這一部分講述如何繪製一些簡單的圖元,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製方法。
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,用於設置線段起點;cairo_line_to()
用於設定線段終點。
#include <gtk/gtk.h>
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;
}
該示例會創建一個支持鼠標交互繪製直線段的 GTK+ 窗口。在窗口中使用鼠標左鍵隨便點幾下,每一次點擊時,光標位置的座標都會被記入長度爲 100 的數組;然後點擊鼠標右鍵,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_line_width (cr, 0.5);
設置顏色爲黑色,線寬爲 0.5pt 爲參數,繪製直線段。
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() 函數會將 cr 中的路徑繪製出來。
G_CALLBACK(clicked),NULL);
設定 button-press-event
事件的回調函數爲
clicked ()
。
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 座標分別記入數組coordx 和
coordy
。
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) 可以繪製形狀的輪廓,填充 (Fill) 則用於向形狀內部灌注顏色。
#include <cairo.h>
#include <gtk/gtk.h>
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;
}
這個示例繪製一個內部填充灰色的圓。
下面對代碼進行解析:
之所以引入這個頭文件,是因爲程序中使用了圓周率常量 M_PI。
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爲繪製圓形的參考尺寸,以實現窗口尺寸變化時,所繪製的圓的尺寸也會相應變化。
cairo_arc (cr, width / 2, height / 2,
(width < height ? width : height) /2 -10,0,
2 * M_PI);
cairo_stroke_preserve (cr);
描繪圓的輪廓。這裏要注意一下 cairo_stroke_preserve () 函數與 cairo_stroke () 函數的區別(最好的辦法是用後者替換一下前者,看看程序執行效果)。cairo_stroke_preserve () 函數會將它繪製的路徑依然保存在 cairo 環境中,而 cairo_stroke () 所繪製的路徑,在繪製完成後,就從 cairo的環境中清除了。
cairo_fill (cr);
對使用 cairo_stroke_preserve () 函數繪製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,數組中的值均爲正數,它們用於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 爲 0,虛線模式就是被禁止了,那所繪製的線是實線。如果數組長度爲 1,則對應着虛實均勻分佈的虛線模式。偏移量是用來設置在虛線的始端在一個虛線週期(包含一個實部單元和一個虛部單元)內的起始位置。
#include <gtk/gtk.h>
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 constdouble dashed1[] ={4.0,1.0 };
static int len1 =sizeof(dashed1) /sizeof(dashed1[0]);
static constdouble dashed2[] ={4.0,10.0, 4.0 };
static int len2 =sizeof(dashed2) /sizeof(dashed2[0]);
static constdouble 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製。
下面分析一下關鍵代碼。
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。
計算數組 dashed1 的長度。
設置虛線模式。
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,分爲三種:
- 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,前者會比後者長一個線寬尺寸。
#include <gtk/gtk.h>
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;
}
該示例繪製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:
設置線的寬度爲 10px。
cairo_move_to (cr, 40, 150);
cairo_line_to (cr, 360, 150);
cairo_stroke (cr);
畫了一條線帽爲 CAIRO_LINE_CAP_ROUND 的直線段。
cairo_line_to (cr, 40, 260);
cairo_stroke (cr);
這是三條豎線之一,用於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 <gtk/gtk.h>
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;
}
該示例採用不同的交合類型繪製了三個矩形。
下面對關鍵代碼進行簡單分析:
cairo_set_line_width (cr, 14);
cairo_set_line_join (cr, CAIRO_LINE_JOIN_MITER);
cairo_stroke (cr);
繪製了一個線寬爲 14px,交合類型爲 CAIRO_LINE_JOIN_MITER 的矩形。