## Vertical Text

Vertical Text — Laying text out in vertical directions

## Synopsis

    enum                PangoGravity;
    enum                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);

## 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

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

Since 1.16

### 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

## 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()`