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[英]C++ Incorrect FPS and deltaTime measuring using std::chrono
[英]Limiting fps with std::chrono
std::chrono::system_clock::time_point m_BeginFrame = std::chrono::system_clock::now();
std::chrono::system_clock::time_point m_EndFrame = std::chrono::system_clock::now();
std::chrono::nanoseconds m_WorkTime = std::chrono::nanoseconds::zero();
std::chrono::nanoseconds m_WaitTime = std::chrono::nanoseconds::zero();
auto invFpsLimit = std::chrono::nanoseconds(1e9 / fpsLimit());
// main loop
while (!glfwWindowShouldClose(m_pScreen->glfwWindow()))
{
m_WaitTime = m_BeginFrame - m_EndFrame;
m_EndFrame = std::chrono::system_clock::now();
m_WorkTime = m_EndFrame - m_BeginFrame;
// if need sleep
if (m_WorkTime < invFpsLimit)
{
std::this_thread::sleep_until(m_BeginFrame + invFpsLimit);
m_DeltaTime = invFpsLimit;
}
else
{
m_DeltaTime = m_WorkTime;
}
m_BeginFrame = std::chrono::system_clock::now();
TRACE(" %f %u %u %u",
((float)1e9 / (float)(m_WaitTime.count() + m_WorkTime.count())),
m_WorkTime.count(),
m_WaitTime.count(),
invFpsLimit.count());
// think and opengl stuff...
}
我正在嘗試使用此代碼限制 fps,但 Fraps 向我顯示了大約 56-57 的值
58.848454 3175500 13817300 16666666
55.259304 3314200 14782300 16666666
58.923698 2321700 14649400 16666666
56.200928 2167400 15625900 16666666
52.774071 3188200 15760500 16666666
50.600887 4899700 14862800 16666666
65.011055 2347500 13034500 16666666
54.966499 2611700 15581200 16666666
55.605911 2653400 15330300 16666666
56.476437 2386100 15320400 16666666
55.280689 2581400 15508100 16666666
56.437550 2355400 15363300 16666666
47.170700 5535900 15663700 16666666
64.713608 3039700 12413000 16666666
56.136570 2941100 14872600 16666666
53.444496 3624200 15086800 16666666
60.074493 2397500 14248500 16666666
51.737339 3777100 15551300 16666666
59.369026 3665800 13178000 16666666
59.355282 3543900 13303800 16666666
54.243759 3961000 14474300 16666666
45.764706 7823500 14027400 16666666
61.242237 6239400 10089200 16666666
73.396652 2013800 11610800 16666666
50.531849 3824000 15965500 16666666
62.895454 2796000 13103400 16666666
60.611572 2360500 14138000 16666666
56.074242 2241700 15591800 16666666
55.143208 2454800 15679800 16666666
我試圖將時鍾更改為 system_clock,使用 sleep_for 而不是 sleep_untill,沒有任何效果
有什么我不知道的關於標准時鍾或我看不到的工作代碼的具體事情嗎?
什么偷了我的fps?
通過使用invFpsLimit
而不是system_clock::now()
更新m_BeginFrame
和m_EndFrame
可以做得更好。 這可能類似於:
#include <iostream>
#include <thread>
int fpsLimit() {return 60;}
int
main()
{
using namespace std::chrono;
using dsec = duration<double>;
auto invFpsLimit = duration_cast<system_clock::duration>(dsec{1./fpsLimit()});
auto m_BeginFrame = system_clock::now();
auto m_EndFrame = m_BeginFrame + invFpsLimit;
unsigned frame_count_per_second = 0;
auto prev_time_in_seconds = time_point_cast<seconds>(m_BeginFrame);
while (true)
{
// Do drawing work ...
// This part is just measuring if we're keeping the frame rate.
// It is not necessary to keep the frame rate.
auto time_in_seconds = time_point_cast<seconds>(system_clock::now());
++frame_count_per_second;
if (time_in_seconds > prev_time_in_seconds)
{
std::cerr << frame_count_per_second << " frames per second\n";
frame_count_per_second = 0;
prev_time_in_seconds = time_in_seconds;
}
// This part keeps the frame rate.
std::this_thread::sleep_until(m_EndFrame);
m_BeginFrame = m_EndFrame;
m_EndFrame = m_BeginFrame + invFpsLimit;
}
}
在 C++17 及以后的版本中,我建議使用round
而不是duration_cast
構造invFpsLimit
以獲得稍微更好的准確性:
auto invFpsLimit = round<system_clock::duration>(dsec{1./fpsLimit()});
要獲得增量,您可以執行以下操作:
unsgined deltaTime = 1000.0f / frame_count_per_fps; // In milliseconds
由於調度或資源爭用延遲,該函數也可能會阻塞更長的時間,直到到達 sleep_time 之后。
如果您確實需要精確的 fps,請使用 vSync,或使用以下循環替換代碼中的 if 塊,該循環在幀時間用完之前會持續執行任何操作:
while (m_WorkTime < invFpsLimit)
{
m_EndFrame = std::chrono::system_clock::now();
m_WorkTime = m_EndFrame - m_BeginFrame;
}
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