TTFFontHeight

TTFFontHeight is a metric used in font rendering to describe the vertical size of a TrueType font as it is laid out on a device. It is primarily used to determine line spacing when rendering multi-line text and is not a fixed property of the font file itself.

TTFFontHeight is a derived value that depends on both the font’s intrinsic metrics and the rendering context. TrueType fonts encode several key metrics, including unitsPerEm, ascent, descent, and lineGap. When these metrics are mapped from font units to device units (pixels or points) at a given point size and DPI, they define the line height that a rendering engine will apply. In practice, TTFFontHeight is often computed from the sum of ascent and descent plus any leading or line gap, scaled to the current display resolution and size.

Because TTFFontHeight depends on the chosen size and pixel density, it can vary between fonts and rendering environments even for the same nominal point size. It is therefore distinct from other related metrics such as the em height (the font’s design grid, typically unitsPerEm) or per-glyph bounding box heights. Some platforms expose a comparable value (for example, tmHeight in Windows via GetTextMetrics), but the exact interpretation can differ across systems and font engines.

Usage and practical notes: TTFFontHeight is primarily used to calculate line spacing and vertical layout when rendering text. Developers should obtain height through the rendering API in use, as it accounts for the specific font, size, and device context. See also TrueType, font metrics, unitsPerEm, ascent, descent, lineGap.