How to limit FPS in a loop with C++?

Question

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

Here is my code:

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  } 

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 

Any thoughts as to why this is happening?

Answer 1

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.

This is what happens:

• 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.

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

This is how it can be done:

#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());     } } 

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