@x_advance: how much the line advances after drawing this glyph when setting text in horizontal direction. @y_advance: how much the line advances after drawing this glyph when setting text in vertical direction. @x_offset: how much the glyph moves on the X-axis before drawing it, this should not affect how much the line advances. @y_offset: how much the glyph moves on the Y-axis before drawing it, this should not affect how much the line advances.
The #hb_glyph_position_t is the structure that holds the positions of the glyph in both horizontal and vertical directions. All positions in #hb_glyph_position_t are relative to the current point.