The Cairo graphics tutorial -------Text in Cairo

In this part of the Cairo graphics tutorial, we will work with text.

Soulmate

In the first example, we will display some lyrics on the GTK+ window.

#include <cairo.h>
#include <gtk/gtk.h>


static gboolean
on_expose_event(GtkWidget *widget,
    GdkEventExpose *event,
    gpointer data)
{
  cairo_t *cr;

  cr = gdk_cairo_create(widget->window);

  cairo_set_source_rgb(cr, 0.1, 0.1, 0.1); 

  cairo_select_font_face(cr, "Purisa",
      CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_BOLD);

  cairo_set_font_size(cr, 13);

  cairo_move_to(cr, 20, 30);
  cairo_show_text(cr, "Most relationships seem so transitory");  
  cairo_move_to(cr, 20, 60);
  cairo_show_text(cr, "They're all good but not the permanent one");

  cairo_move_to(cr, 20, 120);
  cairo_show_text(cr, "Who doesn't long for someone to hold");

  cairo_move_to(cr, 20, 150);
  cairo_show_text(cr, "Who knows how to love you without being told");
  cairo_move_to(cr, 20, 180);
  cairo_show_text(cr, "Somebody tell me why I'm on my own");
  cairo_move_to(cr, 20, 210);
  cairo_show_text(cr, "If there's a soulmate for everyone");

  cairo_destroy(cr);

  return FALSE;
}



int main (int argc, char *argv[])
{
  GtkWidget *window;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(window, "expose-event",
      G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(window, "destroy",
      G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 420, 250); 
  gtk_window_set_title(GTK_WINDOW(window), "Soulmate");
  gtk_widget_set_app_paintable(window, TRUE);

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

In this example, we display part of the lyrics from the Natasha Bedingfields Soulmate song.

 cairo_select_font_face(cr, "Purisa",
    CAIRO_FONT_SLANT_NORMAL,
    CAIRO_FONT_WEIGHT_BOLD);

Here we select the font face. The function takes three parameters, the font family, font slant and the font weight.

 cairo_set_font_size(cr, 13);

Here we specify the font size.

 cairo_move_to(cr, 20, 30);
 cairo_show_text(cr, "Most relationships seem so transitory"); 

We display the text on the window by specifying the position of the text and calling the cairo_show_text() function.

Figure: Soulmate

Letter by letter

In this effect, we will display a text letter by letter. The letters will be drawn with some delay.

#include <cairo.h>
#include <gtk/gtk.h>

gpointer text[7] = { "Z", "e", "t", "C", "o", "d", "e" };
gboolean timer = TRUE;


static gboolean
on_expose_event(GtkWidget *widget,
    GdkEventExpose *event,
    gpointer data)
{
  cairo_t *cr;
  cairo_text_extents_t extents;
  static gint count = 0;

  cr = gdk_cairo_create(widget->window);

  cairo_select_font_face(cr, "Courier",
      CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_BOLD);

  cairo_set_font_size(cr, 35);	
  cairo_set_source_rgb(cr, 0.2, 0.2, 0.2); 

  gint i;
  gint x = 0;

  for (i = 0; i < count; i++) {
      cairo_text_extents(cr, text[i], &extents);
      x += extents.width + 2;
      cairo_move_to(cr, x + 30, 50);
      cairo_show_text(cr, text[i]);  
  }

  count++;

  if (count == 8) {
      timer = FALSE;
      count = 0;
  }

  cairo_destroy(cr);

  return FALSE;
}

static gboolean
time_handler (GtkWidget *widget)
{
  if (widget->window == NULL) return FALSE;

  if (!timer) return FALSE;

  gtk_widget_queue_draw(widget);
  return TRUE;
}


int main (int argc, char *argv[])
{
  GtkWidget *window;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(window, "expose-event",
      G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(window, "destroy",
      G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 300, 90); 
  gtk_window_set_title(GTK_WINDOW(window), "ZetCode");
  gtk_widget_set_app_paintable(window, TRUE);

  g_timeout_add(1000, (GSourceFunc) time_handler, (gpointer) window);

  gtk_widget_show_all(window);


  gtk_main();

  return 0;
}

In our example, we will draw the "ZetCode" string onto the GTK+ window letter by letter with some delay.

