92 lines
2.7 KiB
Plaintext
92 lines
2.7 KiB
Plaintext
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This folder is a subpackage of ttLib. Each module here is a
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specialized TT/OT table converter: they can convert raw data
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to Python objects and vice versa. Usually you don't need to
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use the modules directly: they are imported and used
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automatically when needed by ttLib.
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If you are writing you own table converter the following is
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important.
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The modules here have pretty strange names: this is due to the
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fact that we need to map TT table tags (which are case sensitive)
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to filenames (which on Mac and Win aren't case sensitive) as well
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as to Python identifiers. The latter means it can only contain
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[A-Za-z0-9_] and cannot start with a number.
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ttLib provides functions to expand a tag into the format used here:
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>>> from fontTools import ttLib
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>>> ttLib.tagToIdentifier("FOO ")
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'F_O_O_'
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>>> ttLib.tagToIdentifier("cvt ")
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'_c_v_t'
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>>> ttLib.tagToIdentifier("OS/2")
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'O_S_2f_2'
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>>> ttLib.tagToIdentifier("glyf")
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'_g_l_y_f'
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>>>
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And vice versa:
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>>> ttLib.identifierToTag("F_O_O_")
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'FOO '
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>>> ttLib.identifierToTag("_c_v_t")
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'cvt '
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>>> ttLib.identifierToTag("O_S_2f_2")
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'OS/2'
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>>> ttLib.identifierToTag("_g_l_y_f")
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'glyf'
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>>>
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Eg. the 'glyf' table converter lives in a Python file called:
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_g_l_y_f.py
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The converter itself is a class, named "table_" + expandedtag. Eg:
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class table__g_l_y_f:
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etc.
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Note that if you _do_ need to use such modules or classes manually,
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there are two convenient API functions that let you find them by tag:
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>>> ttLib.getTableModule('glyf')
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<module 'ttLib.tables._g_l_y_f'>
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>>> ttLib.getTableClass('glyf')
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<class ttLib.tables._g_l_y_f.table__g_l_y_f at 645f400>
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>>>
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You must subclass from DefaultTable.DefaultTable. It provides some default
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behavior, as well as a constructor method (__init__) that you don't need to
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override.
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Your converter should minimally provide two methods:
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class table_F_O_O_(DefaultTable.DefaultTable): # converter for table 'FOO '
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def decompile(self, data, ttFont):
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# 'data' is the raw table data. Unpack it into a
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# Python data structure.
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# 'ttFont' is a ttLib.TTfile instance, enabling you to
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# refer to other tables. Do ***not*** keep a reference to
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# it: it will cause a circular reference (ttFont saves
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# a reference to us), and that means we'll be leaking
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# memory. If you need to use it in other methods, just
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# pass it around as a method argument.
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def compile(self, ttFont):
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# Return the raw data, as converted from the Python
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# data structure.
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# Again, 'ttFont' is there so you can access other tables.
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# Same warning applies.
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If you want to support TTX import/export as well, you need to provide two
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additional methods:
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def toXML(self, writer, ttFont):
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# XXX
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def fromXML(self, (name, attrs, content), ttFont):
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# XXX
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