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The Cairo graphics tutorial -------Shapes and fills in Cairo

In this part of the Cairo tutorial, we will create some basic and more advanced shapes. We will fill them withsolid colors, patterns and gradients.

Basic shapes

The Cairo API has several functions to create some basic shapes.

#include <cairo.h>
#include <gtk/gtk.h>
#include <math.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, 0, 0);
  cairo_set_line_width(cr, 1);

  cairo_rectangle(cr, 20, 20, 120, 80);
  cairo_rectangle(cr, 180, 20, 80, 80);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_arc(cr, 330, 60, 40, 0, 2*M_PI);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_arc(cr, 90, 160, 40, M_PI/4, M_PI);
  cairo_close_path(cr);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_translate(cr, 220, 180);
  cairo_scale(cr, 1, 0.7);
  cairo_arc(cr, 0, 0, 50, 0, 2*M_PI);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_destroy(cr);

  return FALSE;
}


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(darea, "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), 390, 240); 

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

In this example, we will create a rectangle, a square, a circle, an arc and an ellipse.

cairo_rectangle(cr, 20, 20, 120, 80);
 cairo_rectangle(cr, 180, 20, 80, 80);

The

cairo_rectangle()

is used to create both squares and rectangles. A square is just a specific type of a rectangle.

cairo_arc(cr, 330, 60, 40, 0, 2*M_PI);

This line creates a circle.

cairo_scale(cr, 1, 0.7);
 cairo_arc(cr, 0, 0, 50, 0, 2*M_PI);

We use the

cairo_scale()

function call to create an ellipse.

The Cairo graphics tutorial -------Shapes and fills in Cairo

Figure: Shapes

Other shapes can be created using a combination of basic primitives.

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


int points[11][2] = { 
    { 0, 85 }, 
    { 75, 75 }, 
    { 100, 10 }, 
    { 125, 75 }, 
    { 200, 85 },
    { 150, 125 }, 
    { 160, 190 },
    { 100, 150 }, 
    { 40, 190 },
    { 50, 125 },
    { 0, 85 } 
};

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, 0, 0);
  cairo_set_line_width(cr, 1);


  gint i;
  for ( i = 0; i < 10; i++ ) {
      cairo_line_to(cr, points[i][0], points[i][1]);
  }

  cairo_close_path(cr);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);


  cairo_move_to(cr, 240, 40);
  cairo_line_to(cr, 240, 160);
  cairo_line_to(cr, 350, 160);
  cairo_close_path(cr);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_move_to(cr, 380, 40);
  cairo_line_to(cr, 380, 160);
  cairo_line_to(cr, 450, 160);
  cairo_curve_to(cr, 440, 155, 380, 145, 380, 40);

  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_stroke_preserve(cr);
  cairo_set_source_rgb(cr, 1, 1, 1);
  cairo_fill(cr);

  cairo_destroy(cr);

  return FALSE;
}


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(darea, "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), 460, 240); 

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

In this example, we create a star object a triangle and a modified triangle. These objects are created using lines and one curve.

gint i;
 for ( i = 0; i < 10; i++ ) {
     cairo_line_to(cr, points[i][0], points[i][1]);
 }

 cairo_close_path(cr);

The star is drawn by joining all the points that are in the points array. The star is finished by calling the

cairo_close_path()

function, which joins the last two points of a star.

cairo_move_to(cr, 380, 40);
 cairo_line_to(cr, 380, 160);
 cairo_line_to(cr, 450, 160);
 cairo_curve_to(cr, 440, 155, 380, 145, 380, 40);

The modified rectangle is a simple combination of two lines and one curve.

The Cairo graphics tutorial -------Shapes and fills in Cairo

Figure: Shapes

Fills

Fills fill the interiors of shapes. Fills can be solid colors, patters or gradients.

Solid colors

A color is an object representing a combination of Red, Green, and Blue (RGB) intensity values. Cairo valid RGB values are in the range 0 to 1.

