 Pen position

In glyph positioning, the pen position is the logical point from which the next glyph is placed. The word pen describes a layout convention: the renderer can draw an outline or a collection of pixels rather than trace a physical pen stroke. Simple horizontal layout places this reference point on the text baseline.^[ref]

Let ${\mathbf{p}}_i$ be the current pen position, ${\mathbf{o}}_i$ a placement offset and ${\mathbf{a}}_i$ the resolved glyph advance. The current glyph's drawing origin and the next logical point are, respectively,For example, in one horizontal coordinate system, ${\mathbf{p}}_i=(10,20)$, ${\mathbf{o}}_i=(-1,2)$ and ${\mathbf{a}}_i=(8,0)$ draw at $(9,22)$ but leave the next logical point at $(18,20)$. Moving the current shape and changing the progression of the run are separate operations.

A glyph's bounding box is therefore not a substitute for its pen position. A left side bearing can leave space between the logical origin and the ink, and a right side bearing records how the ink ends relative to the default advance. Kerning or attachment of a combining character can modify the placement or progression without changing the stored outline.