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Vertical Text

Vertical Text — Laying text out in vertical directions

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

enum PangoGravity

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

The PangoGravity type represents the orientation of glyphs in a segment of text. This is useful when rendering vertical text layouts. In those situations, the layout is rotated using a non-identity PangoMatrix, and then glyph orientation is controlled using PangoGravity. Not every value in this enumeration makes sense for every usage of PangoGravity; for example, PANGO_GRAVITY_AUTO only can be passed to pango_context_set_base_gravity() and can only be returned by pango_context_get_base_gravity().

See also: PangoGravityHint

PANGO_GRAVITY_SOUTH

Glyphs stand upright (default)

PANGO_GRAVITY_EAST

Glyphs are rotated 90 degrees clockwise

PANGO_GRAVITY_NORTH

Glyphs are upside-down

PANGO_GRAVITY_WEST

Glyphs are rotated 90 degrees counter-clockwise

PANGO_GRAVITY_AUTO

Gravity is resolved from the context matrix

Since 1.16


enum PangoGravityHint

typedef enum {
  PANGO_GRAVITY_HINT_NATURAL,
  PANGO_GRAVITY_HINT_STRONG,
  PANGO_GRAVITY_HINT_LINE
} PangoGravityHint;

The PangoGravityHint defines how horizontal scripts should behave in a vertical context. That is, English excerpt in a vertical paragraph for example.

See PangoGravity.

PANGO_GRAVITY_HINT_NATURAL

scripts will take their natural gravity based on the base gravity and the script. This is the default.

PANGO_GRAVITY_HINT_STRONG

always use the base gravity set, regardless of the script.

PANGO_GRAVITY_HINT_LINE

for scripts not in their natural direction (eg. Latin in East gravity), choose per-script gravity such that every script respects the line progression. This means, Latin and Arabic will take opposite gravities and both flow top-to-bottom for example.

Since 1.16


PANGO_GRAVITY_IS_VERTICAL()

#define             PANGO_GRAVITY_IS_VERTICAL(gravity)

Whether a PangoGravity represents vertical writing directions.

gravity :

the PangoGravity to check

Returns :

TRUE if gravity is PANGO_GRAVITY_EAST or PANGO_GRAVITY_WEST, FALSE otherwise.

Since 1.16


pango_gravity_get_for_matrix ()

PangoGravity        pango_gravity_get_for_matrix        (const PangoMatrix *matrix);

Finds the gravity that best matches the rotation component in a PangoMatrix.

matrix :

a PangoMatrix

Returns :

the gravity of matrix, which will never be PANGO_GRAVITY_AUTO, or PANGO_GRAVITY_SOUTH if matrix is NULL

Since 1.16


pango_gravity_get_for_script ()

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

Based on the script, base gravity, and hint, returns actual gravity to use in laying out a single PangoItem.

If base_gravity is PANGO_GRAVITY_AUTO, it is first replaced with the preferred gravity of script. To get the preferred gravity of a script, pass PANGO_GRAVITY_AUTO and PANGO_GRAVITY_HINT_STRONG in.

script :

PangoScript to query

base_gravity :

base gravity of the paragraph

hint :

orientation hint

Returns :

resolved gravity suitable to use for a run of text with script.

Since 1.16


pango_gravity_get_for_script_and_width ()

PangoGravity        pango_gravity_get_for_script_and_width
                                                        (PangoScript script,
                                                         gboolean wide,
                                                         PangoGravity base_gravity,
                                                         PangoGravityHint hint);

Based on the script, East Asian width, base gravity, and hint, returns actual gravity to use in laying out a single character or PangoItem.

This function is similar to pango_gravity_get_for_script() except that this function makes a distinction between narrow/half-width and wide/full-width characters also. Wide/full-width characters always stand <emph>upright</emph>, that is, they always take the base gravity, whereas narrow/full-width characters are always rotated in vertical context.

If base_gravity is PANGO_GRAVITY_AUTO, it is first replaced with the preferred gravity of script.

script :

PangoScript to query

wide :

TRUE for wide characters as returned by g_unichar_iswide()

base_gravity :

base gravity of the paragraph

hint :

orientation hint

Returns :

resolved gravity suitable to use for a run of text with script and wide.

Since 1.26


pango_gravity_to_rotation ()

double              pango_gravity_to_rotation           (PangoGravity gravity);

Converts a PangoGravity value to its natural rotation in radians. gravity should not be PANGO_GRAVITY_AUTO.

Note that pango_matrix_rotate() takes angle in degrees, not radians. So, to call pango_matrix_rotate() with the output of this function you should multiply it by (180. / G_PI).

gravity :

gravity to query

Returns :

the rotation value corresponding to gravity.

Since 1.16

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