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>>>>> tag v1.26.5+1 for changeset b74429b0f5c4
author Franz Glasner <fzglas.hg@dom66.de>
date Sat, 11 Oct 2025 17:17:30 +0200
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# Introduction

Subset preprocessing is a mechanism which can significantly speed up font subsetting operations.
It works by prepopulating datastructures from the source font which can be used in later subsetting
operations to more quickly produce the subset. Preprocessing is useful in cases where multiple subsets
will be cut from the same source font.

# Usage

```c++
hb_face_t* preprocessed = hb_subset_preprocess (source_face);

...

hb_face_t* subset = hb_subset_or_fail (preprocessed, subset_input);
```

# Additional Details

*  A subset produced from a preprocessed face should be identical to a subset produced from only the
   original face. The preprocessor does not change the functionality of the subsetter, just speeds
   things up.

*  The preprocessing operation may take longer than the time it takes to produce a subset from the
   source font. Thus the main performance gains are made when a preprocessed face is reused for
   multiple subsetting operations.

*  Currently the largest performance gains are seen when using a preprocessed face for CFF subsetting.

*  The preprocessed face may contain references to the memory backing the source face. If this memory
   is fully owned by a harfbuzz hb_blob_t* then it will automatically be kept alive for the lifetime
   of the preprocessed face. However, if this memory is not fully owned by a harfbuzz hb_blob_t* then
   it is necessary to ensure that the memory is kept alive for the lifetime of the preprocessed face.


# Performance Improvements

Here is the performance difference of producing a subset with a preprocessed face vs producing
a subset with the source face:

