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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 then 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.
+
+*  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%