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OS/2 - OS/2 and Windows Metrics (Version 3) (OpenType 1.5)

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The OS/2 table consists of a set of metrics that are required in OpenType fonts. The fourth version of the OS/2 table (version 3), follows:

Type Name of Entry Comments
USHORT version 0x0003
SHORT xAvgCharWidth  
USHORT usWeightClass  
USHORT usWidthClass  
USHORT fsType  
SHORT ySubscriptXSize  
SHORT ySubscriptYSize  
SHORT ySubscriptXOffset  
SHORT ySubscriptYOffset  
SHORT ySuperscriptXSize  
SHORT ySuperscriptYSize  
SHORT ySuperscriptXOffset  
SHORT ySuperscriptYOffset  
SHORT yStrikeoutSize  
SHORT yStrikeoutPosition  
SHORT sFamilyClass  
BYTE panose[10]  
ULONG ulUnicodeRange1 Bits 0-31
ULONG ulUnicodeRange2 Bits 32-63
ULONG ulUnicodeRange3 Bits 64-95
ULONG ulUnicodeRange4 Bits 96-127
CHAR achVendID[4]  
USHORT fsSelection  
USHORT usFirstCharIndex  
USHORT usLastCharIndex  
SHORT sTypoAscender  
SHORT sTypoDescender  
SHORT sTypoLineGap  
USHORT usWinAscent  
USHORT usWinDescent  
ULONG ulCodePageRange1 Bits 0-31
ULONG ulCodePageRange2 Bits 32-63
SHORT sxHeight  
SHORT sCapHeight  
USHORT usDefaultChar  
USHORT usBreakChar  
USHORT usMaxContext  

version

Format: 2-byte unsigned short
Units: n/a
Title: OS/2 table version number.
Description: The version number for this OS/2 table.
Comments: The version number allows for identification of the precise contents and layout for the OS/2 table. The version number for this layout is three (3). Versions zero (0, TrueType rev 1.5), one (1, TrueType rev 1.66), and two (2, OpenType rev 1.2) have been used previously.

xAvgCharWidth

Format: 2-byte signed short
Units: Pels / em units
Title: Average weighted escapement.
Description: The Average Character Width parameter specifies the arithmetic average of the escapement (width) of all non-zero width glyphs in the font.
Comments: The value for xAvgCharWidth is calculated by obtaining the arithmetic average of the width of all non-zero width glyphs in the font. Furthermore, it is strongly recommended that implementers do not rely on this value for computing layout for lines of text. Especially, for cases where complex scripts are used.

usWeightClass

Format: 2-byte unsigned short
Title: Weight class.
Description: Indicates the visual weight (degree of blackness or thickness of strokes) of the characters in the font.
Comments:
Value Description C Definition (from windows.h)
100 Thin FW_THIN
200 Extra-light (Ultra-light) FW_EXTRALIGHT
300 Light FW_LIGHT
400 Normal (Regular) FW_NORMAL
500 Medium FW_MEDIUM
600 Semi-bold (Demi-bold) FW_SEMIBOLD
700 Bold FW_BOLD
800 Extra-bold (Ultra-bold) FW_EXTRABOLD
900 Black (Heavy) FW_BLACK

usWidthClass

Format: 2-byte unsigned short
Title: Width class.
Description: Indicates a relative change from the normal aspect ratio (width to height ratio) as specified by a font designer for the glyphs in a font.
Comments: Although every character in a font may have a different numeric aspect ratio, each character in a font of normal width has a relative aspect ratio of one. When a new type style is created of a different width class (either by a font designer or by some automated means) the relative aspect ratio of the characters in the new font is some percentage greater or less than those same characters in the normal font -- it is this difference that this parameter specifies.

