 Font ascent

The font ascent is an upward metric measured from a text baseline and used when allocating vertical space for a font. It describes the font's layout metrics as a whole, rather than the visible ascender of one particular glyph. A short letter, a tall letter and an accented form can therefore share the same font ascent while having different ink bounding boxes.^[ref]

Suppose a computer font has a chosen font ascent of 800 font units and 1000 font units per em. At 20 pixels per em, the scaled upward metric is 16 pixels. This calculation allocates a reference extent; it does not assert that the letters being drawn actually reach 16 pixels above the text baseline.

The field used must be stated. OpenType provides ascender in hhea and sTypoAscender in OS/2, while some older layout paths use other metrics. The sTypoAscender, sTypoDescender and sTypoLineGap fields are intended to work together for typographic line spacing. Their ascent-minus-descent need not equal unitsPerEm.^[ref]

Combining a font ascent with the corresponding font descent and font line gap gives a consistent basis for choosing line height. Mixing fields chosen under different conventions can instead change the spacing even when the nominal font size is unchanged.