#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);

  int width, height;
  gtk_window_get_size(GTK_WINDOW(widget), &width, &height);  


  cairo_set_source_rgb(cr, 0.5, 0.5, 1);
  cairo_rectangle(cr, 20, 20, 100, 100);
  cairo_fill(cr);

  cairo_set_source_rgb(cr, 0.6, 0.6, 0.6);
  cairo_rectangle(cr, 150, 20, 100, 100);
  cairo_fill(cr);
 
  cairo_set_source_rgb(cr, 0, 0.3, 0);
  cairo_rectangle(cr, 20, 140, 100, 100);
  cairo_fill(cr);

  cairo_set_source_rgb(cr, 1, 0, 0.5);
  cairo_rectangle(cr, 150, 140, 100, 100);
  cairo_fill(cr);

  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(G_OBJECT(window), "expose-event",
      G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(G_OBJECT(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), 270, 260); 
  gtk_window_set_title(GTK_WINDOW(window), "colors");

  gtk_widget_set_app_paintable(window, TRUE);
  gtk_widget_show_all(window);

  gtk_main(); 

  return 0;
}

In the example we draw four colored rectangles

cairo_set_source_rgb(cr, 0.5, 0.5, 1);
 cairo_rectangle(cr, 20, 20, 100, 100);
 cairo_fill(cr);

The

cairo_set_source_rgb()

function call sets the source to an opaque color. The parameters are the Red, Green, Blue intensity values. The source is used to fill the interior of a rectangle by calling the

cairo_fill()

function.

The Cairo graphics tutorial -------Shapes and fills in Cairo

Figure: Solid Colors

Patterns

Patterns are complex graphical objects that can fill the shapes.

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

cairo_surface_t *surface1;
cairo_surface_t *surface2;
cairo_surface_t *surface3;
cairo_surface_t *surface4;

static void create_surfaces() {
  surface1 = cairo_image_surface_create_from_png("blueweb.png");
  surface2 = cairo_image_surface_create_from_png("maple.png");
  surface3 = cairo_image_surface_create_from_png("crack.png");
  surface4 = cairo_image_surface_create_from_png("chocolate.png");
}

static void destroy_surfaces() {
  g_print("destroying surfaces");
  cairo_surface_destroy(surface1);
  cairo_surface_destroy(surface2);
  cairo_surface_destroy(surface3);
  cairo_surface_destroy(surface4);
}


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

  cairo_pattern_t *pattern1;
  cairo_pattern_t *pattern2;
  cairo_pattern_t *pattern3;
  cairo_pattern_t *pattern4;

  cr = gdk_cairo_create(widget->window);

  int width, height;
  gtk_window_get_size(GTK_WINDOW(widget), &width, &height);  


  pattern1 = cairo_pattern_create_for_surface(surface1);
  pattern2 = cairo_pattern_create_for_surface(surface2);
  pattern3 = cairo_pattern_create_for_surface(surface3);
  pattern4 = cairo_pattern_create_for_surface(surface4);


  cairo_set_source(cr, pattern1);
  cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
  cairo_rectangle(cr, 20, 20, 100, 100);
  cairo_fill(cr);

  cairo_set_source(cr, pattern2); 
  cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT); 
  cairo_rectangle(cr, 150, 20, 100, 100);
  cairo_fill(cr);

  cairo_set_source(cr, pattern3);
  cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
  cairo_rectangle(cr, 20, 140, 100, 100);
  cairo_fill(cr);

  cairo_set_source(cr, pattern4);
  cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
  cairo_rectangle(cr, 150, 140, 100, 100);
  cairo_fill(cr);

  cairo_pattern_destroy(pattern1);
  cairo_pattern_destroy(pattern2);
  cairo_pattern_destroy(pattern3);
  cairo_pattern_destroy(pattern4);

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

  create_surfaces();

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 270, 260); 
  gtk_window_set_title(GTK_WINDOW(window), "patterns");

  gtk_widget_set_app_paintable(window, TRUE);
  gtk_widget_show_all(window);

  gtk_main();

  destroy_surfaces();

  return 0;
}

In this example we draw four rectangles again. This time, we fill them with some patterns. We use four pattern images from the Gimp image manipulation program. We must retain the original size of those patterns, because we are going to tile them.

We create image surfaces outside the

on_expose_event()

function. It would not be efficient to read from harddisk each time, the window needs to be redrawn.

pattern1 = cairo_pattern_create_for_surface(surface1);

We create a pattern from the surface by calling the

cairo_pattern_create_for_surface()

function.

cairo_set_source(cr, pattern1);
  cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
  cairo_rectangle(cr, 20, 20, 100, 100);
  cairo_fill(cr);

Here we draw our first rectangle. The

cairo_set_source()

tells the Cairo context to use a pattern as a source for drawing. The image patterns may not fit exactly the shape. We set the mode to

CAIRO_EXTEND_REPEAT

, which causes the pattern to be tiled by repeating. The

cairo_rectangle()

creates a rectangular path.Finally,

cairo_fill()

fills the path with the source.