Value Description C Definition % of normal
1 Ultra-condensed FWIDTH_ULTRA_CONDENSED 50
2 Extra-condensed FWIDTH_EXTRA_CONDENSED 62.5
3 Condensed FWIDTH_CONDENSED 75
4 Semi-condensed FWIDTH_SEMI_CONDENSED 87.5
5 Medium (normal) FWIDTH_NORMAL 100
6 Semi-expanded FWIDTH_SEMI_EXPANDED 112.5
7 Expanded FWIDTH_EXPANDED 125
8 Extra-expanded FWIDTH_EXTRA_EXPANDED 150
9 Ultra-expanded FWIDTH_ULTRA_EXPANDED 200

fsType

Format: 2-byte unsigned short
Title: Type flags.
Description: Indicates font embedding licensing rights for the font. Embeddable fonts may be stored in a document. When a document with embedded fonts is opened on a system that does not have the font installed (the remote system), the embedded font may be loaded for temporary (and in some cases, permanent) use on that system by an embedding-aware application. Embedding licensing rights are granted by the vendor of the font.

The OpenType Font Embedding DLL Specification and DLL release notes describe the APIs used to implement support for OpenType font embedding and loading. Applications that implement support for font embedding, either through use of the Font Embedding DLL or through other means, must not embed fonts which are not licensed to permit embedding. Further, applications loading embedded fonts for temporary use (see Preview & Print and Editable embedding below) must delete the fonts when the document containing the embedded font is closed.

This version of the OS/2 table makes bits 0 - 3 a set of exclusive bits. In other words, at most one bit in this range may be set at a time. The purpose is to remove misunderstandings caused by previous behavior of using the least restrictive of the bits that are set.

Bit Bit Mask Description
  0x0000 Installable Embedding: No fsType bit is set. Thus fsType is zero.
Fonts with this setting indicate that they may be embedded and permanently installed on the remote system by an application. The user of the remote system acquires the identical rights, obligations and licenses for that font as the original purchaser of the font, and is subject to the same end-user license agreement, copyright, design patent, and/or trademark as was the original purchaser.
0 0x0001 Reserved, must be zero.
1 0x0002 Restricted License embedding:
Fonts that have only this bit set must not be modified, embedded or exchanged in any manner without first obtaining permission of the legal owner.
Caution: For Restricted License embedding to take effect, it must be the only level of embedding selected.
2 0x0004 Preview & Print embedding: When this bit is set, the font may be embedded, and temporarily loaded on the remote system. Documents containing Preview & Print fonts must be opened "read-only;" no edits can be applied to the document.
3 0x0008 Editable embedding: When this bit is set, the font may be embedded but must only be installed temporarily on other systems. In contrast to Preview & Print fonts, documents containing Editable fonts may be opened for reading, editing is permitted, and changes may be saved.
4-7   Reserved, must be zero.
8 0x0100 No subsetting: When this bit is set, the font may not be subsetted prior to embedding. Other embedding restrictions specified in bits 0-3 and 9 also apply.
9 0x0200 Bitmap embedding only: When this bit is set, only bitmaps contained in the font may be embedded. No outline data may be embedded. If there are no bitmaps available in the font, then the font is considered unembeddable and the embedding services will fail. Other embedding restrictions specified in bits 0-3 and 8 also apply.
10-15   Reserved, must be zero.

ySubscriptXSize

Format: 2-byte signed short
Units: Font design units
Title: Subscript horizontal font size.
Description: The recommended horizontal size in font design units for subscripts for this font.
Comments: If a font has two recommended sizes for subscripts, e.g., numerics and other, the numeric sizes should be stressed. This size field maps to the em square size of the font being used for a subscript. The horizontal font size specifies a font designer’s recommended horizontal font size for subscript characters associated with this font. If a font does not include all of the required subscript characters for an application, and the application can substitute characters by scaling the character of a font or by substituting characters from another font, this parameter specifies the recommended em square for those subscript characters.

For example, if the em square for a font is 2048 and ySubScriptXSize is set to 205, then the horizontal size for a simulated subscript character would be 1/10th the size of the normal character.

ySubscriptYSize

Format: 2-byte signed short
Units: Font design units
Title: Subscript vertical font size.
Description: The recommended vertical size in font design units for subscripts for this font.
Comments: If a font has two recommended sizes for subscripts, e.g. numerics and other, the numeric sizes should be stressed. This size field maps to the emHeight of the font being used for a subscript. The horizontal font size specifies a font designer’s recommendation for horizontal font size of subscript characters associated with this font. If a font does not include all of the required subscript characters for an application, and the application can substitute characters by scaling the characters in a font or by substituting characters from another font, this parameter specifies the recommended horizontal EmInc for those subscript characters.

