这一部分讲述如何绘制一些简单的图元，包括直线、填充与笔画操作、虚线、线端（Cap）与线的交合等图形的绘制方法。
直线段
直线段是非常基础的矢量图形对象。画一条直线段，需要调用两个函数：cairo_move_to() 函数，用于设置线段起点；cairo_line_to() 用于设定线段终点。
#include
#include
double coordx[100];
double coordy[100];
int count = 0;
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, 0.5);
int i, j;
for ( i = 0; i <= count - 1; i++ ) {
for ( j = 0; j <= count -1; j++ ) {
cairo_move_to(cr, coordx[i], coordy[i]);
cairo_line_to(cr, coordx[j], coordy[j]);
}
}
count = 0;
cairo_stroke(cr);
cairo_destroy(cr);
return FALSE;
}
gboolean clicked(GtkWidget *widget, GdkEventButton *event,
gpointer user_data)
{
if (event->button == 1) {
coordx[count] = event->x;
coordy[count++] = event->y;
}
if (event->button == 3) {
gtk_widget_queue_draw(widget);
}
return TRUE;
}
int
main (int argc, char *argv[])
{
GtkWidget *window;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_widget_add_events (window, GDK_BUTTON_PRESS_MASK);
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(window, "button-press-event",
G_CALLBACK(clicked), NULL);
gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_window_set_title(GTK_WINDOW(window), "lines");
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;
}
该示例会创建一个支持鼠标交互绘制直线段的 GTK+ 窗口。在窗口中使用鼠标左键随便点几下，每一次点击时，光标位置的坐标都会被记入长度为 100 的数组；然后点击鼠标右键，所有由鼠标左键点击所得到的点会被彼此连接形成直线段；在窗口中再次点击鼠标右键时，会对窗口绘图区域进行清除。
下面对该示例程序代码进行分析：
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_set_line_width (cr, 0.5);
设置颜色为黑色，线宽为 0.5pt 为参数，绘制直线段。
int i, j;
for ( i = 0; i <= count - 1; i++ ) {
for ( j = 0; j <= count -1; j++ ) {
cairo_move_to(cr, coordx[i], coordy[i]);
cairo_line_to(cr, coordx[j], coordy[j]);
}
}
用 cairo_move_to() 和 cairo_line_to() 函数在 cr 中定义绘图路径 (path)，连接 coordx[] 和 coordy[] 所记录的每个点。
cairo_stroke() 函数会将 cr 中的路径绘制出来。
g_signal_connect(window, "button-press-event",
G_CALLBACK(clicked), NULL);
设定 button-press-event 事件的回调函数为 clicked ()。
if (event->button == 1) {
coordx[count] = event->x;
coordy[count++] = event->y;
}
在 clicked () 函数中，当鼠标左键点击事件发生时，讲光标所在位置的 x 和 y 坐标分别记入数组 coordx 和 coordy。
if (event->button == 3) {
gtk_widget_queue_draw(widget);
}
在 clicked () 函数中，当鼠标右键单击时，调用 gtk_widget_queue_draw () 函数重绘窗口区域。
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描绘 (Stroke) 与填充 (Fill)
描绘 (Stroke) 可以绘制形状的轮廓，填充 (Fill) 则用于向形状内部灌注颜色。
#include
#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_line_width (cr, 9);
cairo_set_source_rgb (cr, 0.69, 0.19, 0);
cairo_arc (cr, width / 2, height / 2,
(width < height ? width : height) / 2 - 10, 0,
2 * M_PI);
cairo_stroke_preserve (cr);
cairo_set_source_rgb (cr, 0.3, 0.4, 0.6);
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), 200, 150);
gtk_widget_set_app_paintable (window, TRUE);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
这个示例绘制一个内部填充灰色的圆。
下面对代码进行解析：
之所以引入这个头文件，是因为程序中使用了圆周率常量 M_PI。
int width, height;
gtk_window_get_size (GTK_WINDOW (widget), &width, &height);
获取窗口的宽度与高度尺寸。程序中将使用这些值作为绘制圆形的参考尺寸，以实现窗口尺寸变化时，所绘制的圆的尺寸也会相应变化。
cairo_set_source_rgb (cr, 0.69, 0.19, 0);
cairo_arc (cr, width / 2, height / 2,
(width < height ? width : height) / 2 - 10, 0,
2 * M_PI);
cairo_stroke_preserve (cr);
描绘圆的轮廓。这里要注意一下 cairo_stroke_preserve () 函数与 cairo_stroke () 函数的区别（最好的办法是用后者替换一下前者，看看程序执行效果）。cairo_stroke_preserve () 函数会将它绘制的路径依然保存在 cairo 环境中，而 cairo_stroke () 所绘制的路径，在绘制完成后，就从 cairo的环境中清除了。
cairo_set_source_rgb (cr, 0.3, 0.4, 0.6);
cairo_fill (cr);
对使用 cairo_stroke_preserve () 函数绘制的路径进行蓝色填充。
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虚线 (Dash)
每条线都可以用不同的虚线笔 (dash pen) 来画。虚线模式是通过 cairo_set_dash () 函数来设定。模式类型通过一个数组来定义，数组中的值均为正数，它们用于设置虚线的虚部分与实部分。数组的长度与偏移量可以在程序中设定。如果数组的长度 为 0，虚线模式就是被禁止了，那所绘制的线是实线。如果数组长度为 1，则对应着虚实均匀分布的虚线模式。偏移量是用来设置在虚线的始端在一个虚线周期（包含一个实部单元和一个虚部单元）内的起始位置。