Benchmark | Delta Time (%)
----------|-----------------
BM_subset/subset_glyphs/Roboto-Regular.ttf/10_median|-56%
BM_subset/subset_glyphs/Roboto-Regular.ttf/64_median|-33%
BM_subset/subset_glyphs/Roboto-Regular.ttf/512_median|-28%
BM_subset/subset_glyphs/Roboto-Regular.ttf/1000_median|-11%
BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/10_median|-56%
BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/64_median|-33%
BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/512_median|-21%
BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/1000_median|-9%
BM_subset/subset_glyphs/Amiri-Regular.ttf/10_median|-67%
BM_subset/subset_glyphs/Amiri-Regular.ttf/64_median|-48%
BM_subset/subset_glyphs/Amiri-Regular.ttf/512_median|-21%
BM_subset/subset_glyphs/Amiri-Regular.ttf/4096_median|-9%
BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/10_median|-66%
BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/64_median|-50%
BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/512_median|-8%
BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/4096_median|-9%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/10_median|-85%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/64_median|-71%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/512_median|-3%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/1400_median|4%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/10_median|-84%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/64_median|-72%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/512_median|0%
BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/1400_median|0%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/10_median|-30%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/64_median|-24%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/512_median|-3%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/1000_median|-3%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/10_median|-30%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/64_median|-24%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/512_median|-3%
BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/1000_median|-5%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/10_median|-96%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/64_median|-90%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/512_median|-74%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/4096_median|-25%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/10000_median|-23%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/10_median|-95%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/64_median|-90%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/512_median|-73%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/4096_median|-24%
BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/10000_median|-11%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/10_median|-84%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/64_median|-77%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/512_median|-70%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/4096_median|-80%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/10000_median|-86%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/10_median|-84%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/64_median|-78%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/512_median|-71%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/4096_median|-86%
BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/10000_median|-88%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/10_median|-59%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/64_median|-55%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/512_median|-67%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/2000_median|-68%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/10_median|-60%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/64_median|-58%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/512_median|-72%
BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/2000_median|-71%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/10_median|-70%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/64_median|-64%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/300_median|-73%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/10_median|-71%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/64_median|-68%
BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/300_median|-72%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/10_median|-90%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/64_median|-82%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/512_median|-31%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/4096_median|-9%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/6000_median|-22%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/10_median|-88%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/64_median|-83%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/512_median|-31%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/4096_median|-16%
BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/6000_median|-18%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/10_median|-44%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/64_median|-18%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/512_median|-2%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/900_median|-6%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/10_median|-45%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/64_median|-17%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/512_median|-15%
BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/900_median|-3%
BM_subset/subset_codepoints/Roboto-Regular.ttf/10_median|-20%
BM_subset/subset_codepoints/Roboto-Regular.ttf/64_median|-16%
BM_subset/subset_codepoints/Roboto-Regular.ttf/512_median|-12%
BM_subset/subset_codepoints/Roboto-Regular.ttf/1000_median|-10%
BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/10_median|-24%
BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/64_median|-14%
BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/512_median|-15%
BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/1000_median|-9%
BM_subset/subset_codepoints/Amiri-Regular.ttf/10_median|-51%
BM_subset/subset_codepoints/Amiri-Regular.ttf/64_median|-37%
BM_subset/subset_codepoints/Amiri-Regular.ttf/512_median|-12%
BM_subset/subset_codepoints/Amiri-Regular.ttf/4096_median|-1%
BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/10_median|-49%
BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/64_median|-35%
BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/512_median|-6%
BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/4096_median|-1%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/10_median|-82%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/64_median|-9%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/512_median|0%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/1400_median|0%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/10_median|-82%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/64_median|-13%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/512_median|-3%
BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/1400_median|2%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/10_median|-40%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/64_median|-26%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/512_median|-5%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/1000_median|3%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/10_median|-43%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/64_median|-24%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/512_median|-2%
BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/1000_median|2%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/10_median|-83%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/64_median|-67%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/512_median|-39%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/4096_median|-20%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/10000_median|-25%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/10_median|-83%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/64_median|-65%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/512_median|-42%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/4096_median|-34%
BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/10000_median|-21%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/10_median|-69%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/64_median|-69%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/512_median|-70%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/4096_median|-84%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/10000_median|-83%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/10_median|-71%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/64_median|-68%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/512_median|-70%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/4096_median|-86%
BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/10000_median|-88%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/10_median|-45%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/64_median|-48%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/512_median|-57%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/2000_median|-66%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/10_median|-43%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/64_median|-50%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/512_median|-63%
BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/2000_median|-72%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/10_median|-69%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/64_median|-66%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/300_median|-74%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/10_median|-70%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/64_median|-71%
BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/300_median|-75%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/10_median|-66%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/64_median|-46%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/512_median|-15%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/4096_median|-5%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/6000_median|-16%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/10_median|-66%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/64_median|-45%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/512_median|-14%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/4096_median|-11%
BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/6000_median|-27%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/10_median|-38%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/64_median|-9%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/512_median|-3%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/900_median|-16%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/10_median|-39%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/64_median|-12%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/512_median|-4%
BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/900_median|-2%
BM_subset/instance/MPLUS1-Variable.ttf/10_median|-68%
BM_subset/instance/MPLUS1-Variable.ttf/64_median|-45%
BM_subset/instance/MPLUS1-Variable.ttf/512_median|-18%
BM_subset/instance/MPLUS1-Variable.ttf/4096_median|-2%
BM_subset/instance/MPLUS1-Variable.ttf/6000_median|4%
BM_subset/instance/MPLUS1-Variable.ttf/nohinting/10_median|-69%
BM_subset/instance/MPLUS1-Variable.ttf/nohinting/64_median|-46%
BM_subset/instance/MPLUS1-Variable.ttf/nohinting/512_median|-11%
BM_subset/instance/MPLUS1-Variable.ttf/nohinting/4096_median|4%
BM_subset/instance/MPLUS1-Variable.ttf/nohinting/6000_median|-5%
BM_subset/instance/RobotoFlex-Variable.ttf/10_median|-34%
BM_subset/instance/RobotoFlex-Variable.ttf/64_median|-12%
BM_subset/instance/RobotoFlex-Variable.ttf/512_median|6%
BM_subset/instance/RobotoFlex-Variable.ttf/900_median|-6%
BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/10_median|-33%
BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/64_median|-11%
BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/512_median|3%
BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/900_median|0%