Cairo Rendering

Cairo Rendering — Rendering with the Cairo backend

Synopsis




                    PangoCairoFont;
                    PangoCairoFontMap;
PangoFontMap*       pango_cairo_font_map_get_default    (void);
PangoFontMap*       pango_cairo_font_map_new            (void);
PangoFontMap*       pango_cairo_font_map_new_for_font_type
                                                        (cairo_font_type_t fonttype);
cairo_font_type_t   pango_cairo_font_map_get_font_type  (PangoCairoFontMap *fontmap);
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);
cairo_scaled_font_t* pango_cairo_font_get_scaled_font   (PangoCairoFont *font);
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                (*PangoCairoShapeRendererFunc)      (cairo_t *cr,
                                                         PangoAttrShape *attr,
                                                         gboolean do_path,
                                                         gpointer data);
void                pango_cairo_context_set_shape_renderer
                                                        (PangoContext *context,
                                                         PangoCairoShapeRendererFunc func,
                                                         gpointer data,
                                                         GDestroyNotify dnotify);
PangoCairoShapeRendererFunc pango_cairo_context_get_shape_renderer
                                                        (PangoContext *context,
                                                         gpointer *data);
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;
}


Details

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


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.

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_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_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().

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.

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.

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).

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_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().

font :

a PangoFont from a PangoCairoFontMap

Returns :

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

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).

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.

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_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.

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.

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.

context :

a PangoContext, from pango_cairo_font_map_create_context()

func :

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

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.

context :

a PangoContext, from pango_cairo_font_map_create_context()

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.

Since 1.18


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 from a PangoCairoFontMap

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 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.

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