Cairo Rendering

Cairo Rendering — Rendering with the Cairo backend

Types and Values

Object Hierarchy

    GInterface
    ├── PangoCairoFont
    ╰── PangoCairoFontMap

Prerequisites

PangoCairoFont requires PangoFont.

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

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#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;
}




Functions

pango_cairo_font_map_get_default ()

PangoFontMap *
pango_cairo_font_map_get_default (void);

Gets a default PangoCairoFontMap to use with Cairo.

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.

The default Cairo fontmap can be changed by using pango_cairo_font_map_set_default(). This can be used to change the Cairo font backend that the default fontmap uses for example.

Note that since Pango 1.32.6, the default fontmap is per-thread. Each thread gets its own default fontmap. In this way, PangoCairo can be used safely from multiple threads.

Returns

the default PangoCairo fontmap for the current thread. This object is owned by Pango and must not be freed.

[transfer none]

Since: 1.10


pango_cairo_font_map_set_default ()

void
pango_cairo_font_map_set_default (PangoCairoFontMap *fontmap);

Sets a default PangoCairoFontMap to use with Cairo.

This can be used to change the Cairo font backend that the default fontmap uses for example. The old default font map is unreffed and the new font map referenced.

Note that since Pango 1.32.6, the default fontmap is per-thread. This function only changes the default fontmap for the current thread. Default fontmaps of exisiting threads are not changed. Default fontmaps of any new threads will still be created using pango_cairo_font_map_new().

A value of NULL for fontmap will cause the current default font map to be released and a new default font map to be created on demand, using pango_cairo_font_map_new().

Parameters

fontmap

The new default font map, or NULL.

[nullable]

Since: 1.22


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.

You can override the type of backend returned by using an environment variable PANGOCAIRO_BACKEND. Supported types, based on your build, are fc (fontconfig), win32, and coretext. If requested type is not available, NULL is returned. Ie. this is only useful for testing, when at least two backends are compiled in.

Returns

the newly allocated PangoFontMap, which should be freed with g_object_unref().

[transfer full]

Since: 1.10


pango_cairo_font_map_new_for_font_type ()

PangoFontMap *
pango_cairo_font_map_new_for_font_type
                               (cairo_font_type_t fonttype);

Creates a new PangoCairoFontMap object of the type suitable to be used with cairo font backend of type fonttype .

In most cases one should simply use pango_cairo_font_map_new() , or in fact in most of those cases, just use pango_cairo_font_map_get_default() .

Parameters

fonttype

desired cairo_font_type_t

 

Returns

the newly allocated PangoFontMap of suitable type which should be freed with g_object_unref(), or NULL if the requested cairo font backend is not supported / compiled in.

[transfer full][nullable]

Since: 1.18


pango_cairo_font_map_get_font_type ()

cairo_font_type_t
pango_cairo_font_map_get_font_type (PangoCairoFontMap *fontmap);

Gets the type of Cairo font backend that fontmap uses.

Parameters

fontmap

a PangoCairoFontMap

 

Returns

the cairo_font_type_t cairo font backend type

Since: 1.18


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).

Parameters

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()

Parameters

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);

pango_cairo_font_map_create_context has been deprecated since version 1.22 and should not be used in newly-written code.

Use pango_font_map_create_context() instead.

Create a PangoContext for the given fontmap.

[skip]

Parameters

fontmap

a PangoCairoFontMap

 

Returns

the newly created context; free with g_object_unref().

Since: 1.10


pango_cairo_font_get_scaled_font ()

cairo_scaled_font_t *
pango_cairo_font_get_scaled_font (PangoCairoFont *font);

Gets the cairo_scaled_font_t used by font . The scaled font can be referenced and kept using cairo_scaled_font_reference().

Parameters

font

a PangoFont from a PangoCairoFontMap

 

Returns

the cairo_scaled_font_t used by font , or NULL if font is NULL.

[nullable]

Since: 1.18


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).

Parameters

context

a PangoContext, from a pangocairo font map

 

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()

Parameters

context

a PangoContext, from a pangocairo font map

 

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.

Parameters

context

a PangoContext, from a pangocairo font map

 

options

a cairo_font_options_t, or NULL to unset any previously set options. A copy is made.

