[GUI] Support gui.fps_limit and reduce idle power consumption

docs/gui.rst

@@ -339,6 +339,19 @@ A *event filter* is a list combined of *key*, *type* and *(type, key)* tuple, e.
         mouse_x, mouse_y = gui.get_cursor_pos()
 
 
+.. attribute:: gui.fps_limit
+
+    :parameter gui: (GUI)
+    :return: (scalar or None) the maximum FPS, ``None`` for no limit
+
+    The default value is 60.
+
+    For example, to restrict FPS to be below 24, simply ``gui.fps_limit = 24``.
+    This helps reduce the overload on your hardware especially when you're
+    using OpenGL on your intergrated GPU which could make desktop slow to
+    response.
+
+
 GUI Widgets
 -----------
 

examples/game_of_life.py

@@ -66,6 +66,7 @@ def render():
 
 
 gui = ti.GUI('Game of Life', (img_size, img_size))
+gui.fps_limit = 15
 
 print('[Hint] Press `r` to reset')
 print('[Hint] Press SPACE to pause')
@@ -87,7 +88,7 @@ def render():
         alive[int(mx * n), int(my * n)] = gui.is_pressed(gui.LMB)
         paused = True
 
-    if not paused and gui.frame % 4 == 0:
+    if not paused:
         run()
 
     render()

python/taichi/misc/gui.py

@@ -414,6 +414,20 @@ def running(self, value):
         elif not self.core.should_close:
             self.core.should_close = 1
 
+    @property
+    def fps_limit(self):
+        if self.core.frame_delta_limit == 0:
+            return None
+        else:
+            return 1 / self.core.frame_delta_limit
+
+    @fps_limit.setter
+    def fps_limit(self, value):
+        if value is None:
+            self.core.frame_delta_limit = 0
+        else:
+            self.core.frame_delta_limit = 1 / value
+
 
 def rgb_to_hex(c):
     to255 = lambda x: np.clip(np.int32(x * 255), 0, 255)

taichi/gui/gui.h

@@ -491,7 +491,7 @@ class GUI : public GUIBase {
   std::string window_name;
   int width, height;
   int frame_id = 0;
-  const int fps = 60;
+  real frame_delta_limit = 1.0 / 60;
   float64 start_time;
   Array2D<Vector4> buffer;
   std::vector<real> last_frame_interval;
@@ -806,12 +806,12 @@ class GUI : public GUIBase {
   void update() {
     frame_id++;
     redraw_widgets();
-    while (taichi::Time::get_time() < last_frame_time + 1 / (real)fps)
-      ;
+    taichi::Time::wait_until(last_frame_time + frame_delta_limit);
+    auto this_frame_time = taichi::Time::get_time();
     if (last_frame_time != 0) {
-      last_frame_interval.push_back(taichi::Time::get_time() - last_frame_time);
+      last_frame_interval.push_back(this_frame_time - last_frame_time);
     }
-    last_frame_time = taichi::Time::get_time();
+    last_frame_time = this_frame_time;
     redraw();
     // Some old examples / users don't even provide a `break` statement for us
     // to terminate loop. So we have to terminate the program with RuntimeError

taichi/python/export_visual.cpp

@@ -26,6 +26,7 @@ void export_visual(py::module &m) {
       .value("Release", Type::release);
   py::class_<GUI>(m, "GUI")
       .def(py::init<std::string, Vector2i>())
+      .def_readwrite("frame_delta_limit", &GUI::frame_delta_limit)
       .def_readwrite("should_close", &GUI::should_close)
       .def("get_canvas", &GUI::get_canvas, py::return_value_policy::reference)
       .def("set_img",

taichi/system/timer.cpp

@@ -69,6 +69,27 @@ void Time::sleep(double s) {
   Time::usleep(s * 1e6_f64);
 }
 
+void Time::wait_until(double t) {
+#ifndef _WIN64  // we can't release CPU slice due to the lack of
+                // microsecond (us) sleep on Windows... sadly.
+  double dt;
+  if (t < Time::get_time()) {
+    return;
+  }
+  do {  // use system-provided sleep for large scale sleeping:
+    dt = t - Time::get_time();
+    if (dt <= 0) {
+      return;
+    }
+    Time::sleep(dt * (dt < 4e-2_f64 ? 0.02 : 0.4));
+  } while (dt > 2e-4_f64);  // until dt <= 200us
+#endif
+
+  // use an EBFE loop for small scale waiting:
+  while (Time::get_time() < t - 1e-6_f64)
+    ;  // until dt <= 1us
+}
+
 double Time::Timer::get_time() {
   return Time::get_time();
 }

taichi/system/timer.h

@@ -34,8 +34,8 @@ TI_NAMESPACE_BEGIN
 class Time {
  public:
   static double get_time();
-
   static uint64 get_cycles();
+  static void wait_until(double t);
 
   static void usleep(double us);
   static void sleep(double s);