[subset] Add short document on subset preprocessing.
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Garret Rieger
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bc87fe952e
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# Introduction
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Subset preprocessing is a mechanism which can significantly speed up font subsetting operations.
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It works by prepopulating datastructures from the source font which can be used in later subsetting
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operations to more quickly produce the subset. Preprocessing is useful in cases where multiple subsets
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will be cut from the same source font.
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# Usage
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```c++
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hb_face_t* preprocessed = hb_subset_preprocess (source_face);
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...
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hb_face_t* subset = hb_subset_or_fail (preprocessed, subset_input);
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```
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# Additional Details
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* A subset produced from a preprocessed face should be identical to a subset produced from only the
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original face. The preprocessor does not change the functionality of the subsetter, just speeds
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things up.
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* The preprocessing operation may take longer then the time it takes to produce a subset from the
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source font. Thus the main performance gains are made when a preprocessed face is reused for multiple
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subsetting operations.
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* The preprocessed face may contain references to the memory backing the source face. If this memory
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is fully owned by a harfbuzz hb_blob_t* then it will automatically be kept alive for the lifetime
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of the preprocessed face. However, if this memory is not fully owned by a harfbuzz hb_blob_t* then
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it is necessary to ensure that the memory is kept alive for the lifetime of the preprocessed face.
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# Performance Improvements
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Here is the performance difference of producing a subset with a preprocessed face vs producing
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a subset with the source face:
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Benchmark | Delta Time (%)
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----------|-----------------
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BM_subset/subset_glyphs/Roboto-Regular.ttf/10_median|-56%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/64_median|-33%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/512_median|-28%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/1000_median|-11%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/10_median|-56%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/64_median|-33%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/512_median|-21%
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BM_subset/subset_glyphs/Roboto-Regular.ttf/nohinting/1000_median|-9%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/10_median|-67%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/64_median|-48%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/512_median|-21%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/4096_median|-9%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/10_median|-66%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/64_median|-50%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/512_median|-8%
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BM_subset/subset_glyphs/Amiri-Regular.ttf/nohinting/4096_median|-9%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/10_median|-85%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/64_median|-71%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/512_median|-3%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/1400_median|4%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/10_median|-84%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/64_median|-72%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/512_median|0%
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BM_subset/subset_glyphs/NotoNastaliqUrdu-Regular.ttf/nohinting/1400_median|0%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/10_median|-30%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/64_median|-24%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/512_median|-3%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/1000_median|-3%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/10_median|-30%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/64_median|-24%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/512_median|-3%
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BM_subset/subset_glyphs/NotoSansDevanagari-Regular.ttf/nohinting/1000_median|-5%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/10_median|-96%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/64_median|-90%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/512_median|-74%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/4096_median|-25%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/10000_median|-23%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/10_median|-95%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/64_median|-90%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/512_median|-73%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/4096_median|-24%
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BM_subset/subset_glyphs/Mplus1p-Regular.ttf/nohinting/10000_median|-11%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/10_median|-84%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/64_median|-77%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/512_median|-70%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/4096_median|-80%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/10000_median|-86%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/10_median|-84%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/64_median|-78%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/512_median|-71%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/4096_median|-86%
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BM_subset/subset_glyphs/SourceHanSans-Regular_subset.otf/nohinting/10000_median|-88%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/10_median|-59%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/64_median|-55%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/512_median|-67%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/2000_median|-68%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/10_median|-60%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/64_median|-58%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/512_median|-72%
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BM_subset/subset_glyphs/SourceSansPro-Regular.otf/nohinting/2000_median|-71%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/10_median|-70%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/64_median|-64%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/300_median|-73%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/10_median|-71%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/64_median|-68%
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BM_subset/subset_glyphs/AdobeVFPrototype.otf/nohinting/300_median|-72%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/10_median|-90%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/64_median|-82%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/512_median|-31%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/4096_median|-9%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/6000_median|-22%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/10_median|-88%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/64_median|-83%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/512_median|-31%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/4096_median|-16%
