26 Pango Interaction

Using Pango in GDK

26.1 Overview

Pango is the text layout system used by GDK and GTK+. The functions and types in this section are used to render Pango objects to GDK. drawables, and also extend the set of Pango attributes to include stippling and embossing.

Creating a <pango-layout> object is the first step in rendering text, and requires getting a handle to a <pango-context>. For GTK+ programs, you'll usually want to use gtk-widget-get-pango-context, or gtk-widget-create-pango-layout, rather than using the lowlevel gdk-pango-context-get-for-screen. Once you have a <pango-layout>, you can set the text and attributes of it with Pango functions like pango-layout-set-text and get its size with pango-layout-get-size. (Note that Pango uses a fixed point system internally, so converting between Pango units and pixels using PANGO_SCALE or the pango-pixels macro.)

Rendering a Pango layout is done most simply with gdk-draw-layout; you can also draw pieces of the layout with gdk-draw-layout or gdk-draw-glyphs. <gdk-pango-renderer> is a subclass of <pango-renderer> that is used internally to implement these functions. Using it directly or subclassing it can be useful in some cases. See the <gdk-pango-renderer> documentation for details.

     
     #define RADIUS 100
     #define N_WORDS 10
     #define FONT "Sans Bold 18"
     
     GdkScreen *screen = gdk_drawable_get_screen (drawable);
     PangoRenderer *renderer;
     GdkGC *gc;
     
     PangoMatrix matrix = PANGO_MATRIX_INIT;
     PangoContext *context;
     PangoLayout *layout;
     PangoFontDescription *desc;
     
     double device_radius;
     int width, height;
     int i;
     
     /* Get the default renderer for the screen, and set it up for drawing  */
     renderer = gdk_pango_renderer_get_default (screen);
     gdk_pango_renderer_set_drawable (GDK_PANGO_RENDERER (renderer), drawable);
     
     gc = gdk_gc_new (drawable);
     gdk_pango_renderer_set_gc (GDK_PANGO_RENDERER (renderer), gc);
     
     /* Set up a transformation matrix so that the user space coordinates for
      * where we are drawing are [-RADIUS, RADIUS], [-RADIUS, RADIUS]
      * We first center, then change the scale */
     gdk_drawable_get_size (drawable, &width, &height);
     device_radius = MIN (width, height) / 2.;
     
     pango_matrix_translate (&matrix,
                             device_radius + (width - 2 * device_radius) / 2,
                             device_radius + (height - 2 * device_radius) / 2);
     pango_matrix_scale (&matrix, device_radius / RADIUS, device_radius / RADIUS);
     
     /* Create a PangoLayout, set the font and text */
     context = gdk_pango_context_get_for_screen (screen);
     layout = pango_layout_new (context);
     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++)
       {
         GdkColor color;
         PangoMatrix rotated_matrix = matrix;
         int width, height;
         double angle = (360. * i) / N_WORDS;
     
         /* Gradient from red at angle == 60 to blue at angle == 300 */
         color.red   = 65535 * (1 + cos ((angle - 60) * M_PI / 180.)) / 2;
         color.green = 0;
         color.blue  = 65535  - color.red;
     
         gdk_pango_renderer_set_override_color (GDK_PANGO_RENDERER (renderer),
                                                PANGO_RENDER_PART_FOREGROUND, &color);
     
         pango_matrix_rotate (&rotated_matrix, angle);
     
         pango_context_set_matrix (context, &rotated_matrix);
     
         /* Inform Pango to re-layout the text with the new transformation matrix */
         pango_layout_context_changed (layout);
     
         pango_layout_get_size (layout, &width, &height);
         pango_renderer_draw_layout (renderer, layout,
                                     - width / 2, - RADIUS * PANGO_SCALE);
       }
     
     /* Clean up default renderer, since it is shared */
     gdk_pango_renderer_set_override_color (GDK_PANGO_RENDERER (renderer),
                                            PANGO_RENDER_PART_FOREGROUND, NULL);
     gdk_pango_renderer_set_drawable (GDK_PANGO_RENDERER (renderer), NULL);
     gdk_pango_renderer_set_gc (GDK_PANGO_RENDERER (renderer), NULL);
     
     /* free the objects we created */
     g_object_unref (layout);
     g_object_unref (context);
     g_object_unref (gc);

26.2 Usage