c - 如何在没有 GPU 的情况下加快 Arm 上 Gtk2+ 的 Cairo 绘画速度

Question

我在配备 A53(1.5GHz) CPU 但 Linux 上不支持 GPU 的 Arm 设备上运行 Gtk2+(配置了 DirectFB，没有 X 系统)。它的帧缓冲区大小为:3840*2160。现在，我需要在屏幕上画一些线、圆……，就像 Windows 画板一样。于是，我尝试用Cairo来做，但是我发现Cairo画的速度太慢了!从左上到右下画一条线需要400ms!由于鼠标移动速度太快，并且在每次运动事件时，我都需要更新整个屏幕并重新绘制线条，但绘制时间太长，因此运动事件的响应太慢。我不知道是否有一些方法可以加快速度，使绘图任务像MS Paint一样顺利。我也尝试过 gdk_draw_line，但是更新也花费了太长的时间...下面是一些代码:

#include <gtk/gtk.h>
#include <time.h>

static gint prex, prey, nowx, nowy;
static GtkWidget *window = NULL;

static gboolean on_button_press_event(GtkWidget *widget, GdkEventButton *event, gpointer data)
{
  if (event->button == 1) {
      prex = event->x;
      prey = event->y;
  }
  return TRUE;
}

static gboolean on_button_release_event(GtkWidget *widget, GdkEventButton *event, gpointer data)
{
  if (event->button == 1) {
    prex = 0;
    prey = 0;
  }
  return TRUE;
}

static gboolean on_motion_notify_event(GtkWidget *widget, GdkEventMotion *event, gpointer data)
{
  GdkModifierType state;
  int minx, miny, maxx, maxy;
  GdkRectangle update_rect;

  gdk_window_get_pointer(event->window, &nowx, &nowy, &state);
  if (state & GDK_BUTTON1_MASK) {
    if (prex != 0 && prey != 0) {
      minx = prex < nowx ? prex : nowx;
      miny = prey < nowy ? prey : nowy;
      maxx = prex < nowx ? nowx : prex;
      maxy = prey < nowy ? nowy : prey;

      update_rect.x = minx;
      update_rect.y = miny;
      update_rect.width = maxx - minx;
      update_rect.height = maxy - miny;

      gdk_window_invalidate_rect(widget->window, &update_rect, FALSE);
    }
  }
  return TRUE;
}

void on_expose_event(GtkWidget *widget, GdkEventMotion *event, gpointer data)
{
  struct timeval tv_start, tv_end;
  cairo_t *cr;

  cr = gdk_cairo_create(widget->window);
  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_set_line_width(cr, 1);

  cairo_move_to(cr, prex, prey);
  cairo_line_to(cr, nowx, nowy);

  gettimeofday(&tv_start, NULL);
  cairo_stroke(cr);
  gettimeofday(&tv_end, NULL);
  printf("draw use time: %dus\n", (tv_end.tv_sec-tv_start.tv_sec)*1000000 + (tv_end.tv_usec-tv_start.tv_usec));
  //this takes almost 400ms when I draw a line from the upper left to lower right.
  cairo_destroy(cr);
  return FALSE;
}

int main (int argc,char* argv[])
{
  GtkWidget *canvas;

  gtk_init(&argc,&argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_window_set_default_size(GTK_WINDOW(window), 3840, 2160);
  g_signal_connect(G_OBJECT(window), "delete_event", G_CALLBACK(gtk_main_quit), NULL);
  g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(gtk_main_quit), NULL);

  canvas = gtk_drawing_area_new();
  gtk_widget_set_size_request(canvas, 3840, 2160);
  gtk_container_add(GTK_CONTAINER(window), canvas);

  g_signal_connect(canvas, "expose_event", G_CALLBACK(on_expose_event), NULL);
  g_signal_connect(canvas, "motion_notify_event", G_CALLBACK(on_motion_notify_event), NULL);
  g_signal_connect(canvas, "button_press_event", G_CALLBACK(on_button_press_event), NULL);
  g_signal_connect(canvas, "button_release_event", G_CALLBACK(on_button_release_event), NULL);

  gtk_widget_set_events(canvas, gtk_widget_get_events(canvas)
    | GDK_LEAVE_NOTIFY_MASK | GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK
    | GDK_POINTER_MOTION_MASK | GDK_POINTER_MOTION_HINT_MASK);

  gtk_widget_show_all(window);
  gtk_main();
  return FALSE;
}

Answer 1

仅当最后一次鼠标移动事件发生在 KL 毫秒之前，但不超过 MN 毫秒(当然，KL < MN)时，才尝试重新绘制线条。通过这种方式，您可以减少计算需求，同时仍然提供响应能力。