Cairo backends

Cairo backends

The Cairo library supports various backends. In this section of the Cairo graphics tutorial, we will use Cairo to create a PNG image, PDF file, SVG file and we will draw on a GTK window.

PNG image

In the first example, we will create a PNG image.

#include <cairo.h>

int main(void)
{
  cairo_surface_t *surface;
  cairo_t *cr;

  surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 390, 60);
  cr = cairo_create(surface);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Disziplin ist Macht.");

  cairo_surface_write_to_png(surface, "image.png");

  cairo_destroy(cr);
  cairo_surface_destroy(surface);

  return 0;
}

This example is a small console application, that will create a PNG image.

#include <cairo.h>

In this header file, we will find declarations of our functions and constants.

cairo_surface_t *surface;
cairo_t *cr;

Here we declare a surface and a Cairo context.

surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 390, 60);
cr = cairo_create(surface);

We create a surface and a Cairo context. The surface is an 390x60 px image.

cairo_set_source_rgb(cr, 0, 0, 0);

We will draw in black ink.

cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
    CAIRO_FONT_WEIGHT_NORMAL);
cairo_set_font_size(cr, 40.0);

We choose a font type and set its size.

cairo_move_to(cr, 10.0, 50.0);
cairo_show_text(cr, "Disziplin ist Macht.");

We move to a (10.0, 50.0) position within the image and draw the text.

cairo_surface_write_to_png(surface, "image.png");

This function call creates the PNG image.

cairo_destroy(cr);
cairo_surface_destroy(surface);

In the end, we clean the resources.

PDF file

In the second example, we will use the Cairo library to create a simple PDF file.

#include <cairo.h>
#include <cairo-pdf.h>

int main(void) 
{
  cairo_surface_t *surface;
  cairo_t *cr;

  surface = cairo_pdf_surface_create("pdffile.pdf", 504, 648);
  cr = cairo_create(surface);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face (cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size (cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Disziplin ist Macht.");

  cairo_show_page(cr);

  cairo_surface_destroy(surface);
  cairo_destroy(cr);

  return 0;
}

We must open the pdf file in a pdf viewer. Linux users can use KPDF or Evince viewers.

surface = cairo_pdf_surface_create("pdffile.pdf", 504, 648);

To render a pdf file, we must create a pdf surface using the cairo_pdf_surface_create() function call. The size of the pdf file is specified in points, which is a standard in typesetting.

cairo_show_page(cr);

The cairo_show_page() finishes rendering of the pdf file.

Figure: PDF file in Evince

SVG file

The next example creates a simple SVG (Scalable Vector Graphics) file. The SVG is one of the hottest technologies these days.

#include <cairo.h>
#include <cairo-svg.h> 
 
int main(void) 
{
  cairo_surface_t *surface;
  cairo_t *cr;

  surface = cairo_svg_surface_create("svgfile.svg", 390, 60);
  cr = cairo_create(surface);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Disziplin ist Macht.");

  cairo_surface_destroy(surface);
  cairo_destroy(cr);

  return 0;
}

We can use Firefox, Opera or Inkscape programs to open the svgfile.svg file.

surface = cairo_svg_surface_create("svgfile.svg", 390, 60);

To create a SVG file in Cairo, we must create a svg surface using the cairo_svg_surface_create() function call.

cr = cairo_create(surface);

A Cairo context is created from a SVG surface.
The Rest of the code is identical to the previous examples.

SVG file in Chrome

GTK Window

In the last example, we will draw on the GTK window. This backend will be used throughout the rest of the tutorial.

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

static void do_drawing(cairo_t *);

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr, 
    gpointer user_data)
{      
  cr = gdk_cairo_create(gtk_widget_get_window(widget));
  do_drawing(cr);
  cairo_destroy(cr);

  return FALSE;
}

static void do_drawing(cairo_t *cr)
{
  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Disziplin ist Macht.");    
}


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

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  darea = gtk_drawing_area_new();
  gtk_container_add(GTK_CONTAINER(window), darea);

  g_signal_connect(G_OBJECT(darea), "draw", 
      G_CALLBACK(on_draw_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), 400, 90); 
  gtk_window_set_title(GTK_WINDOW(window), "GTK window");

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

The example pops up a centered GTK window, on which we draw the "Disziplin ist Macht" text.

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

We include the necessary Cairo and GTK headers.

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr, 
    gpointer user_data)
{      
  cr = gdk_cairo_create(gtk_widget_get_window(widget));
  do_drawing(cr);
  cairo_destroy(cr);

  return FALSE;
}

In the on_draw_event() function, we create a Cairo context. It is a graphics object on which we perform the drawing operations. Actual drawing is delegated to the do_drawing() function. After the drawing, the cairo context is destroyed.

static void do_drawing(cairo_t *cr)
{
  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Disziplin ist Macht.");    
}

The drawing is done in the do_drawing() function.

darea = gtk_drawing_area_new();
gtk_container_add(GTK_CONTAINER(window), darea);

We create a GtkDrawingArea widget and add it to the container window. It is used for custom drawing.

g_signal_connect(G_OBJECT(darea), "draw", 
    G_CALLBACK(on_draw_event), NULL); 

When the GtkDrawingArea widget needs to be redrawn, it emits the draw signal. We connect that signal to the on_draw_event() callback.

Figure: GTK window

In this chapter we have covered supported Cairo backends.