TTFFontLineSkip

TTFFontLineSkip is a term used in some text rendering systems to denote the vertical spacing applied between lines when drawing text from TrueType fonts. In practice, it represents the line height or leading that separates the baselines of successive lines, helping to maintain legibility and a consistent vertical rhythm across different fonts and sizes.

TrueType fonts carry vertical metrics that influence TTFFontLineSkip. Key metrics include ascent, descent, and line gap (often represented as sTypoLineGap or the lineGap in the hhea/OS/2 tables). The line skip used by a renderer is typically derived from these metrics, commonly approximated as ascent minus descent plus the line gap, after converting font units to device units. Different frameworks may apply additional adjustments for display scaling, DPI, or user-specified line-height settings.

Usage and impact: TTFFontLineSkip is a fundamental parameter in text layout engines, affecting how much vertical space is left between lines. It can be computed automatically from the font’s metrics or overridden by explicit line-height settings in an application or stylesheet. Properly calibrated line skip helps prevent glyphs from overlapping between lines and contributes to a readable typographic hierarchy.

Compatibility and considerations: The exact value and interpretation of TTFFontLineSkip are not strictly standardized and can vary by platform, rendering engine, and font. Because font metrics differ between typefaces and can change with font versions, applications may need to adjust or clamp the value to maintain consistent layout across fonts.

See also: line height, leading, font metrics, ascent, descent, line gap, TrueType font, HINT: Typographic metrics, OS/2 and hhea tables.