 gpointer text[7] = { "Z", "e", "t", "C", "o", "d", "e" };

We create an array of strings.

 cairo_select_font_face(cr, "Courier",
    CAIRO_FONT_SLANT_NORMAL,
    CAIRO_FONT_WEIGHT_BOLD);

We select a Courier font face.

 for (i = 0; i < count; i++) {
    cairo_text_extents(cr, text[i], &extents);
    x += extents.width + 2;
    cairo_move_to(cr, x + 30, 50);
    cairo_show_text(cr, text[i]);  
 }

Here we draw the text letter by letter. The extents.width gives us the width of the current letter.

Figure: ZetCode

Puff

In the following example, we create a puff effect. The example will display a growing centered text, that will gradully fade out from some point. This is a very common effect, which you can often see in flash animations.

#include <cairo.h>
#include <gtk/gtk.h>


gpointer text[7] = { "Z", "e", "t", "C", "o", "d", "e" };
gboolean timer = TRUE;


static gboolean
on_expose_event(GtkWidget *widget,
    GdkEventExpose *event,
    gpointer data)
{
  cairo_t *cr;
  cairo_text_extents_t extents;

  static gdouble alpha = 1.0;
  static gdouble size = 1;


  gint x = widget->allocation.width / 2;
  gint y = widget->allocation.height / 2;

  cr = gdk_cairo_create(widget->window);

  cairo_set_source_rgb(cr, 0.5, 0, 0); 
  cairo_paint(cr);

  cairo_select_font_face(cr, "Courier",
      CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_BOLD);

  size += 0.8;

  if (size > 20) {
      alpha -= 0.01;
  }

  cairo_set_font_size(cr, size);

  cairo_set_source_rgb(cr, 1, 1, 1); 

  cairo_text_extents(cr, "ZetCode", &extents);
  cairo_move_to(cr, x - extents.width/2, y);
  cairo_text_path(cr, "ZetCode");
  cairo_clip(cr);
  cairo_stroke(cr);
  cairo_paint_with_alpha(cr, alpha);

  if (alpha <= 0) {
      timer = FALSE;
  }

  cairo_destroy(cr);

  return FALSE;
}

static gboolean
time_handler (GtkWidget *widget)
{
  if (widget->window == NULL) return FALSE;

  if (!timer) return FALSE;

  gtk_widget_queue_draw(widget);

  return TRUE;
}


int main (int argc, char *argv[])
{
  GtkWidget *window;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(window, "expose-event",
      G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(window, "destroy",
      G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 350, 200); 
  gtk_window_set_title(GTK_WINDOW(window), "puff");
  gtk_widget_set_app_paintable(window, TRUE);

  g_timeout_add(14, (GSourceFunc) time_handler, (gpointer) window);

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

The example creates a growing and fading text on the GTK+ window.

 gint x = widget->allocation.width / 2;
 gint y = widget->allocation.height / 2;

Coordinates of the middle point.

 cairo_set_source_rgb(cr, 0.5, 0, 0); 
 cairo_paint(cr);

We set the background color to dark red color.

 size += 0.8;

Each cycle, the font size will grow by 0.8 units.

 if (size > 20) {
     alpha -= 0.01;
 }

The fading out begins after the font size is bigger than 20.

 cairo_text_extents(cr, "ZetCode", &extents);

We get the text metrics.

 cairo_move_to(cr, x - extents.width/2, y);

We use the text metrics to center the text on the window.

 cairo_text_path(cr, "ZetCode");
 cairo_clip(cr);

We get the path of the text and set the current clip region to it.

 cairo_stroke(cr);
 cairo_paint_with_alpha(cr, alpha);

We paint the current path with and take alpha value into account.

Figure: Puff

---

This chapter covered text in Cairo.