Cairo Rendering

## Cairo Rendering Cairo Rendering — Rendering with the Cairo backend |

## Synopsis

                PangoCairoFontMap;
    PangoFontMap* pango_cairo_font_map_new      (void);
    PangoFontMap* pango_cairo_font_map_get_default
                                                (void);
    void        pango_cairo_font_map_set_resolution
                                                (PangoCairoFontMap *fontmap,
                                                 double dpi);
    double      pango_cairo_font_map_get_resolution
                                                (PangoCairoFontMap *fontmap);
    PangoContext* pango_cairo_font_map_create_context
                                                (PangoCairoFontMap *fontmap);
    void        pango_cairo_context_set_resolution
                                                (PangoContext *context,
                                                 double dpi);
    double      pango_cairo_context_get_resolution
                                                (PangoContext *context);
    void        pango_cairo_context_set_font_options
                                                (PangoContext *context,
                                                 const cairo_font_options_t *options);
    const cairo_font_options_t* pango_cairo_context_get_font_options
                                                (PangoContext *context);
    void        pango_cairo_update_context      (cairo_t *cr,
                                                 PangoContext *context);
    PangoLayout* pango_cairo_create_layout      (cairo_t *cr);
    void        pango_cairo_update_layout       (cairo_t *cr,
                                                 PangoLayout *layout);
    void        pango_cairo_show_glyph_string   (cairo_t *cr,
                                                 PangoFont *font,
                                                 PangoGlyphString *glyphs);
    void        pango_cairo_show_layout_line    (cairo_t *cr,
                                                 PangoLayoutLine *line);
    void        pango_cairo_show_layout         (cairo_t *cr,
                                                 PangoLayout *layout);
    void        pango_cairo_show_error_underline
                                                (cairo_t *cr,
                                                 double x,
                                                 double y,
                                                 double width,
                                                 double height);
    void        pango_cairo_glyph_string_path   (cairo_t *cr,
                                                 PangoFont *font,
                                                 PangoGlyphString *glyphs);
    void        pango_cairo_layout_line_path    (cairo_t *cr,
                                                 PangoLayoutLine *line);
    void        pango_cairo_layout_path         (cairo_t *cr,
                                                 PangoLayout *layout);
    void        pango_cairo_error_underline_path
                                                (cairo_t *cr,
                                                 double x,
                                                 double y,
                                                 double width,
                                                 double height);

## Object Hierarchy

      GInterface
       +----PangoCairoFontMap

## Prerequisites

PangoCairoFontMap requires PangoFontMap.

## Description

The Cairo library is a vector graphics library with a powerful rendering model. It has such features as anti-aliased primitives, alpha-compositing, and gradients. Multiple backends for Cairo are available, to allow rendering to images, to PDF files, and to the screen on X and on other windowing systems. The functions in this section allow using Pango to render to Cairo surfaces.

Using Pango with Cairo is straightforward. A PangoContext created with `pango_cairo_font_map_create_context()` can be used on any Cairo context (cairo_t), but needs to be updated to match the current transformation matrix and target surface of the Cairo context using `pango_cairo_update_context()`. The convenience functions `pango_cairo_create_layout()` and `pango_cairo_update_layout()` handle the common case where the program doesn't need to manipulate the properties of the PangoContext.

When you get the metrics of a layout or of a piece of a layout using functions such as `pango_layout_get_extents()`, the reported metrics are in user-space coordinates. If a piece of text is 10 units long, and you call cairo_scale (cr, 2.0), it still is more-or-less 10 units long. However, the results will be affected by hinting (that is, the process of adjusting the text to look good on the pixel grid), so you shouldn't assume they are completely independent of the current transformation matrix. Note that the basic metrics functions in Pango report results in integer Pango units. To get to the floating point units used in Cairo divide by `PANGO_SCALE`.