[nullable]

Since: 1.10


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_context_set_font_options(). This function does not report options that are derived from the target surface by pango_cairo_update_context()

Parameters

context

a PangoContext, from a pangocairo font map

 

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.

[nullable]

Since: 1.10


PangoCairoShapeRendererFunc ()

void
(*PangoCairoShapeRendererFunc) (cairo_t *cr,
                                PangoAttrShape *attr,
                                gboolean do_path,
                                gpointer data);

Function type for rendering attributes of type PANGO_ATTR_SHAPE with Pango's Cairo renderer.

Parameters

cr

a Cairo context with current point set to where the shape should be rendered

 

attr

the PANGO_ATTR_SHAPE to render

 

do_path

whether only the shape path should be appended to current path of cr and no filling/stroking done. This will be set to TRUE when called from pango_cairo_layout_path() and pango_cairo_layout_line_path() rendering functions.

 

data

user data passed to pango_cairo_context_set_shape_renderer()

 

pango_cairo_context_set_shape_renderer ()

void
pango_cairo_context_set_shape_renderer
                               (PangoContext *context,
                                PangoCairoShapeRendererFunc func,
                                gpointer data,
                                GDestroyNotify dnotify);

Sets callback function for context to use for rendering attributes of type PANGO_ATTR_SHAPE. See PangoCairoShapeRendererFunc for details.

Parameters

context

a PangoContext, from a pangocairo font map

 

func

Callback function for rendering attributes of type PANGO_ATTR_SHAPE, or NULL to disable shape rendering.

[nullable]

data

User data that will be passed to func .

 

dnotify

Callback that will be called when the context is freed to release data , or NULL.

 

Since: 1.18


pango_cairo_context_get_shape_renderer ()

PangoCairoShapeRendererFunc
pango_cairo_context_get_shape_renderer
                               (PangoContext *context,
                                gpointer *data);

Sets callback function for context to use for rendering attributes of type PANGO_ATTR_SHAPE. See PangoCairoShapeRendererFunc for details.

Retrieves callback function and associated user data for rendering attributes of type PANGO_ATTR_SHAPE as set by pango_cairo_context_set_shape_renderer(), if any.

[skip]

Parameters

context

a PangoContext, from a pangocairo font map

 

data

Pointer to gpointer to return user data

 

Returns

the shape rendering callback previously set on the context, or NULL if no shape rendering callback have been set.

[nullable]

Since: 1.18


pango_cairo_create_context ()

PangoContext *
pango_cairo_create_context (cairo_t *cr);

Creates a context object set up to match the current transformation and target surface of the Cairo context. This context can then be used to create a layout using pango_layout_new().

This function is a convenience function that creates a context using the default font map, then updates it to cr . If you just need to create a layout for use with cr and do not need to access PangoContext directly, you can use pango_cairo_create_layout() instead.

Parameters

cr

a Cairo context

 

Returns

the newly created PangoContext. Free with g_object_unref().

[transfer full]

Since: 1.22


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.

Parameters

cr

a Cairo context

 

context

a PangoContext, from a pangocairo font map

 

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.

Parameters

cr

a Cairo context

 

Returns

the newly created PangoLayout. Free with g_object_unref().

[transfer full]

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.

Parameters

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.

Parameters

cr

a Cairo context

 

font

a PangoFont from a PangoCairoFontMap

 

glyphs

a PangoGlyphString

 

Since: 1.10


pango_cairo_show_glyph_item ()

void
pango_cairo_show_glyph_item (cairo_t *cr,
                             const char *text,
                             PangoGlyphItem *glyph_item);

Draws the glyphs in glyph_item in the specified cairo context, embedding the text associated with the glyphs in the output if the output format supports it (PDF for example), otherwise it acts similar to pango_cairo_show_glyph_string().

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

Note that text is the start of the text for layout, which is then indexed by glyph_item->item->offset .

Parameters

cr

a Cairo context

 

text

the UTF-8 text that glyph_item refers to

 

glyph_item

a PangoGlyphItem

 

Since: 1.22


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.

Parameters

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

Parameters

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)

Parameters

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.

Parameters

cr

a Cairo context

 

font

a PangoFont from a PangoCairoFontMap

 

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.

Parameters

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.

Parameters

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)

Parameters

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

Types and Values

PangoCairoFont

typedef struct _PangoCairoFont PangoCairoFont;

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

Since: 1.18


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