这一部分，讲述一些基本的以及较为高级的形状绘制及其纯色 (solid color)、图案 (pattern) 与渐变 (gradient) 填充方法。

基本形状

Cairo 提供了几个用于绘制基本形状的函数。

#include

#include

#include

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;

}

这个示例，绘制了矩形、正方形、圆、圆弧和椭圆。

下面对关键代码简单分析：

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

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

绘制矩形与正方形。正方形在 cairo 中是矩形的一种特例。

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

画了一个圆，圆心为 (330, 60)px，半径为 40px。Cairo 所谓的圆，其实是起始角为 0 度，终止角为 360 度的弧线。

cairo_scale (cr, 1, 0.7);

cairo_arc (cr, 0, 0, 50, 0, 2 * M_PI);

画椭圆的方法也与画圆类似，只是需要先设定长轴与短轴的比例，在本例中为 1:0.7。

复杂的图形

复杂的图形是由简单的图形拼凑出来的，譬如下面这个绘制圆角矩形的程序。

#include

#include

#include

static void

draw_round_rectangle (cairo_t * cr,

double x, double y,

double width, double height, double r)

{

cairo_move_to (cr, x + r, y);

cairo_line_to (cr, x + width - r, y);

cairo_move_to (cr, x + width, y + r);

cairo_line_to (cr, x + width, y + height - r);

cairo_move_to (cr, x + width - r, y + height);

cairo_line_to (cr, x + r, y + height);

cairo_move_to (cr, x, y + height - r);

cairo_line_to (cr, x, y + r);

cairo_arc (cr, x + r, y + r, r, M_PI, 3 * M_PI / 2.0);

cairo_arc (cr, x + width - r, y + r, r, 3 * M_PI / 2, 2 * M_PI);

cairo_arc (cr, x + width - r, y + height - r, r, 0, M_PI / 2);

cairo_arc (cr, x + r, y + height - r, r, M_PI / 2, M_PI);

}

static gboolean

on_expose_event (GtkWidget * widget,

GdkEventExpose * event, gpointer data)

{

cairo_t *cr;

int width, height;

double w, h, x, y, r;

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

x = width / 5.0;

y = height / 5.0;

w = 3 * width / 5.0;

h = 3 * height / 5.0;

r = h / 4.0;

cr = gdk_cairo_create (widget->window);

cairo_set_source_rgb (cr, 0.8, 0.4, 0);

cairo_set_line_width (cr, 6);

draw_round_rectangle (cr, x, y, w, h, r);

cairo_stroke_preserve (cr);

cairo_set_source_rgb (cr, 0.8, 0.8, 0.2);

cairo_fill (cr);

cairo_destroy (cr);

g_print ("test\n");

return FALSE;

}

static gboolean

on_configure_event (GtkWidget * widget,

GdkEventConfigure * event, gpointer data)

{

gdk_window_invalidate_rect (widget->window,

&widget->allocation,

FALSE);

return FALSE;

}

int

main (int argc, char *argv[])

{

GtkWidget *window;

GtkWidget *darea;

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);

g_signal_connect(G_OBJECT(window), "configure-event",

G_CALLBACK(on_configure_event), NULL);

gtk_window_set_position (GTK_WINDOW (window),

GTK_WIN_POS_CENTER);

gtk_window_set_default_size (GTK_WINDOW (window), 400, 300);

gtk_widget_set_app_paintable (window, TRUE);

gtk_widget_show_all (window);

gtk_main ();

return 0;

}

注：因为 "The cairo graphics tutorial" 在这一部分所提供的示例程序不具代表性，因此写了这个程序。

该示例程序绘制了一个可跟随窗口尺寸进行缩放变化的圆角矩形。

自定义的 draw_round_rectangle () 函数利用 Cairo 提供的基本图元函数，利用直线段与圆弧拼凑出圆角矩形。on_configure_event () 函数用于响应窗口尺寸变化事件，在其中调用 gdk_window_invalidate_rect () 函数让窗口绘图区域失效，并产生窗口重绘制事件(即 expose 事件)。

填充 (Fill)

虽然上一篇已经讲述了一些有关填充的知识，但这里所讲述的内容是与形状相关的。填充可分为三种类型：纯色、图案、渐变。

纯色 (Solid color)

对象的颜色是采用红 (R)、绿 (G)、蓝 (B) 三原色描述的，Cairo 的 RGB 取值是从 0 到 1 的双精浮点数。

#include

#include

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;

}

该示例绘制了 4 个正方形，分别采用四种不同颜色进行填充。这个例子，由于很简单，就不再像原作者那样自作多情的分析了。

图案 (Pattern)

所谓图案填充，就是将图片填充到形状内部。

#include

#include

#include

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_arc (cr, 200, 70, 50, 0, 2 * M_PI);

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;

}

该示例，载入 4 张图片，分别填充至三个矩形与一个圆形内部区域。所使用的 4 幅图，均采用 GIMP 制作。程序中，图片的外观 (surface) 实在 on_expose_event () 函数中创建的，这并不是很妥当，因为窗口每次被重绘时，都需要从硬盘中读取图片。

pattern1 = cairo_pattern_create_for_surface (surface1);

由图片外观创建一个图案。

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);

这里，绘制第一个矩形。cairo_set_source () 函数通知 Cairo 环境，让它使用一份图案作为源 (source)。图片所形成的图案或许并不适合于形状，当使用 cairo_pattern_set_extend () 函数讲图案填充模式设为 CAIRO_EXTEND_REPEAT 时，可以让图案像瓦片那样填充于形状内部。cairo_rectangle () 函数创建一个矩形路径，cairo_fill () 函数将已经准备好的图案填充到矩形路径所构成的封闭区域中。

渐变 (Gradient)

在计算机图形学中，渐变是形状由明到暗或者从一种颜色向另一种颜色的平滑过度。在 2D 绘图与渲染程序中，渐变通常被用于创造多彩的背景与一些特效，比如光影的仿真。

#include

#include

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;

}

在这一示例程序中，我们绘制了三个具有不同渐变风格的矩形。

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

这里，创建了一个线性渐变图案。参数设定了绘制渐变方向的直线，在示例中，它是一条竖线。

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);

定义了渐变图案的断点。在示例中，渐变图案表现为黑色与黄色的过渡。通过添加两个黑色断点和一个黄色断点，就可以构成一个水平方向的渐变图案，颜色 的变化方向则是沿竖直方向。渐变图案从矩形的上端至下端，开始是黑色，到 1/10 宽度时，黑色便停止了，然后就是由黑色向黄色的渐变渲染；到达矩形中部时，黄色达到饱和状态。黄色断点会在 9/10 宽度处终止，最后的 1/10 又是黑色。