For example, if the em square for a font is 2048 and ySubScriptYSize is set to 205, then the vertical size for a simulated subscript character would be 1/10th the size of the normal character.

ySubscriptXOffset

Format: 2-byte signed short
Units: Font design units
Title: Subscript x offset.
Description: The recommended horizontal offset in font design untis for subscripts for this font.
Comments: The Subscript X Offset parameter specifies a font designer’s recommended horizontal offset -- from the character origin of the font to the character origin of the subscript’s character -- for subscript characters associated with this font. If a font does not include all of the required subscript characters for an application, and the application can substitute characters, this parameter specifies the recommended horizontal position from the character escapement point of the last character before the first subscript character. For upright characters, this value is usually zero; however, if the characters of a font have an incline (italic characters) the reference point for subscript characters is usually adjusted to compensate for the angle of incline.

ySubscriptYOffset

Format: 2-byte signed short
Units: Font design units
Title: Subscript y offset.
Description: The recommended vertical offset in font design units from the baseline for subscripts for this font.
Comments: The Subscript Y Offset parameter specifies a font designer’s recommended vertical offset from the character baseline to the character baseline for subscript characters associated with this font. Values are expressed as a positive offset below the character baseline. If a font does not include all of the required subscript for an application, this parameter specifies the recommended vertical distance below the character baseline for those subscript characters.

ySuperscriptXSize

Format: 2-byte signed short
Units: Font design units
Title: Superscript horizontal font size.
Description: The recommended horizontal size in font design units for superscripts for this font.
Comments: If a font has two recommended sizes for subscripts, e.g., numerics and other, the numeric sizes should be stressed. This size field maps to the em square size of the font being used for a subscript. The horizontal font size specifies a font designer’s recommended horizontal font size for superscript characters associated with this font. If a font does not include all of the required superscript characters for an application, and the application can substitute characters by scaling the character of a font or by substituting characters from another font, this parameter specifies the recommended em square for those superscript characters.

For example, if the em square for a font is 2048 and ySuperScriptXSize is set to 205, then the horizontal size for a simulated superscript character would be 1/10th the size of the normal character.

ySuperscriptYSize

Format: 2-byte signed short
Units: Font design units
Title: Superscript vertical font size.
Description: The recommended vertical size in font design units for superscripts for this font.
Comments: If a font has two recommended sizes for subscripts, e.g., numerics and other, the numeric sizes should be stressed. This size field maps to the emHeight of the font being used for a subscript. The vertical font size specifies a font designer’s recommended vertical font size for superscript characters associated with this font. If a font does not include all of the required superscript characters for an application, and the application can substitute characters by scaling the character of a font or by substituting characters from another font, this parameter specifies the recommended EmHeight for those superscript characters.

For example, if the em square for a font is 2048 and ySuperScriptYSize is set to 205, then the vertical size for a simulated superscript character would be 1/10th the size of the normal character.

ySuperscriptXOffset

Format: 2-byte signed short
Units: Font design units
Title: Superscript x offset.
Description: The recommended horizontal offset in font design units for superscripts for this font.
Comments: The Superscript X Offset parameter specifies a font designer’s recommended horizontal offset -- from the character origin to the superscript character’s origin for the superscript characters associated with this font. If a font does not include all of the required superscript characters for an application, this parameter specifies the recommended horizontal position from the escapement point of the character before the first superscript character. For upright characters, this value is usually zero; however, if the characters of a font have an incline (italic characters) the reference point for superscript characters is usually adjusted to compensate for the angle of incline.

ySuperscriptYOffset

Format: 2-byte signed short
Units: Font design units
Title: Superscript y offset.
Description: The recommended vertical offset in font design units from the baseline for superscripts for this font.
Comments: The Superscript Y Offset parameter specifies a font designer’s recommended vertical offset -- from the character baseline to the superscript character’s baseline associated with this font. Values for this parameter are expressed as a positive offset above the character baseline. If a font does not include all of the required superscript characters for an application, this parameter specifies the recommended vertical distance above the character baseline for those superscript characters.

yStrikeoutSize

Format: 2-byte signed short
Units: Font design units
Title: Strikeout size.
Description: Width of the strikeout stroke in font design units.
Comments: This field should normally be the width of the em dash for the current font. If the size is one, the strikeout line will be the line represented by the strikeout position field. If the value is two, the strikeout line will be the line represented by the strikeout position and the line immediately above the strikeout position. For a Roman font with a 2048 em square, 102 is suggested.

yStrikeoutPosition

Format: 2-byte signed short
Units: Font design units
Title: Strikeout position.
Description: The position of the top of the strikeout stroke relative to the baseline in font design units.
Comments: Positive values represent distances above the baseline, while negative values represent distances below the baseline. A value of zero falls directly on the baseline, while a value of one falls one pel above the baseline. The value of strikeout position should not interfere with the recognition of standard characters, and therefore should not line up with crossbars in the font. For a Roman font with a 2048 em square, 460 is suggested.

sFamilyClass

Format: 2-byte signed short
Title: Font-family class and subclass.
Description: This parameter is a classification of font-family design.
Comments: The font class and font subclass are registered values assigned by IBM to each font family. This parameter is intended for use in selecting an alternate font when the requested font is not available. The font class is the most general and the font subclass is the most specific. The high byte of this field contains the family class, while the low byte contains the family subclass. More information about this field.

Panose

Format: 10 byte array
Title: PANOSE classification number
International: Additional specifications are required for PANOSE to classify non-Latin character sets.
Description: This 10 byte series of numbers is used to describe the visual characteristics of a given typeface. These characteristics are then used to associate the font with other fonts of similar appearance having different names. The variables for each digit are listed below. The Panose values are fully described in the Panose “greybook” reference, currently owned by Agfa-Monotype.
Comments: The PANOSE definition contains ten digits each of which currently describes up to sixteen variations. Windows uses bFamilyType, bSerifStyle and bProportion in the font mapper to determine family type. It also uses bProportion to determine if the font is monospaced. If the font is a symbol font, the first byte of the PANOSE number (bFamilyType) must be set to "pictorial." Good PANOSE values in fonts are very valuable to users of the Windows fonts folder. The specification for assigning PANOSE values is located at https://monotype.github.io/panose/

Type Name
BYTE bFamilyType;
BYTE bSerifStyle;
BYTE bWeight;
BYTE bProportion;
BYTE bContrast;
BYTE bStrokeVariation;
BYTE bArmStyle;
BYTE bLetterform;
BYTE bMidline;
BYTE bXHeight;

ulUnicodeRange1 (Bits 0-31)
ulUnicodeRange2 (Bits 32-63)
ulUnicodeRange3 (Bits 64-95)
ulUnicodeRange4 (Bits 96-127)

Format: 32-bit unsigned long(4 copies) totaling 128 bits.
Title: Unicode Character Range
Description: This field is used to specify the Unicode blocks or ranges encompassed by the font file in the 'cmap' subtable for platform 3, encoding ID 1 (Microsoft platform). If the bit is set (1) then the Unicode range is considered functional. If the bit is clear (0) then the range is not considered functional. Each of the bits is treated as an independent flag and the bits can be set in any combination. The determination of "functional" is left up to the font designer, although character set selection should attempt to be functional by ranges if at all possible.

All reserved fields must be zero. Each long is in Big-Endian form. See the Basic Multilingual Plane of ISO/IEC 10646-1 or the Unicode Standard v.3.0 for the list of Unicode ranges and characters.

