Learning_GTK4_tree/docs/reference/gdk/pango.md

Title: Pango Interaction

Pango is the text layout system used by GDK and GTK. The functions and types in this section are used to obtain clip regions for PangoLayouts, and to get PangoContexts that can be used with GDK.

Using Pango in GDK

Creating a PangoLayout object is the first step in rendering text, and requires getting a handle to a PangoContext. For GTK programs, you’ll usually want to use [method@Gtk.Widget.get_pango_context], or [method@Gtk.Widget.create_pango_layout]. Once you have a PangoLayout, you can set the text and attributes of it with Pango functions like [method@Pango.Layout.set_text] and get its size with [method@Pango.Layout.get_size].

Note: 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 [func@PangoCairo.show_layout]; you can also draw pieces of the layout with [func@PangoCairo.show_layout_line].

Draw transformed text with Pango and cairo

#define RADIUS 100
#define N_WORDS 10
#define FONT "Sans Bold 18"

PangoContext *context;
PangoLayout *layout;
PangoFontDescription *desc;

double radius;
int width, height;
int i;

// 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

width = gdk_surface_get_width (surface);
height = gdk_surface_get_height (surface);
radius = MIN (width, height) / 2.;

cairo_translate (cr,
                 radius + (width - 2 * radius) / 2,
                 radius + (height - 2 * radius) / 2);
                 cairo_scale (cr, radius / RADIUS, radius / RADIUS);

// Create a PangoLayout, set the font and text
context = gdk_pango_context_get_for_display (display);
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++)
  {
    double red, green, blue;
    double angle = 2 * G_PI * i / n_words;

    cairo_save (cr);

    // Gradient from red at angle == 60 to blue at angle == 300
    red = (1 + cos (angle - 60)) / 2;
    green = 0;
    blue = 1 - red;

    cairo_set_source_rgb (cr, red, green, blue);
    cairo_rotate (cr, angle);

    // Inform Pango to re-layout the text with the new transformation matrix
    pango_cairo_update_layout (cr, layout);

    pango_layout_get_size (layout, &width, &height);

    cairo_move_to (cr, - width / 2 / PANGO_SCALE, - DEFAULT_TEXT_RADIUS);
    pango_cairo_show_layout (cr, layout);

    cairo_restore (cr);
  }

g_object_unref (layout);
g_object_unref (context);

The example code above will yield the following result: