Drawing to the screen uses too much CPU

Drawing to the screen uses too much CPU

Tobias Dammers

Member #2,604

August 2002

Why are you using the unreliable and unpredictable (in terms of accuracy) multithread-timing facilities at all? Here's what I usually do:
- I use a high-resolution timer like QueryPerformanceCounter (Linux has gettimeofday() which is supposed to be as exact; since I'm on a windows box, I haven't investigated this to any extent though). Wrote a nice wrapper (once, for all my future projects) around the various interfaces, falling back on allegro timers if all else fails.
- In my main loop, I first read the timer delta and accumulate it. I decide independently if I need to update logic (if (timer) > 0 for variable-delta, while (timer >= timeslice) for fixed-delta) and graphics (cap to, say, 60 Hz or whatever you like, or don't cap and just update only if logic has changed). After that, I sleep (with a user option to turn this off).
Works beautifully.

---
Me make music: Triofobie
---
"We need Tobias and his awesome trombone, too." - Johan Halmén

BAF

Member #2,981

December 2002

Would you mind posting your framework?

BAF.zone | SantaHack!

Tobias Dammers

Member #2,604

August 2002

I might release a little library soon. In the meantime, here's my timer api.
(timer.h)

#ifndef TIMER_H
#define TIMER_H

void init_timer(bool use_qpc = false);
void stop_timer();
void start_timer(int timer_ms);
float get_timer_delta();
const char* get_timing_method_string();
float get_timer_accuracy();

#endif

(timer.cpp)

1#include <allegro.h>
2#ifdef ALLEGRO_WINDOWS
3  #include <winalleg.h>
4#endif
5#include "timer.h"
6 
7bool initialized = false;
8 
9int timer = 0;
10int timer_ms = 0;
11 
12#ifdef ALLEGRO_WINDOWS
13LARGE_INTEGER last_perf_count_li;
14LARGE_INTEGER perf_count_li;
15LARGE_INTEGER perf_freq_li;
16unsigned long long last_perf_count;
17unsigned long long perf_count;
18unsigned long long perf_freq;
19 
20unsigned long long li_to_ll(LARGE_INTEGER li) {
21  unsigned long long ll = li.HighPart;
22  ll = ll << 32;
23  ll |= li.LowPart;
24  return ll;
25}
26#endif
27 
28bool qpc_mode = false;
29 
30void timer_proc() {
31  ++timer;
32}
33END_OF_FUNCTION(timer_proc);
34 
35void init_timer(bool use_qpc) {
36#ifdef ALLEGRO_WINDOWS
37  if (use_qpc)
38    qpc_mode = QueryPerformanceFrequency(&perf_freq_li);
39  else
40#endif
41    qpc_mode = false;
42  
43  if (qpc_mode)
44    perf_freq = li_to_ll(perf_freq_li);
45  
46  if (!qpc_mode) {
47    LOCK_VARIABLE(timer);
48    LOCK_FUNCTION(timer_proc);
49    install_timer();
50  }
51  
52  initialized = true;
53}
54 
55void stop_timer() {
56  if (!qpc_mode)
57    remove_int(timer_proc);
58}
59 
60void start_timer(int _timer_ms) {
61  if (qpc_mode) {
62#ifdef ALLEGRO_WINDOWS
63    QueryPerformanceCounter(&last_perf_count_li);
64    last_perf_count = li_to_ll(last_perf_count_li);
65#endif
66  }
67  else {
68    stop_timer();
69    timer_ms = _timer_ms;
70    install_int(timer_proc, timer_ms);
71  }
72}
73 
74float get_timer_delta() {
75  if (qpc_mode) {
76#ifdef ALLEGRO_WINDOWS
77    QueryPerformanceCounter(&perf_count_li);
78    perf_count = li_to_ll(perf_count_li);
79    float result = (float)((double)(perf_count - last_perf_count) / (double)perf_freq);
80    last_perf_count = perf_count;
81    return result;
82#else
83    return NULL;
84#endif
85  }
86  else {
87    float result = (float)timer * (float)timer_ms * 0.001f;
88    timer = 0;
89    return result;
90  }
91}
92 
93const char* get_timing_method_string() {
94  if (!initialized)
95    return "Not initialized!";
96  if (qpc_mode)
97#ifdef ALLEGRO_WINDOWS
98    return "QueryPerformanceCounter";
99#else
100    return "Something's terribly wrong";
101#endif
102  return "Allegro timer routines";
103}
104 
105float get_timer_accuracy() {
106  if (qpc_mode)
107#ifdef ALLEGRO_WINDOWS
108    return (float)perf_freq;
109#else
110    return 0.0f;
111#endif
112  if (timer_ms)
113    return 1000.0f / (float)timer_ms;
114  return 0.0f;
115}

And here's what the main loop does:
(snip from scene.cpp)

1bool SCENE::run() {
bool looping = true;
float dt = 0.0f;
float paintt = 0.0f;
float min_paintt = 0.5f;
fps = 0.0f;
logic_frametime = 0.0f;
render_frametime = 0.0f;
logic_frames = 0;
display_info = false;

get_timer_delta();

while (looping) {
  while (keypressed()) {
    switch (readkey() >> 8) {
17// removed irrelevant code here
      case KEY_ESC:
        looping = false;
        break;
    }
  }
  
  dt = get_timer_delta();
  ++logic_frames;
  tick(dt);
  
  paint();
  
  paintt += dt;
  
  if (paintt > min_paintt) {
    fps = (float)logic_frames / paintt;
    logic_frames = 0;
    paintt = 0.0f;
    render_frametime = 1.0f / fps;
    logic_frametime = render_frametime;
  }
  
  if (level_completed)
    looping = false;
  rest(1);
}
return level_completed;
45}

Makes sense?

---
Me make music: Triofobie
---
"We need Tobias and his awesome trombone, too." - Johan Halmén