Bit Description
0 Basic Latin
1 Latin-1 Supplement
2 Latin Extended-A
3 Latin Extended-B
4 IPA Extensions
5 Spacing Modifier Letters
6 Combining Diacritical Marks
7 Greek and Coptic
8 Reserved for Unicode SubRanges
9 Cyrillic
  Cyrillic Supplementary
10 Armenian
11 Hebrew
12 Reserved for Unicode SubRanges
13 Arabic
14 Reserved for Unicode SubRanges
15 Devanagari
16 Bengali
17 Gurmukhi
18 Gujarati
19 Oriya
20 Tamil
21 Telugu
22 Kannada
23 Malayalam
24 Thai
25 Lao
26 Georgian
27 Reserved for Unicode SubRanges
28 Hangul Jamo
29 Latin Extended Additional
30 Greek Extended
31 General Punctuation
32 Superscripts And Subscripts
33 Currency Symbols
34 Combining Diacritical Marks For Symbols
35 Letterlike Symbols
36 Number Forms
37 Arrows
  Supplemental Arrows-A
  Supplemental Arrows-B
38 Mathematical Operators
  Supplemental Mathematical Operators
  Miscellaneous Mathematical Symbols-A
  Miscellaneous Mathematical Symbols-B
39 Miscellaneous Technical
40 Control Pictures
41 Optical Character Recognition
42 Enclosed Alphanumerics
43 Box Drawing
44 Block Elements
45 Geometric Shapes
46 Miscellaneous Symbols
47 Dingbats
48 CJK Symbols And Punctuation
49 Hiragana
50 Katakana
  Katakana Phonetic Extensions
51 Bopomofo
  Bopomofo Extended
52 Hangul Compatibility Jamo
53 Reserved for Unicode SubRanges
54 Enclosed CJK Letters And Months
55 CJK Compatibility
56 Hangul Syllables
57 Non-Plane 0 *
58 Reserved for Unicode SubRanges
59 CJK Unified Ideographs
  CJK Radicals Supplement
  Kangxi Radicals
  Ideographic Description Characters
  CJK Unified Ideograph Extension A
  CJK Unified Ideographs Extension B
  Kanbun
60 Private Use Area
61 CJK Compatibility Ideographs
  CJK Compatibility Ideographs Supplement
62 Alphabetic Presentation Forms
63 Arabic Presentation Forms-A
64 Combining Half Marks
65 CJK Compatibility Forms
66 Small Form Variants
67 Arabic Presentation Forms-B
68 Halfwidth And Fullwidth Forms
69 Specials
70 Tibetan
71 Syriac
72 Thaana
73 Sinhala
74 Myanmar
75 Ethiopic
76 Cherokee
77 Unified Canadian Aboriginal Syllabics
78 Ogham
79 Runic
80 Khmer
81 Mongolian
82 Braille Patterns
83 Yi Syllables
  Yi Radicals
84 Tagalog
  Hanunoo
  Buhid
  Tagbanwa
85 Old Italic
86 Gothic
87 Deseret
88 Byzantine Musical Symbols
  Musical Symbols
89 Mathematical Alphanumeric Symbols
90 Private Use (plane 15)
  Private Use (plane 16)
91 Variation Selectors
92 Tags
93-127 Reserved for Unicode SubRanges
Note: * Setting bit 57 implies that there is at least one codepoint beyond the Basic Multilingual Plane that is supported by this font.

achVendID

Format: 4-byte character array
Title: Font Vendor Identification
Description: The four character identifier for the vendor of the given type face.
Comments: This is not the royalty owner of the original artwork. This is the company responsible for the marketing and distribution of the typeface that is being classified. It is reasonable to assume that there will be 6 vendors of ITC Zapf Dingbats for use on desktop platforms in the near future (if not already). It is also likely that the vendors will have other inherent benefits in their fonts (more kern pairs, unregularized data, hand hinted, etc.). This identifier will allow for the correct vendor’s type to be used over another, possibly inferior, font file. The Vendor ID value is not required.

Microsoft has assigned values for some font suppliers as listed below. Uppercase vendor ID’s are reserved by Microsoft. Other suppliers can choose their own mixed case or lowercase ID’s, or leave the field blank.

