool command from the MuPDF toolset to process PDF files, resulting in text extraction failure due to abnormal font glyph index mapping. This problem is commonly found in scanned documents processed through Optical Character Recognition (OCR). __TOC__ == Problem Description == When mutool processes certain PDF files, it generates the following error message: <pre>warning: FT_Get_Advance(HiddenHor...") |
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<pre>warning: FT_Get_Advance(HiddenHorzOCR,126): invalid glyph index | <pre>warning: FT_Get_Advance(HiddenHorzOCR,126): invalid glyph index | ||
Error!: Invalid PDF format</pre> | Error!: Invalid PDF format</pre> | ||
This error causes the text extraction process to abort, preventing normal retrieval of PDF content. The issue is particularly prone to occur in scanned documents containing OCR hidden text layers | This error causes the text extraction process to abort, preventing normal retrieval of PDF content. The issue is particularly prone to occur in scanned documents containing OCR hidden text layers <ref>PyMuPDF Development Team (2023). “Font errors during document processing”. GitHub Discussions.https://github.com/pymupdf/PyMuPDF/discussions/2385</ref>. | ||
== Technical Background == | == Technical Background == | ||
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=== PDF Font Embedding Mechanism === | === PDF Font Embedding Mechanism === | ||
To ensure consistent cross-platform display, PDF files typically embed the fonts used within the document. To reduce file size, '''font subsetting''' technology is commonly employed, embedding only the glyphs actually used | To ensure consistent cross-platform display, PDF files typically embed the fonts used within the document. To reduce file size, '''font subsetting''' technology is commonly employed, embedding only the glyphs actually used <ref>Adobe Systems (2008). ''PDF Reference, sixth edition: Adobe Portable Document Format version 1.7''. https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf</ref>. | ||
=== FreeType Library === | === FreeType Library === | ||
mutool relies on the FreeType library for font rendering. The <code>FT_Get_Advance()</code> function is responsible for obtaining the advance width of glyphs, a critical parameter for text layout | mutool relies on the FreeType library for font rendering. The <code>FT_Get_Advance()</code> function is responsible for obtaining the advance width of glyphs, a critical parameter for text layout <ref>The FreeType Project (2024). “FreeType-2 API Reference: Quick retrieval of advance values”. https://freetype.org/freetype2/docs/reference/ft2-quick_advance.html</ref>. When a provided glyph index does not exist in the font, the function returns an error. | ||
== Root Cause == | == Root Cause == | ||
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Different PDF engines’ approaches to handling font errors result in varying processing outcomes for the same document across different tools | Different PDF engines’ approaches to handling font errors result in varying processing outcomes for the same document across different tools <ref>Artifex Software (2024). “MuPDF Documentation”. https://mupdf.readthedocs.io/_/downloads/en/1.26.1/pdf/</ref>. | ||
== Solutions == | == Solutions == | ||
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== | == See Also == | ||
* [[PDF conversion]] | |||
* [[Text Extraction]] | |||
== References == | |||
<references /> | |||
[[Category: PDF Processing]] | [[Category: PDF Processing]] | ||
[[Category: Document Conversion]] | [[Category: Document Conversion]] | ||
[[Category: Revised with LLMs]] | [[Category: Revised with LLMs]] | ||