
@c %start of fragment

@node Cairo Rendering
@chapter Cairo Rendering
Rendering with the Cairo backend

@section Overview
The @uref{http://cairographics.org,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 @code{<pango-context>} created with
@code{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 @code{pango-cairo-update-context}.
The convenience functions @code{pango-cairo-create-layout} and
@code{pango-cairo-update-layout} handle the common case where the program
doesn't need to manipulate the properties of the @code{<pango-context>}.

When you get the metrics of a layout or of a piece of a layout using functions
such as @code{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 @samp{PANGO_SCALE}.

@example 

#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;
@}
@end example

@section Usage
@include defuns-pangocairo.xml.texi

@c %end of fragment