For a list of registered Vendor id’s see our Registered ‘vendors’ links page.

fsSelection

Format: 2-byte bit field.
Title: Font selection flags.
Description: Contains information concerning the nature of the font patterns, as follows:

Bit # macStyle bit C definition Description
0 bit 1 ITALIC Font contains Italic characters, otherwise they are upright.
1   UNDERSCORE Characters are underscored.
2   NEGATIVE Characters have their foreground and background reversed.
3   OUTLINED Outline (hollow) characters, otherwise they are solid.
4   STRIKEOUT Characters are overstruck.
5 bit 0 BOLD Characters are emboldened.
6   REGULAR Characters are in the standard weight/style for the font.
Comments: All undefined bits must be zero.

This field contains information on the original design of the font. Bits 0 & 5 can be used to determine if the font was designed with these features or whether some type of machine simulation was performed on the font to achieve this appearance. Bits 1-4 are rarely used bits that indicate the font is primarily a decorative or special purpose font.

If bit 6 is set, then bits 0 and 5 must be clear, else the behavior is undefined. As noted above, the settings of bits 0 and 1 must be reflected in the macStyle bits in the 'head' table. While bit 6 on implies that bits 0 and 1 of macStyle are clear (along with bits 0 and 5 of fsSelection), the reverse is not true. Bits 0 and 1 of macStyle (and 0 and 5 of fsSelection) may be clear and that does not give any indication of whether or not bit 6 of fsSelection is clear (e.g., Arial Light would have all bits cleared; it is not the regular version of Arial).

usFirstCharIndex

Format: 2-byte USHORT
Description: The minimum Unicode index (character code) in this font, according to the cmap subtable for platform ID 3 and platform- specific encoding ID 0 or 1. For most fonts supporting Win-ANSI or other character sets, this value would be 0x0020. This field cannot represent supplementary character values (codepoints greater than 0xFFFF). Fonts that support supplementary characters should set the value in this field to 0xFFFF if the minimum index value is a supplementary character.

usLastCharIndex

Format: 2-byte USHORT
Description: The maximum Unicode index (character code) in this font, according to the cmap subtable for platform ID 3 and encoding ID 0 or 1. This value depends on which character sets the font supports. This field cannot represent supplementary character values (codepoints greater than 0xFFFF). Fonts that support supplementary characters should set the value in this field to 0xFFFF.

sTypoAscender

Format: SHORT
Description: The typographic ascender for this font. Remember that this is not the same as the Ascender value in the 'hhea' table, which Apple defines in a far different manner. One good source for sTypoAscender in Latin based fonts is the Ascender value from an AFM file. For CJK fonts see below.

The suggested usage for sTypoAscender is that it be used in conjunction with unitsPerEm to compute a typographically correct default line spacing. The goal is to free applications from Macintosh or Windows-specific metrics which are constrained by backward compatibility requirements. These new metrics, when combined with the character design widths, will allow applications to lay out documents in a typographically correct and portable fashion. These metrics will be exposed through Windows APIs. Macintosh applications will need to access the 'sfnt' resource and parse it to extract this data from the "OS/2" table.

For CJK (Chinese, Japanese, and Korean) fonts that are intended to be used for vertical writing (in addition to horizontal writing), the required value for sTypoAscender is that which describes the top of the of the ideographic em-box. For example, if the ideographic em-box of the font extends from coordinates 0,-120 to 1000,880 (that is, a 1000x1000 box set 120 design units below the Latin baseline), then the value of sTypoAscender must be set to 880. Failing to adhere to these requirements will result in incorrect vertical layout.

Also see the Recommendations Section for more on this field.

sTypoDescender

Format: SHORT
Description: The typographic descender for this font. Remember that this is not the same as the Descender value in the 'hhea' table, which Apple defines in a far different manner. One good source for sTypoDescender in Latin based fonts is the Descender value from an AFM file. For CJK fonts see below.

The suggested usage for sTypoDescender is that it be used in conjunction with unitsPerEm to compute a typographically correct default line spacing. The goal is to free applications from Macintosh or Windows-specific metrics which are constrained by backward compatability requirements. These new metrics, when combined with the character design widths, will allow applications to lay out documents in a typographically correct and portable fashion. These metrics will be exposed through Windows APIs. Macintosh applications will need to access the 'sfnt' resource and parse it to extract this data from the "OS/2" table (unless Apple exposes the 'OS/2' table through a new API).