The Cairo graphics tutorial -------Shapes and fills in Cairo

Figure: Patterns

Gradients

In computer graphics, gradient is a smooth blending of shades from light to dark or from one color to another. In 2D drawing programs and paint programs, gradients are used to create colorful backgrounds and special effects as well as to simulate lights and shadows. (answers.com)

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

static gboolean
on_expose_event(GtkWidget *widget,
    GdkEventExpose *event,
    gpointer data)
{
  cairo_t *cr;
  cairo_pattern_t *pat1;
  cairo_pattern_t *pat2;
  cairo_pattern_t *pat3;

  cr = gdk_cairo_create(widget->window);

  pat1 = cairo_pattern_create_linear(0.0, 0.0,  350.0, 350.0);

  gdouble j;
  gint count = 1;
  for ( j = 0.1; j < 1; j += 0.1 ) {
      if (( count % 2 ))  {
          cairo_pattern_add_color_stop_rgb(pat1, j, 0, 0, 0);
      } else { 
          cairo_pattern_add_color_stop_rgb(pat1, j, 1, 0, 0);
      }
   count++;
  }

  cairo_rectangle(cr, 20, 20, 300, 100);
  cairo_set_source(cr, pat1);
  cairo_fill(cr);


  pat2 = cairo_pattern_create_linear(0.0, 0.0,  350.0, 0.0);

  gdouble i;
  count = 1;
  for ( i = 0.05; i < 0.95; i += 0.025 ) {
      if (( count % 2 ))  {
          cairo_pattern_add_color_stop_rgb(pat2, i, 0, 0, 0);
      } else { 
          cairo_pattern_add_color_stop_rgb(pat2, i, 0, 0, 1);
      }
   count++;
  }

  cairo_rectangle(cr, 20, 140, 300, 100);
  cairo_set_source(cr, pat2);
  cairo_fill(cr);


  pat3 = cairo_pattern_create_linear(20.0, 260.0, 20.0, 360.0);

  cairo_pattern_add_color_stop_rgb(pat3, 0.1, 0, 0, 0);
  cairo_pattern_add_color_stop_rgb(pat3, 0.5, 1, 1, 0);
  cairo_pattern_add_color_stop_rgb(pat3, 0.9, 0, 0, 0);

  cairo_rectangle(cr, 20, 260, 300, 100);
  cairo_set_source(cr, pat3);
  cairo_fill(cr);

  cairo_pattern_destroy(pat1);
  cairo_pattern_destroy(pat2);
  cairo_pattern_destroy(pat3);

  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(G_OBJECT(window), "expose-event",
      G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(G_OBJECT(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), 340, 390); 
  gtk_window_set_title(GTK_WINDOW(window), "gradients");

  gtk_widget_set_app_paintable(window, TRUE);
  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

In our example, we draw three rectangles with three different gradients.

pat3 = cairo_pattern_create_linear(20.0, 260.0, 20.0, 360.0);

Here we create a linear gradient pattern. The parameters specify the line, along which we draw thegradient. In our case it is a vertical line.

cairo_pattern_add_color_stop_rgb(pat3, 0.1, 0, 0, 0);
 cairo_pattern_add_color_stop_rgb(pat3, 0.5, 1, 1, 0);
 cairo_pattern_add_color_stop_rgb(pat3, 0.9, 0, 0, 0);

We define color stops to produce our gradient pattern. In this case, the gradient is a blending of black and yellowcolors. By adding two black and one yellow stops, we create a horizontal gradient pattern.What these stops actually mean? In our case, we begin with black color, which will stop at 1/10 of the size. Then we beginto gradually paint in yellow, which will culminate at the centre of the shape. The yellow color stops at 9/10 of the size, where we begin painting in black again, until the end.

The Cairo graphics tutorial -------Shapes and fills in Cairo

Figure: Gradients

This chapter covered Cairo shapes and fills.

gtk cairo callback signal object colors image
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