Cairo 图形指南 (7) —— 合成

时间：2010-05-06 来源：kine1314

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;
}

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);

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);

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_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);

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,
};

这就是传说中的那 6 种合成操作。