For CJK (Chinese, Japanese, and Korean) fonts that are intended to be used for vertical writing (in addition to horizontal writing), the required value for sTypoDescender is that which describes the bottom of the of the ideographic em-box. For example, if the ideographic em-box of the font extends from coordinates 0,-120 to 1000,880 (that is, a 1000x1000 box set 120 design units below the Latin baseline), then the value of sTypoDescender must be set to -120. Failing to adhere to these requirements will result in incorrect vertical layout.

Also see the Recommendations Section for more on this field.

sTypoLineGap

Format: 2-byte SHORT
Description: The typographic line gap for this font. Remember that this is not the same as the LineGap value in the 'hhea' table, which Apple defines in a far different manner.

The suggested usage for usTypoLineGap is that it be used in conjunction with unitsPerEm to compute a typographically correct default line spacing. Typical values average 7-10% of units per em. The goal is to free applications from Macintosh or Windows-specific metrics which are constrained by backward compatability requirements (see chapter, "Recommendations for OpenType Fonts"). These new metrics, when combined with the character design widths, will allow applications to lay out documents in a typographically correct and portable fashion. These metrics will be exposed through Windows APIs. Macintosh applications will need to access the 'sfnt' resource and parse it to extract this data from the "OS/2" table (unless Apple exposes the 'OS/2' table through a new API)

usWinAscent

Format: 2-byte USHORT
Description: The ascender metric for Windows. This, too, is distinct from Apple’s Ascender value and from the usTypoAscender values. usWinAscent is computed as the yMax for all characters in the Windows ANSI character set. usWinAscent is used to compute the Windows font height and default line spacing. For platform 3 encoding 0 fonts, it is the same as yMax. Windows will clip the bitmap of any portion of a glyph that appears above this value. Some applications use this value to determine default line spacing. This is strongly discouraged. The typographic ascender, descender and line gap fields in conjunction with unitsPerEm should be used for this purpose. Developers should set this field keeping the above factors in mind.
If any clipping is unacceptable, then the value should be set to yMax.
However, if a developer desires to provide appropriate default line spacing using this field, for those applications that continue to use this field for doing so (against OpenType recommendations), then the value should be set appropriately. In such a case, it may result in some glyph bitmaps being clipped.

usWinDescent

Format: 2-byte USHORT
Description: The descender metric for Windows. This, too, is distinct from Apple’s Descender value and from the usTypoDescender values. usWinDescent is computed as the -yMin for all characters in the Windows ANSI character set. usWinDescent is used to compute the Windows font height and default line spacing. For platform 3 encoding 0 fonts, it is the same as -yMin. Windows will clip the bitmap of any portion of a glyph that appears below this value. Some applications use this value to determine default line spacing. This is strongly discouraged. The typographic ascender, descender and line gap fields in conjunction with unitsPerEm should be used for this purpose. Developers should set this field keeping the above factors in mind.
If any clipping is unacceptable, then the value should be set to yMin.
However, if a developer desires to provide appropriate default line spacing using this field, for those applications that continue to use this field for doing so (against OpenType recommendations), then the value should be set appropriately. In such a case, it may result in some glyph bitmaps being clipped.

ulCodePageRange1 Bits 0-31
ulCodePageRange2 Bits 32-63

Format: 32-bit unsigned long(2 copies) totaling 64 bits.
Title: Code Page Character Range
Description: This field is used to specify the code pages encompassed by the font file in the 'cmap' subtable for platform 3, encoding ID 1 (Microsoft platform). If the font file is encoding ID 0, then the Symbol Character Set bit should be set. If the bit is set (1) then the code page is considered functional. If the bit is clear (0) then the code page is not considered functional. Each of the bits is treated as an independent flag and the bits can be set in any combination. The determination of "functional" is left up to the font designer, although character set selection should attempt to be functional by code pages if at all possible.

