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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().
#define PANGO_GRAVITY_IS_IMPROPER(gravity)
Whether a PangoGravity represents a gravity that results in reversal of text direction.
Since: 1.32
#define PANGO_GRAVITY_IS_VERTICAL(gravity)
Whether a PangoGravity represents vertical writing directions.
Since: 1.16
PangoGravity
pango_gravity_get_for_matrix (const PangoMatrix *matrix);
Finds the gravity that best matches the rotation component in a PangoMatrix.
the gravity of matrix
, which will never be
PANGO_GRAVITY_AUTO, or PANGO_GRAVITY_SOUTH if matrix
is NULL
Since: 1.16
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 |
Since: 1.16
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 upright, 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 |
|
|
base_gravity |
base gravity of the paragraph |
|
hint |
orientation hint |
Since: 1.26
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).
Since: 1.16
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
Since: 1.16
The PangoGravityHint defines how horizontal scripts should behave in a vertical context. That is, English excerpt in a vertical paragraph for example.
See PangoGravity.
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scripts will take their natural gravity based on the base gravity and the script. This is the default. |
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always use the base gravity set, regardless of the script. |
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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_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()