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README

1This is a README for the font compression reference code. There are several
2compression related modules in this repository.
3
4brotli/ contains reference code for the Brotli byte-level compression
5algorithm. Note that it is licensed under an Apache 2 license.
6
7src/ contains prototype Java code for compressing fonts.
8
9cpp/ contains prototype C++ code for decompressing fonts.
10
11docs/ contains documents describing the proposed compression format.
12
13= How to run the compression test tool =
14
15This document documents how to run the compression reference code. At this
16writing, the code, while it is intended to produce a bytestream that can be
17reconstructed into a working font, the reference decompression code is not
18done, and the exact format of that bytestream is subject to change.
19
20== Building the tool ==
21
22On a standard Unix-style environment, it should be as simple as running “ant”.
23
24The tool depends on sfntly for much of the font work. The lib/ directory
25contains a snapshot jar. If you want to use the latest sfntly sources, then cd
26to the java subdirectory, run “ant”, then copy these files dist/lib/sfntly.jar
27dist/tools/conversion/eot/eotconverter.jar and
28dist.tools/conversion/woff/woffconverter.jar to $(thisproject)/lib:
29
30dist/lib/sfntly.jar dist/tools/conversion/eot/eotconverter.jar
31dist.tools/conversion/woff/woffconverter.jar
32
33There’s also a dependency on guava (see references below).
34
35The dependencies are subject to their own licenses.
36
37== Setting up the test ==
38
39A run of the tool evaluates a “base” configuration plus one or more test
40configurations, for each font. It measures the file size of the test as a ratio
41over the base file size, then graphs the value of that ratio sorted across all
42files given on the command line.
43
44The test parameters are set by command line options (an improvement from the
45last snapshot). The base is set by the -b command line option, and the
46additional tests are specified by repeated -x command line options (see below).
47
48Each test is specified by a string description. It is a colon-separated list of
49stages. The final stage is entropy compression and can be one of “gzip”,
50“lzma”, “bzip2”, “woff”, “eot” (with actual wire-format MTX compression), or
51“uncomp” (for raw, uncompressed TTF’s). Also, the new wire-format draft
52WOFF2 spec is available as "woff2", and takes an entropy coding as an
53optional argument, as in "woff2/gzip" or "woff2/lzma".
54
55Other stages may optionally include subparameters (following a slash, and
56comma-separated). The stages are:
57
58glyf: performs glyf-table preprocessing based on MTX. There are subparameters:
591. cbbox (composite bounding box). When specified, the bounding box for
60composite glyphs is included, otherwise stripped 2. sbbox (simple bounding
61box). When specified, the bounding box for simple glyphs is included 3. code:
62the bytecode is separated out into a separate stream 4. triplet: triplet coding
63(as in MTX) is used 5. push: push sequences are separated; if unset, pushes are
64kept inline in the bytecode 6. reslice: components of the glyf table are
65separated into individual streams, taking the MTX idea of separating the
66bytecodes further.
67
68hmtx: strips lsb’s from the hmtx table. Based on the idea that lsb’s can be
69reconstructed from bbox.
70
71hdmx: performs the delta coding on hdmx, essentially the same as MTX.
72
73cmap: compresses cmap table: wire format representation is inverse of cmap
74table plus exceptions (one glyph encoded by multiple character codes).
75
76kern: compresses kern table (not robust, intended just for rough testing).
77
78strip: the subparameters are a list of tables to be stripped entirely
79(comma-separated).
80
81The string roughly corresponding to MTX is:
82
83glyf/cbbox,code,triplet,push,hop:hdmx:gzip
84
85Meaning: glyph encoding is used, with simple glyph bboxes stripped (but
86composite glyph bboxes included), triplet coding, push sequences, and hop
87codes. The hdmx table is compressed. And finally, gzip is used as the entropy
88coder.
89
90This differs from MTX in a number of small ways: LZCOMP is not exactly the same
91as gzip. MTX uses three separate compression streams (the base font including
92triplet-coded glyph data), the bytecodes, and the push sequences, while this
93test uses a single stream. MTX also compresses the CVT table (an upper bound on
94the impact of this can be estimated by testing strip/cvt)
95
96Lastly, as a point of methodology, the code by default strips the “dsig” table,
97which would be invalidated by any non-bit-identical change to the font data. If
98it is desired to keep this table, add the “keepdsig” stage.
99
100The string representing the currently most aggressive optimization level is:
101
102glyf/triplet,code,push,reslice:hdmx:hmtx:cmap:kern:lzma
103
104In addition to the MTX one above, it strips the bboxes from composite glyphs,
105reslices the glyf table, compresses the htmx, cmap, and kern tables, and uses
106lzma as the entropy coding.
107
108The string corresponding to the current WOFF Ultra Condensed draft spec
109document is:
110
111glyf/cbbox,triplet,code,reslice:woff2/lzma
112
113The current C++ codebase can roundtrip compressed files as long as no per-table
114entropy coding is specified, as below (this will be fixed soon).
115
116glyf/cbbox,triplet,code,reslice:woff2
117
118
119== Running the tool ==
120
121java -jar build/jar/compression.jar *.ttf > chart.html
122
123The tool takes a list of OpenType fonts on the commandline, and generates an
124HTML chart, which it simply outputs to stdout. This chart uses the Google Chart
125API for plotting.
126
127Options:
128
129-b <desc>
130
131Sets the baseline experiment description.
132
133[ -x <desc> ]...
134
135Sets an experiment description. Can be used multiple times.
136
137-o
138
139Outputs the actual compressed file, substituting ".wof2" for ".ttf" in
140the input file name. Only useful when a single -x parameter is specified.
141
142= Decompressing the fonts =
143
144See the cpp/ directory (including cpp/README) for the C++ implementation of
145decompression. This code is based on OTS, and successfully roundtrips the
146basic compression as described in the draft spec.
147
148= References =
149
150sfntly: http://code.google.com/p/sfntly/ Guava:
151http://code.google.com/p/guava-libraries/ MTX:
152http://www.w3.org/Submission/MTX/
153
154Also please refer to documents (currently Google Docs):
155
156WOFF Ultra Condensed file format: proposals and discussion of wire format
157issues (PDF is in docs/ directory)
158
159WIFF Ultra Condensed: more discussion of results and compression techniques.
160This tool was used to prepare the data in that document.
161