Example 1. Using Pango with Cairo

    #include <math.h>
    #include <pango/pangocairo.h>

    static void
    draw_text (cairo_t *cr)
    {
    #define RADIUS 150
    #define N_WORDS 10
    #define FONT "Sans Bold 27"

      PangoLayout *layout;
      PangoFontDescription *desc;
      int i;

      /* Center coordinates on the middle of the region we are drawing
       */
      cairo_translate (cr, RADIUS, RADIUS);

      /* Create a PangoLayout, set the font and text */
      layout = pango_cairo_create_layout (cr);

      pango_layout_set_text (layout, "Text", -1);
      desc = pango_font_description_from_string (FONT);
      pango_layout_set_font_description (layout, desc);
      pango_font_description_free (desc);

      /* Draw the layout N_WORDS times in a circle */
      for (i = 0; i < N_WORDS; i++)
        {
          int width, height;
          double angle = (360. * i) / N_WORDS;
          double red;

          cairo_save (cr);

          /* Gradient from red at angle == 60 to blue at angle == 240 */
          red   = (1 + cos ((angle - 60) * G_PI / 180.)) / 2;
          cairo_set_source_rgb (cr, red, 0, 1.0 - red);

          cairo_rotate (cr, angle * G_PI / 180.);

          /* Inform Pango to re-layout the text with the new transformation */
          pango_cairo_update_layout (cr, layout);

          pango_layout_get_size (layout, &width, &height);
          cairo_move_to (cr, - ((double)width / PANGO_SCALE) / 2, - RADIUS);
          pango_cairo_show_layout (cr, layout);

          cairo_restore (cr);
        }

      /* free the layout object */
      g_object_unref (layout);
    }

    int main (int argc, char **argv)
    {
      cairo_t *cr;
      char *filename;
      cairo_status_t status;
      cairo_surface_t *surface;

      if (argc != 2)
        {
          g_printerr ("Usage: cairosimple OUTPUT_FILENAME\n");
          return 1;
        }

      filename = argv[1];

      surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32,
    					2 * RADIUS, 2 * RADIUS);
      cr = cairo_create (surface);

      cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
      cairo_paint (cr);
      draw_text (cr);
      cairo_destroy (cr);

      status = cairo_surface_write_to_png (surface, filename);
      cairo_surface_destroy (surface);

      if (status != CAIRO_STATUS_SUCCESS)
        {
          g_printerr ("Could not save png to 's'\n", filename);
          return 1;
        }

      return 0;
    }

Figure 2. Output of Example 1, “Using Pango with Cairo”

## Details

### PangoCairoFontMap

    typedef struct _PangoCairoFontMap PangoCairoFontMap;

PangoCairoFontMap is an interface exported by font maps for use with Cairo. The actual type of the font map will depend on the particular font technology Cairo was compiled to use.

Since 1.10

### pango_cairo_font_map_new ()

    PangoFontMap* pango_cairo_font_map_new      (void);

Creates a new PangoCairoFontMap object; a fontmap is used to cache information about available fonts, and holds certain global parameters such as the resolution. In most cases, you can use `pango_cairo_font_map_get_default()` instead.

Note that the type of the returned object will depend on the particular font backend Cairo was compiled to use; You generally should only use the PangoFontMap and PangoCairoFontMap interfaces on the returned object.

Returns : | the newly allocated PangoFontMap, which should be freed with `g_object_unref()`.

Since 1.10

### pango_cairo_font_map_get_default ()

    PangoFontMap* pango_cairo_font_map_get_default
                                                (void);

Gets a default font map to use with Cairo.

Returns : | the default Cairo fontmap for Pango. This object is owned by Pango and must not be freed.

Since 1.10

### pango_cairo_font_map_set_resolution ()

    void        pango_cairo_font_map_set_resolution
                                                (PangoCairoFontMap *fontmap,
                                                 double dpi);

Sets the resolution for the fontmap. This is a scale factor between points specified in a PangoFontDescription and Cairo units. The default value is 96, meaning that a 10 point font will be 13 units high. (10 * 96. / 72. = 13.3).

`fontmap` : | a PangoCairoFontMap
`dpi` : | the resolution in "dots per inch". (Physical inches aren't actually involved; the terminology is conventional.)

