Cairo Rendering
Rendering with the Cairo backend
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.
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;
}
Output of <xref linkend="rotated-example"/>
@Returns:
@fontmap:
@Returns:
@fonttype:
@Returns:
@fontmap:
@Returns:
@fontmap:
@dpi:
@fontmap:
@Returns:
@fontmap:
@Returns:
@font:
@Returns:
@context:
@dpi:
@context:
@Returns:
@context:
@options:
@context:
@Returns:
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()
@context:
@func:
@data:
@dnotify:
@context:
@data:
@Returns:
@cr:
@Returns:
@cr:
@context:
@cr:
@Returns:
@cr:
@layout:
@cr:
@font:
@glyphs:
@cr:
@text:
@glyph_item:
@cr:
@line:
@cr:
@layout:
@cr:
@x:
@y:
@width:
@height:
@cr:
@font:
@glyphs:
@cr:
@line:
@cr:
@layout:
@cr:
@x:
@y:
@width:
@height: