Description

Since 1.16, Pango is able to correctly lay vertical text out. In fact, it can set layouts of mixed vertical and non-vertical text. This section describes the types used for setting vertical text parameters.

The way this is implemented is through the concept of gravity. Gravity of normal Latin text is south. A gravity value of east means that glyphs will be rotated ninety degrees counterclockwise. So, to render vertical text one needs to set the gravity and rotate the layout using the matrix machinery already in place. This has the huge advantage that most algorithms working on a PangoLayout do not need any change as the assumption that lines run in the X direction and stack in the Y direction holds even for vertical text layouts.

Applications should only need to set base gravity on PangoContext in use, and let Pango decide the gravity assigned to each run of text. This automatically handles text with mixed scripts. A very common use is to set the context base gravity to auto using pango_context_set_base_gravity() and rotate the layout normally. Pango will make sure that Asian languages take the right form, while other scripts are rotated normally.

The correct way to set gravity on a layout is to set it on the context associated with it using pango_context_set_base_gravity(). The context of a layout can be accessed using pango_layout_get_context(). The currently set base gravity of the context can be accessed using pango_context_get_base_gravity() and the resolved gravity of it using pango_context_get_gravity(). The resolved gravity is the same as the base gravity for the most part, except that if the base gravity is set to PANGO_GRAVITY_AUTO, the resolved gravity will depend on the current matrix set on context, and is derived using pango_gravity_get_for_matrix().

The next thing an application may want to set on the context is the gravity hint. A PangoGravityHint instructs how different scripts should react to the set base gravity.

Font descriptions have a gravity property too, that can be set using pango_font_description_set_gravity() and accessed using pango_font_description_get_gravity(). However, those are rarely useful from application code and are mainly used by PangoLayout internally.

Last but not least, one can create PangoAttributes for gravity and gravity hint using pango_attr_gravity_new() and pango_attr_gravity_hint_new().

Details

typedef enum {
  PANGO_GRAVITY_SOUTH,
  PANGO_GRAVITY_EAST,
  PANGO_GRAVITY_NORTH,
  PANGO_GRAVITY_WEST,
  PANGO_GRAVITY_AUTO
} PangoGravity;

typedef enum {
  PANGO_GRAVITY_HINT_NATURAL,
  PANGO_GRAVITY_HINT_STRONG,
  PANGO_GRAVITY_HINT_LINE
} PangoGravityHint;

#define             PANGO_GRAVITY_IS_VERTICAL(gravity)

PangoGravity        pango_gravity_get_for_matrix        (const PangoMatrix *matrix);

PangoGravity        pango_gravity_get_for_script        (PangoScript script,
                                                         PangoGravity base_gravity,
                                                         PangoGravityHint hint);

double              pango_gravity_to_rotation           (PangoGravity gravity);

See Also

pango_context_get_base_gravity(), pango_context_set_base_gravity(), pango_context_get_gravity(), pango_context_get_gravity_hint(), pango_context_set_gravity_hint(), pango_font_description_set_gravity(), pango_font_description_get_gravity(), pango_attr_gravity_new(), pango_attr_gravity_hint_new()