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BM_subset/subset_glyphs/MPLUS1-Variable.ttf/nohinting/6000_median|-18%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/10_median|-44%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/64_median|-18%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/512_median|-2%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/900_median|-6%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/10_median|-45%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/64_median|-17%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/512_median|-15%
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BM_subset/subset_glyphs/RobotoFlex-Variable.ttf/nohinting/900_median|-3%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/10_median|-20%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/64_median|-16%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/512_median|-12%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/1000_median|-10%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/10_median|-24%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/64_median|-14%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/512_median|-15%
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BM_subset/subset_codepoints/Roboto-Regular.ttf/nohinting/1000_median|-9%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/10_median|-51%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/64_median|-37%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/512_median|-12%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/4096_median|-1%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/10_median|-49%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/64_median|-35%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/512_median|-6%
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BM_subset/subset_codepoints/Amiri-Regular.ttf/nohinting/4096_median|-1%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/10_median|-82%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/64_median|-9%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/512_median|0%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/1400_median|0%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/10_median|-82%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/64_median|-13%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/512_median|-3%
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BM_subset/subset_codepoints/NotoNastaliqUrdu-Regular.ttf/nohinting/1400_median|2%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/10_median|-40%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/64_median|-26%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/512_median|-5%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/1000_median|3%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/10_median|-43%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/64_median|-24%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/512_median|-2%
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BM_subset/subset_codepoints/NotoSansDevanagari-Regular.ttf/nohinting/1000_median|2%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/10_median|-83%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/64_median|-67%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/512_median|-39%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/4096_median|-20%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/10000_median|-25%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/10_median|-83%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/64_median|-65%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/512_median|-42%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/4096_median|-34%
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BM_subset/subset_codepoints/Mplus1p-Regular.ttf/nohinting/10000_median|-21%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/10_median|-69%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/64_median|-69%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/512_median|-70%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/4096_median|-84%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/10000_median|-83%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/10_median|-71%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/64_median|-68%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/512_median|-70%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/4096_median|-86%
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BM_subset/subset_codepoints/SourceHanSans-Regular_subset.otf/nohinting/10000_median|-88%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/10_median|-45%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/64_median|-48%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/512_median|-57%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/2000_median|-66%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/10_median|-43%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/64_median|-50%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/512_median|-63%
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BM_subset/subset_codepoints/SourceSansPro-Regular.otf/nohinting/2000_median|-72%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/10_median|-69%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/64_median|-66%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/300_median|-74%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/10_median|-70%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/64_median|-71%
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BM_subset/subset_codepoints/AdobeVFPrototype.otf/nohinting/300_median|-75%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/10_median|-66%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/64_median|-46%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/512_median|-15%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/4096_median|-5%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/6000_median|-16%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/10_median|-66%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/64_median|-45%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/512_median|-14%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/4096_median|-11%
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BM_subset/subset_codepoints/MPLUS1-Variable.ttf/nohinting/6000_median|-27%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/10_median|-38%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/64_median|-9%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/512_median|-3%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/900_median|-16%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/10_median|-39%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/64_median|-12%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/512_median|-4%
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BM_subset/subset_codepoints/RobotoFlex-Variable.ttf/nohinting/900_median|-2%
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BM_subset/instance/MPLUS1-Variable.ttf/10_median|-68%
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BM_subset/instance/MPLUS1-Variable.ttf/64_median|-45%
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BM_subset/instance/MPLUS1-Variable.ttf/512_median|-18%
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BM_subset/instance/MPLUS1-Variable.ttf/4096_median|-2%
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BM_subset/instance/MPLUS1-Variable.ttf/6000_median|4%
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BM_subset/instance/MPLUS1-Variable.ttf/nohinting/10_median|-69%
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BM_subset/instance/MPLUS1-Variable.ttf/nohinting/64_median|-46%
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BM_subset/instance/MPLUS1-Variable.ttf/nohinting/512_median|-11%
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BM_subset/instance/MPLUS1-Variable.ttf/nohinting/4096_median|4%
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BM_subset/instance/MPLUS1-Variable.ttf/nohinting/6000_median|-5%
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BM_subset/instance/RobotoFlex-Variable.ttf/10_median|-34%
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BM_subset/instance/RobotoFlex-Variable.ttf/64_median|-12%
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BM_subset/instance/RobotoFlex-Variable.ttf/512_median|6%
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BM_subset/instance/RobotoFlex-Variable.ttf/900_median|-6%
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BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/10_median|-33%
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BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/64_median|-11%
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BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/512_median|3%
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BM_subset/instance/RobotoFlex-Variable.ttf/nohinting/900_median|0%
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