Symbol character sets have a special meaning. If the symbol bit (31) is set, and the font file contains a 'cmap' subtable for platform of 3 and encoding ID of 1, then all of the characters in the Unicode range 0xF000 - 0xF0FF (inclusive) will be used to enumerate the symbol character set. If the bit is not set, any characters present in that range will not be enumerated as a symbol character set.

All reserved fields must be zero. Each long is in Big-Endian form.

Bit Code Page Description
0 1252 Latin 1
1 1250 Latin 2: Eastern Europe
2 1251 Cyrillic
3 1253 Greek
4 1254 Turkish
5 1255 Hebrew
6 1256 Arabic
7 1257 Windows Baltic
8 1258 Vietnamese
9-15   Reserved for Alternate ANSI
16 874 Thai
17 932 JIS/Japan
18 936 Chinese: Simplified chars--PRC and Singapore
19 949 Korean Wansung
20 950 Chinese: Traditional chars--Taiwan and Hong Kong
21 1361 Korean Johab
22-28   Reserved for Alternate ANSI & OEM
29   Macintosh Character Set (US Roman)
30   OEM Character Set
31   Symbol Character Set
32-47   Reserved for OEM
48 869 IBM Greek
49 866 MS-DOS Russian
50 865 MS-DOS Nordic
51 864 Arabic
52 863 MS-DOS Canadian French
53 862 Hebrew
54 861 MS-DOS Icelandic
55 860 MS-DOS Portuguese
56 857 IBM Turkish
57 855 IBM Cyrillic; primarily Russian
58 852 Latin 2
59 775 MS-DOS Baltic
60 737 Greek; former 437 G
61 708 Arabic; ASMO 708
62 850 WE/Latin 1
63 437 US

sxHeight

Format: SHORT
Description: This metric specifies the distance between the baseline and the approximate height of non-ascending lowercase letters measured in FUnits. This value would normally be specified by a type designer but in situations where that is not possible, for example when a legacy font is being converted, the value may be set equal to the top of the unscaled and unhinted glyph bounding box of the glyph encoded at U+0078 (LATIN SMALL LETTER X). If no glyph is encoded in this position the field should be set to 0.

This metric, if specified, can be used in font substitution: the xHeight value of one font can be scaled to approximate the apparent size of another.

sCapHeight

Format: SHORT
Description: This metric specifies the distance between the baseline and the approximate height of uppercase letters measured in FUnits. This value would normally be specified by a type designer but in situations where that is not possible, for example when a legacy font is being converted, the value may be set equal to the top of the unscaled and unhinted glyph bounding box of the glyph encoded at U+0048 (LATIN CAPITAL LETTER H). If no glyph is encoded in this position the field should be set to 0.

This metric, if specified, can be used in systems that specify type size by capital height measured in millimeters. It can also be used as an alignment metric; the top of a drop capital, for instance, can be aligned to the sCapHeight metric of the first line of text.

usDefaultChar

Format: USHORT
Description: Whenever a request is made for a character that is not in the font, Windows provides this default character. If the value of this field is zero, glyph ID 0 is to be used for the default character otherwise this is the Unicode encoding of the glyph that Windows uses as the default character. This field cannot represent supplementary character values (codepoints greater than 0xFFFF).

usBreakChar

Format: USHORT
Description: This is the Unicode encoding of the glyph that Windows uses as the break character. The break character is used to separate words and justify text. Most fonts specify ‘space’ as the break character. This field cannot represent supplementary character values (codepoints greater than 0xFFFF).

usMaxContext

Format: USHORT
Description: The maximum length of a target glyph context for any feature in this font. For example, a font which has only a pair kerning feature should set this field to 2. If the font also has a ligature feature in which the glyph sequence ‘f f i’ is substituted by the ligature ‘ffi’, then this field should be set to 3. This field could be useful to sophisticated line-breaking engines in determining how far they should look ahead to test whether something could change that effects the line breaking. For chaining contextual lookups, the length of the string (covered glyph) + (input sequence) + (lookahead sequence) should be considered.