這一部分講述如何繪制一些簡單的圖元，包括直線、填充與筆畫操作、虛線、線端（Cap）與線的交合等圖形的繪制方法。
直線段
直線段是非常基礎的矢量圖形對象。畫一條直線段，需要調用兩個函數：cairo_move_to()
函數，用于設置線段起點；cairo_line_to() 用于設定線段終點。
#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 1; i++ ) {
for ( j = 0; j -1; j++ ) {
cairo_move_to(cr, coordx, coordy);
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;
}
該示例會創建一個支持鼠標交互繪制直線段的 GTK+
窗口。在窗口中使用鼠標左鍵隨便點幾下，每一次點擊時，光標位置的坐標都會被記入長度為 100
的數組；然后點擊鼠標右鍵，所有由鼠標左鍵點擊所得到的點會被彼此連接形成直線段；在窗口中再次點擊鼠標右鍵時，會對窗口繪圖區域進行清除。
下面對該示例程序代碼進行分析：
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_set_line_width (cr,
0.5);
設置顏色為黑色，線寬為 0.5pt 為參數，繪制直線段。
int i, j;
for ( i =
0; i 1; i++ ) {
for ( j = 0; j -1; j++ ) {
cairo_move_to(cr, coordx, coordy);
cairo_line_to(cr, coordx[j], coordy[j]);
}
}
用 cairo_move_to() 和 cairo_line_to() 函數在 cr 中定義繪圖路徑 (path)，連接
coordx[] 和 coordy[] 所記錄的每個點。
cairo_stroke(cr);
cairo_stroke() 函數會將 cr 中的路徑繪制出來。
g_signal_connect(window, "button-press-event",
G_CALLBACK(clicked), NULL);
設定 button-press-event 事件的回調函數為 clicked ()。
if (event->button == 1) {
coordx[count]
= event->x;
coordy[count++] = event->y;
}
在 clicked () 函數中，當鼠標左鍵點擊事件發生時，講光標所在位置的 x 和 y 坐標分別記入數組
coordx 和 coordy。
if (event->button == 3) {
gtk_widget_queue_draw(widget);
}
在 clicked ()
函數中，當鼠標右鍵單擊時，調用 gtk_widget_queue_draw () 函數重繪窗口區域。
描繪 (Stroke) 與填充 (Fill)
描繪 (Stroke) 可以繪制形狀的輪廓，填充 (Fill)
則用于向形狀內部灌注顏色。
#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 ) / 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;
}
這個示例繪制一個內部填充灰色的圓。
下面對代碼進行解析：
#include
之所以引入這個頭文件，是因為程序中使用了圓周率常量 M_PI。
int width, height;
gtk_window_get_size (GTK_WINDOW (widget), &width,
&height);
獲取窗口的寬度與高度尺寸。程序中將使用這些值作為繪制圓形的參考尺寸，以實現窗口尺寸變化時，所繪制的圓的尺寸也會相應變化。
cairo_set_source_rgb (cr, 0.69, 0.19, 0);
cairo_arc (cr, width
/ 2, height / 2,
(width ) / 2 - 10, 0,
2 * M_PI);
cairo_stroke_preserve (cr);
描繪圓的輪廓。這里要注意一下
cairo_stroke_preserve () 函數與 cairo_stroke ()
函數的區別（最好的辦法是用后者替換一下前者，看看程序執行效果）。cairo_stroke_preserve () 函數會將它繪制的路徑依然保存在 cairo
環境中，而 cairo_stroke () 所繪制的路徑，在繪制完成后，就從 cairo的環境中清除了。
cairo_set_source_rgb (cr, 0.3, 0.4, 0.6);
cairo_fill (cr);
對使用
cairo_stroke_preserve () 函數繪制的路徑進行藍色填充。
虛線 (Dash)
每條線都可以用不同的虛線筆 (dash pen) 來畫。虛線模式是通過 cairo_set_dash ()
函數來設定。模式類型通過一個數組來定義，數組中的值均為正數，它們用于設置虛線的虛部分與實部分。數組的長度與偏移量可以在程序中設定。如果數組的長度 為
0，虛線模式就是被禁止了，那所繪制的線是實線。如果數組長度為
1，則對應著虛實均勻分布的虛線模式。偏移量是用來設置在虛線的始端在一個虛線周期（包含一個實部單元和一個虛部單元）內的起始位置。
#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;
}
該示例演示了三種虛線模式的設置及繪制。
下面分析一下關鍵代碼。
static const double
dashed1[] = { 4.0, 1.0 };
設定第一條虛線的模式，它的實部是 4 個像素，虛部是 1 個像素。
static
int len1 = sizeof (dashed1) / sizeof (dashed1[0]);
計算數組 dashed1 的長度。
cairo_set_dash (cr, dashed1, len1, 0);
設置虛線模式。
darea =
gtk_drawing_area_new ();
gtk_container_add (GTK_CONTAINER (window),
darea);
這次，我們是在 drawing_area 部件上繪圖，不再是窗口區域了。
線帽 (Line caps)
線帽是針對直線段的端點形狀而言的，分為三種：
- CAIRO_LINE_CAP_SQUARE
- CAIRO_LINE_CAP_ROUND
- CAIRO_LINE_CAP_BUTT
對應形狀如下圖所示：
同一條直線段，CAIRO_LINE_CAP_SQUARE 線帽與 CAIRO_LINE_CAP_BUTT
線帽會導致直線段長度有所差別，前者會比后者長一個線寬尺寸。
#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;
}
該示例繪制三條具有不同線帽的直線段，同時也展示了不同線帽對線的長度的影響。
下面對關鍵代碼進行簡單分析：
cairo_set_line_width (cr, 10);
設置線的寬度為 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);
畫了一條線帽為
CAIRO_LINE_CAP_ROUND 的直線段。
cairo_move_to (cr, 40, 40);
cairo_line_to (cr, 40, 260);
cairo_stroke
(cr);
這是三條豎線之一，用于表現線帽對線的長度的影響。
線的交合 (Line joins)
線的交合存在以下三種風格：
- CAIRO_LINE_JOIN_MITER
- CAIRO_LINE_JOIN_BEVEL
- CAIRO_LINE_JOIN_ROUND
對應形狀如下圖所示。
#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;
}
該示例采用不同的交合類型繪制了三個矩形。
下面對關鍵代碼進行簡單分析：
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);
繪制了一個線寬為 14px，交合類型為 CAIRO_LINE_JOIN_MITER 的矩形。
本文來自ChinaUnix博客，如果查看原文請點： |