Cairo 图形指南 (7) —— 合成
合成 (Compositing) 可以将一些单独的源 (source) 所生成可视元素组合成到单幅图像中去，主要用于创建所有的可视元素作为同一场景的一部分这样一种假象。合成在电影工业中被广为使用，用于创建人群、不寻常 的世界等画面，因为这样的画面在现实中的制作代价非常昂贵甚至难以实现。
Cairo 图形库提供了 14 种不同的合成操作。
#include <cairo.h>
#include <gtk/gtk.h>
static void
draw (cairo_t * cr, gint x, gint w, gint h, cairo_operator_t op)
{
cairo_t *first_cr, *second_cr;
cairo_surface_t *first, *second;
first = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
second = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
first_cr = cairo_create (first);
cairo_set_source_rgb (first_cr, 0, 0, 0.4);
cairo_rectangle (first_cr, x, 20, 50, 50);
cairo_fill (first_cr);
second_cr = cairo_create (second);
cairo_set_source_rgb (second_cr, 0.5, 0.5, 0);
cairo_rectangle (second_cr, x + 10, 40, 50, 50);
cairo_fill (second_cr);
cairo_set_operator (first_cr, op);
cairo_set_source_surface (first_cr, second, 0, 0);
cairo_paint (first_cr);
cairo_set_source_surface (cr, first, 0, 0);
cairo_paint (cr);
cairo_surface_destroy (first);
cairo_surface_destroy (second);
cairo_destroy (first_cr);
cairo_destroy (second_cr);
}
static gboolean
on_expose_event (GtkWidget * widget,
GdkEventExpose * event, gpointer data)
{
cairo_t *cr;
gint w, h;
gint x, y;
cairo_operator_t oper[] = {
CAIRO_OPERATOR_DEST_OVER,
CAIRO_OPERATOR_DEST_IN,
CAIRO_OPERATOR_OUT,
CAIRO_OPERATOR_ADD,
CAIRO_OPERATOR_ATOP,
CAIRO_OPERATOR_DEST_ATOP,
};
gtk_window_get_size (GTK_WINDOW (widget), &w, &h);
cr = gdk_cairo_create (widget->window);
gint i;
for (x = 20, y = 20, i = 0; i < 6; x += 80, i++) {
draw (cr, x, w, h, oper[i]);
}
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), 510, 120);
gtk_widget_set_app_paintable (window, TRUE);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
#include <gtk/gtk.h>
static void
draw (cairo_t * cr, gint x, gint w, gint h, cairo_operator_t op)
{
cairo_t *first_cr, *second_cr;
cairo_surface_t *first, *second;
first = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
second = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
first_cr = cairo_create (first);
cairo_set_source_rgb (first_cr, 0, 0, 0.4);
cairo_rectangle (first_cr, x, 20, 50, 50);
cairo_fill (first_cr);
second_cr = cairo_create (second);
cairo_set_source_rgb (second_cr, 0.5, 0.5, 0);
cairo_rectangle (second_cr, x + 10, 40, 50, 50);
cairo_fill (second_cr);
cairo_set_operator (first_cr, op);
cairo_set_source_surface (first_cr, second, 0, 0);
cairo_paint (first_cr);
cairo_set_source_surface (cr, first, 0, 0);
cairo_paint (cr);
cairo_surface_destroy (first);
cairo_surface_destroy (second);
cairo_destroy (first_cr);
cairo_destroy (second_cr);
}
static gboolean
on_expose_event (GtkWidget * widget,
GdkEventExpose * event, gpointer data)
{
cairo_t *cr;
gint w, h;
gint x, y;
cairo_operator_t oper[] = {
CAIRO_OPERATOR_DEST_OVER,
CAIRO_OPERATOR_DEST_IN,
CAIRO_OPERATOR_OUT,
CAIRO_OPERATOR_ADD,
CAIRO_OPERATOR_ATOP,
CAIRO_OPERATOR_DEST_ATOP,
};
gtk_window_get_size (GTK_WINDOW (widget), &w, &h);
cr = gdk_cairo_create (widget->window);
gint i;
for (x = 20, y = 20, i = 0; i < 6; x += 80, i++) {
draw (cr, x, w, h, oper[i]);
}
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), 510, 120);
gtk_widget_set_app_paintable (window, TRUE);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
示例程序中，显示了两个正方形的 6 种合成效果。
first = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
second = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
second = cairo_surface_create_similar (cairo_get_target (cr),
CAIRO_CONTENT_COLOR_ALPHA,
w, h);
先创建两个外观 (surface)。
first_cr = cairo_create (first);
cairo_set_source_rgb (first_cr, 0, 0, 0.4);
cairo_rectangle (first_cr, x, 20, 50, 50);
cairo_fill (first_cr);
cairo_set_source_rgb (first_cr, 0, 0, 0.4);
cairo_rectangle (first_cr, x, 20, 50, 50);
cairo_fill (first_cr);
对第一个外观生成的一个 cairo 环境，并在其中创建一个矩形。
cairo_set_operator (first_cr, op);
cairo_set_source_surface (first_cr, second, 0, 0);
cairo_paint (first_cr);
cairo_set_source_surface (first_cr, second, 0, 0);
cairo_paint (first_cr);
cairo_set_operator () 函数设置 first_cr 所容许的合成操作。cairo_set_source_surface () 函数基于 second 外观创建图案，并将其作为 first_cr 环境中的源。调用 cairo_paint () 函数显示最终合成结果（也就是输出到 cr 环境）。
cairo_set_source_surface (cr, first, 0, 0);
cairo_paint (cr);
cairo_paint (cr);
最后，在 GTK+ 窗口上显示结果。
cairo_operator_t oper[] = {
CAIRO_OPERATOR_DEST_OVER,
CAIRO_OPERATOR_DEST_IN,
CAIRO_OPERATOR_OUT,
CAIRO_OPERATOR_ADD,
CAIRO_OPERATOR_ATOP,
CAIRO_OPERATOR_DEST_ATOP,
};
CAIRO_OPERATOR_DEST_OVER,
CAIRO_OPERATOR_DEST_IN,
CAIRO_OPERATOR_OUT,
CAIRO_OPERATOR_ADD,
CAIRO_OPERATOR_ATOP,
CAIRO_OPERATOR_DEST_ATOP,
};
这就是传说中的那 6 种合成操作。
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