Since 1.10

### pango_cairo_font_map_get_resolution ()

    double      pango_cairo_font_map_get_resolution
                                                (PangoCairoFontMap *fontmap);

Gets the resolution for the fontmap. See `pango_cairo_font_map_set_resolution()`

`fontmap` : | a PangoCairoFontMap
Returns : | the resolution in "dots per inch"

Since 1.10

### pango_cairo_font_map_create_context ()

    PangoContext* pango_cairo_font_map_create_context
                                                (PangoCairoFontMap *fontmap);

Create a PangoContext for the given fontmap.

`fontmap` : | a PangoCairoFontMap
Returns : | the newly created context; free with `g_object_unref()`.

Since 1.10

### pango_cairo_context_set_resolution ()

    void        pango_cairo_context_set_resolution
                                                (PangoContext *context,
                                                 double dpi);

Sets the resolution for the context. This is a scale factor between points specified in a PangoFontDescription and Cairo units. The default value is 96, meaning that a 10 point font will be 13 units high. (10 * 96. / 72. = 13.3).

`context` : | a PangoContext, from `pango_cairo_font_map_create_context()`
`dpi` : | the resolution in "dots per inch". (Physical inches aren't actually involved; the terminology is conventional.) A 0 or negative value means to use the resolution from the font map.

Since 1.10

### pango_cairo_context_get_resolution ()

    double      pango_cairo_context_get_resolution
                                                (PangoContext *context);

Gets the resolution for the context. See `pango_cairo_context_set_resolution()`

`context` : | a PangoContext, from `pango_cairo_font_map_create_context()`
Returns : | the resolution in "dots per inch". A negative value will be returned if no resolution has previously been set.

Since 1.10

### pango_cairo_context_set_font_options ()

    void        pango_cairo_context_set_font_options
                                                (PangoContext *context,
                                                 const cairo_font_options_t *options);

Sets the font options used when rendering text with this context. These options override any options that `pango_cairo_update_context()` derives from the target surface.

`context` : | a PangoContext, from `pango_cairo_font_map_create_context()`
`options` : | a cairo_font_options_t, or `NULL` to unset any previously set options. A copy is made.

### pango_cairo_context_get_font_options ()

    const cairo_font_options_t* pango_cairo_context_get_font_options
                                                (PangoContext *context);

Retrieves any font rendering options previously set with `pango_cairo_font_map_set_font_options()`. This functions not report options that are derived from the target surface by `pango_cairo_update_context()`

`context` : | a PangoContext, from `pango_cairo_font_map_create_context()`
Returns : | the font options previously set on the context, or `NULL` if no options have been set. This value is owned by the context and must not be modified or freed.

### pango_cairo_update_context ()

    void        pango_cairo_update_context      (cairo_t *cr,
                                                 PangoContext *context);

Updates a PangoContext previously created for use with Cairo to match the current transformation and target surface of a Cairo context. If any layouts have been created for the context, it's necessary to call `pango_layout_context_changed()` on those layouts.

`cr` : | a Cairo context
`context` : | a PangoContext, from `pango_cairo_font_map_create_context()`

Since 1.10

### pango_cairo_create_layout ()

    PangoLayout* pango_cairo_create_layout      (cairo_t *cr);

Creates a layout object set up to match the current transformation and target surface of the Cairo context. This layout can then be used for text measurement with functions like `pango_layout_get_size()` or drawing with functions like `pango_cairo_show_layout()`. If you change the transformation or target surface for `cr`, you need to call `pango_cairo_update_layout()`

This function is the most convenient way to use Cairo with Pango, however it is slightly inefficient since it creates a separate PangoContext object for each layout. This might matter in an application that was laying out large amounts of text.

`cr` : | a Cairo context
Returns : | the newly created PangoLayout. Free with `g_object_unref()`.

Since 1.10

### pango_cairo_update_layout ()

    void        pango_cairo_update_layout       (cairo_t *cr,
                                                 PangoLayout *layout);

Updates the private PangoContext of a PangoLayout created with `pango_cairo_create_layout()` to match the current transformation and target surface of a Cairo context.

