如何在C ++循环中限制FPS? [英] How to limit FPS in a loop with C++?

问题描述

我正在尝试使用带有chrono和线程的C ++在执行相交检查的循环中限制每秒的帧数.

I'm trying to limit the frames per second in a loop that is performing intersection checking, using C++ with chrono and thread.

这是我的代码:

std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
std::chrono::system_clock::time_point lastFrame = std::chrono::system_clock::now();

while (true)
{
    // Maintain designated frequency of 5 Hz (200 ms per frame)
    now = std::chrono::system_clock::now();
    std::chrono::duration<double, std::milli> delta = now - lastFrame;
    lastFrame = now;

    if (delta.count() < 200.0)
    {
        std::chrono::duration<double, std::milli> delta_ms(200.0 - delta.count());
        auto delta_ms_duration = std::chrono::duration_cast<std::chrono::milliseconds>(delta_ms);
        std::this_thread::sleep_for(std::chrono::milliseconds(delta_ms_duration.count()));
    }

    printf("Time: %f \n", delta.count());

    // Perform intersection test

}

我遇到的问题是，delta的所有其他输出都显示出微不足道的数量，而不是我要达到的〜200 ms/帧:

The problem I'm having is that every other output of delta is showing miniscule amounts, rather than the ~200 ms / frame I'm aiming for:

Time: 199.253200
Time: 2.067700
Time: 199.420400
Time: 2.408100
Time: 199.494200
Time: 2.306200
Time: 199.586800
Time: 2.253400
Time: 199.864000
Time: 2.156500
Time: 199.293800
Time: 2.075500
Time: 201.787500
Time: 4.426600
Time: 197.304100
Time: 4.530500
Time: 198.457200
Time: 3.482000
Time: 198.365300
Time: 3.415400
Time: 198.467400
Time: 3.595000
Time: 199.730100
Time: 3.373400

关于这种情况为什么发生的任何想法?

Any thoughts as to why this is happening?

推荐答案

如果考虑代码的工作原理，您会发现它与您编写代码的方式完全一样.Delta由于代码中的逻辑错误而发生振荡.

If you think about how your code works, you'll find out that it works exactly how you wrote it. Delta oscillates because of a logical mistake in the code.

会发生这种情况:

• 我们从 delta == 0 开始.
• 因为增量小于 200 ，所以您将代码休眠 200-delta(0)== 200 ms.
• 现在，增量本身变得接近 200 (因为您已经测量了睡眠时间以及实际工作)，并且您睡眠了 200-delta(200)== 0 ms.
• 此后，循环重复进行.

• We start with delta == 0.
• Because the delta is smaller than 200, you code sleeps 200 - delta(0) == 200 ms.
• Now, the delta itself becomes close to 200 (because you've measured that sleep time as well as an actual work) and you sleep 200 - delta(200) == 0 ms.
• After that the cycle repeats.

要解决此问题，您无需测量睡眠时间.

To fix the problem you need to not measure the sleep time.

这是可以完成的方法:

#include <iostream>
#include <cstdio>
#include <chrono>
#include <thread>

std::chrono::system_clock::time_point a = std::chrono::system_clock::now();
std::chrono::system_clock::time_point b = std::chrono::system_clock::now();

int main()
{
    while (true)
    {
        // Maintain designated frequency of 5 Hz (200 ms per frame)
        a = std::chrono::system_clock::now();
        std::chrono::duration<double, std::milli> work_time = a - b;

        if (work_time.count() < 200.0)
        {
            std::chrono::duration<double, std::milli> delta_ms(200.0 - work_time.count());
            auto delta_ms_duration = std::chrono::duration_cast<std::chrono::milliseconds>(delta_ms);
            std::this_thread::sleep_for(std::chrono::milliseconds(delta_ms_duration.count()));
        }

        b = std::chrono::system_clock::now();
        std::chrono::duration<double, std::milli> sleep_time = b - a;

        // Your code here

        printf("Time: %f \n", (work_time + sleep_time).count());
    }
}

这段代码为我提供了稳定的增量序列:

This code gives me a steady sequence of deltas:

Time: 199.057206 
Time: 199.053581 
Time: 199.064718 
Time: 199.053515 
Time: 199.053307 
Time: 199.053415 
Time: 199.053164 
Time: 199.053511 
Time: 199.053280 
Time: 199.053283    

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