PangoCairo – 1.0: Rendering with Cairo
Description: Reference for PangoCairo-1.0: Rendering with Cairo

# Rendering with Cairo

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>

    #define RADIUS 150
    #define N_WORDS 10
    #define FONT "Sans Bold 27"

    static void
    draw_text (cairo_t *cr)
    {
      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;
    }

Once you build and run the example code above, you should see the following result: