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.