`cr` : | a Cairo context
`layout` : | a PangoLayout, from `pango_cairo_create_layout()`

Since 1.10

### pango_cairo_show_glyph_string ()

    void        pango_cairo_show_glyph_string   (cairo_t *cr,
                                                 PangoFont *font,
                                                 PangoGlyphString *glyphs);

Draws the glyphs in `glyphs` in the specified cairo context. The origin of the glyphs (the left edge of the baseline) will be drawn at the current point of the cairo context.

`cr` : | a Cairo context
`font` : | a PangoFont
`glyphs` : | a PangoGlyphString

Since 1.10

### pango_cairo_show_layout_line ()

    void        pango_cairo_show_layout_line    (cairo_t *cr,
                                                 PangoLayoutLine *line);

Draws a PangoLayoutLine in the specified cairo context. The origin of the glyphs (the left edge of the line) will be drawn at the current point of the cairo context.

`cr` : | a Cairo context
`line` : | a PangoLayoutLine

Since 1.10

### pango_cairo_show_layout ()

    void        pango_cairo_show_layout         (cairo_t *cr,
                                                 PangoLayout *layout);

Draws a PangoLayoutLine in the specified cairo context. The top-left corner of the PangoLayout will be drawn at the current point of the cairo context.

`cr` : | a Cairo context
`layout` : | a Pango layout

Since 1.10

### pango_cairo_show_error_underline ()

    void        pango_cairo_show_error_underline
                                                (cairo_t *cr,
                                                 double x,
                                                 double y,
                                                 double width,
                                                 double height);

Draw a squiggly line in the specified cairo context that approximately covers the given rectangle in the style of an underline used to indicate a spelling error. (The width of the underline is rounded to an integer number of up/down segments and the resulting rectangle is centered in the original rectangle)

`cr` : | a Cairo context
`x` : | The X coordinate of one corner of the rectangle
`y` : | The Y coordinate of one corner of the rectangle
`width` : | Non-negative width of the rectangle
`height` : | Non-negative height of the rectangle

Since 1.14

### pango_cairo_glyph_string_path ()

    void        pango_cairo_glyph_string_path   (cairo_t *cr,
                                                 PangoFont *font,
                                                 PangoGlyphString *glyphs);

Adds the glyphs in `glyphs` to the current path in the specified cairo context. The origin of the glyphs (the left edge of the baseline) will be at the current point of the cairo context.

`cr` : | a Cairo context
`font` : | a PangoFont
`glyphs` : | a PangoGlyphString

Since 1.10

### pango_cairo_layout_line_path ()

    void        pango_cairo_layout_line_path    (cairo_t *cr,
                                                 PangoLayoutLine *line);

Adds the text in PangoLayoutLine to the current path in the specified cairo context. The origin of the glyphs (the left edge of the line) will be at the current point of the cairo context.

`cr` : | a Cairo context
`line` : | a PangoLayoutLine

Since 1.10

### pango_cairo_layout_path ()

    void        pango_cairo_layout_path         (cairo_t *cr,
                                                 PangoLayout *layout);

Adds the text in a PangoLayout to the current path in the specified cairo context. The top-left corner of the PangoLayout will be at the current point of the cairo context.

`cr` : | a Cairo context
`layout` : | a Pango layout

Since 1.10

### pango_cairo_error_underline_path ()

    void        pango_cairo_error_underline_path
                                                (cairo_t *cr,
                                                 double x,
                                                 double y,
                                                 double width,
                                                 double height);

Add a squiggly line to the current path in the specified cairo context that approximately covers the given rectangle in the style of an underline used to indicate a spelling error. (The width of the underline is rounded to an integer number of up/down segments and the resulting rectangle is centered in the original rectangle)

`cr` : | a Cairo context
`x` : | The X coordinate of one corner of the rectangle
`y` : | The Y coordinate of one corner of the rectangle
`width` : | Non-negative width of the rectangle
`height` : | Non-negative height of the rectangle

Since 1.14