Que signifient hheaAscender, hheaDescender, hheaLineGap, et quel est leur impact ?

typoAscender

Integer Ascender value. Corresponds to the OpenType OS/2 table sTypoAscender field. The typographic ascender for this font. Remember that this is not the same asthe 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 applicationsfrom Macintosh or Windows-specific metrics which are constrained by backward compatibility requirements. These new metrics, when combined with the character design widths, willallow applications to lay out documents in a typographically correct and portable fashion. These metrics will be exposed through Windows APIs. Macintosh applications will need toaccess 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 thatwhich 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 1000 × 1000 boxset 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 verticallayout.’

typoDescender

integer Descender value. Corresponds to the OpenType OS/2 table sTypoDescender field. ‘The typographic descender for this font. Remember that this is not the same asthe 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 AFMfile. 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 designwidths, will allow applications to lay out documents in a typographically correct and portable fashion. These metrics will be exposed through Windows APIs. Macintosh applicationswill 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 thatwhich 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 1000 × 1000box 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 verticallayout.’

typoLineGap

Integer Line gap value. Corresponds to the OpenType OS/2 table sTypoLineGap field. ‘The typographic line gap for this font. Remember that this is not the same as theLineGap 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. These newmetrics, when combined with the character design widths, will allow applications to lay out documents in a typographically correct and portable fashion.’

hheaAscender

Integer Ascender value. Corresponds to the OpenType hhea table Ascender field. ‘Typographic ascent (distance from baseline of highest ascender).’

hheaDescender

Integer Descender value. Corresponds to the OpenType hhea table Descender field. ‘Typographic descent (distance from baseline of lowest descender).’

hheaLineGap

Integer Line gap value. Corresponds to the OpenType hhea table LineGap field. ‘Typographic line gap. Negative LineGap values are treated as zero in Windows 3.1,

winAscent

non-negative integer Ascender value. Corresponds to the OpenType OS/2 table usWinAscent field.

winDescent

non-negative integer Descender value. Corresponds to the OpenType OS/2 table usWinDescent field.