#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_rgba (cr, 0, 0, 0, 1);
static const double dashed1[] = { 4.0, 1.0 };
static int len1 = sizeof (dashed1) / sizeof (dashed1[0]);
static const double dashed2[] = { 4.0, 10.0, 4.0 };
static int len2 = sizeof (dashed2) / sizeof (dashed2[0]);
static const double dashed3[] = { 1.0 };
cairo_set_line_width (cr, 1.5);
cairo_set_dash (cr, dashed1, len1, 0);
cairo_move_to (cr, 40, 60);
cairo_line_to (cr, 360, 60);
cairo_stroke (cr);
cairo_set_dash (cr, dashed2, len2, 10);
cairo_move_to (cr, 40, 120);
cairo_line_to (cr, 360, 120);
cairo_stroke (cr);
cairo_set_dash (cr, dashed3, 1, 0);
cairo_move_to (cr, 40, 180);
cairo_line_to (cr, 360, 180);
cairo_stroke (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), 400, 300);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
该示例演示了三种虚线模式的设置及绘制。
下面分析一下关键代码。
static const double dashed1[] = { 4.0, 1.0 };
设定第一条虚线的模式，它的实部是 4 个像素，虚部是 1 个像素。
static int len1 = sizeof (dashed1) / sizeof (dashed1[0]);
计算数组 dashed1 的长度。
cairo_set_dash (cr, dashed1, len1, 0);
设置虚线模式。
darea = gtk_drawing_area_new ();
gtk_container_add (GTK_CONTAINER (window), darea);
这次，我们是在 drawing_area 部件上绘图，不再是窗口区域了。
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线帽 (Line caps)
线帽是针对直线段的端点形状而言的，分为三种：
- CAIRO_LINE_CAP_SQUARE
- CAIRO_LINE_CAP_ROUND
- CAIRO_LINE_CAP_BUTT
对应形状如下图所示：
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同一条直线段，CAIRO_LINE_CAP_SQUARE 线帽与 CAIRO_LINE_CAP_BUTT 线帽会导致直线段长度有所差别，前者会比后者长一个线宽尺寸。
#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_rgba (cr, 0, 0, 0, 1);
cairo_set_line_width (cr, 10);
cairo_set_line_cap (cr, CAIRO_LINE_CAP_BUTT);
cairo_move_to (cr, 40, 60);
cairo_line_to (cr, 360, 60);
cairo_stroke (cr);
cairo_set_line_cap (cr, CAIRO_LINE_CAP_ROUND);
cairo_move_to (cr, 40, 150);
cairo_line_to (cr, 360, 150);
cairo_stroke (cr);
cairo_set_line_cap (cr, CAIRO_LINE_CAP_SQUARE);
cairo_move_to (cr, 40, 240);
cairo_line_to (cr, 360, 240);
cairo_stroke (cr);
cairo_set_line_width (cr, 1.5);
cairo_move_to (cr, 40, 40);
cairo_line_to (cr, 40, 260);
cairo_stroke (cr);
cairo_move_to (cr, 360, 40);
cairo_line_to (cr, 360, 260);
cairo_stroke (cr);
cairo_move_to (cr, 365, 40);
cairo_line_to (cr, 365, 260);
cairo_stroke (cr);
cairo_destroy (cr);
return FALSE;
}
该示例绘制三条具有不同线帽的直线段，同时也展示了不同线帽对线的长度的影响。
下面对关键代码进行简单分析：
cairo_set_line_width (cr, 10);
设置线的宽度为 10px。
cairo_set_line_cap (cr, CAIRO_LINE_CAP_ROUND);
cairo_move_to (cr, 40, 150);
cairo_line_to (cr, 360, 150);
cairo_stroke (cr);
画了一条线帽为 CAIRO_LINE_CAP_ROUND 的直线段。
cairo_move_to (cr, 40, 40);
cairo_line_to (cr, 40, 260);
cairo_stroke (cr);
这是三条竖线之一，用于表现线帽对线的长度的影响。
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线的交合 (Line joins)
线的交合存在以下三种风格：
- CAIRO_LINE_JOIN_MITER
- CAIRO_LINE_JOIN_BEVEL
- CAIRO_LINE_JOIN_ROUND
对应形状如下图所示。
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#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, 0.1, 0, 0);
cairo_rectangle (cr, 30, 30, 100, 100);
cairo_set_line_width (cr, 14);
cairo_set_line_join (cr, CAIRO_LINE_JOIN_MITER);
cairo_stroke (cr);
cairo_rectangle (cr, 160, 30, 100, 100);
cairo_set_line_width (cr, 14);
cairo_set_line_join (cr, CAIRO_LINE_JOIN_BEVEL);
cairo_stroke (cr);
cairo_rectangle (cr, 100, 160, 100, 100);
cairo_set_line_width (cr, 14);
cairo_set_line_join (cr, CAIRO_LINE_JOIN_ROUND);
cairo_stroke (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), 300, 280);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
该示例采用不同的交合类型绘制了三个矩形。
下面对关键代码进行简单分析：
cairo_rectangle (cr, 30, 30, 100, 100);
cairo_set_line_width (cr, 14);
cairo_set_line_join (cr, CAIRO_LINE_JOIN_MITER);
cairo_stroke (cr);
绘制了一个线宽为 14px，交合类型为 CAIRO_LINE_JOIN_MITER 的矩形。
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