OS/2 - OS/2 and Windows Metrics (Version 3) (OpenType 1.5)
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: |
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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.
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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.
|
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/
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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.
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| 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:
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| 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.
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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. |
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OpenType specification