c++ - 为什么每帧 4 像素上的 80 fps 对我来说滞后？

Question

所以我正在开发打砖 block ，但运动对我来说是滞后的。
我不知道为什么它会滞后，因为我听说 60 fps 是眼罩。
关于我应该做什么的任何建议，因为我不想以 100 fps 编程游戏
因为如果我能以 60 fps 的速度对它们进行编程，我处理计算的时间就更少了？
这是我的上限框架代码。

//Cap the frame rate                
if( fps.get_ticks() < 1000 / FRAMES_PER_SECOND )   
{   
    SDL_Delay( ( 1000 / FRAMES_PER_SECOND ) - fps.get_ticks() );   
}

如果你好奇的话，这是我的全部代码

/*This source code copyrighted by Lazy Foo' Productions (2004-2012)
and may not be redistributed without written permission.*/

//The headers
#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include <string>

//The screen attributes
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

//The frame rate
const int FRAMES_PER_SECOND = 40;

//The dimensions of the dot
const int DOT_WIDTH = 20;
const int DOT_HEIGHT = 20;

//The surfaces
SDL_Surface *dot = NULL;
SDL_Surface *screen = NULL;

//The event structure
SDL_Event event;

//The dot that will move around on the screen
class Dot
{
    private:
    //The X and Y offsets of the dot
    int x, y;

    //The velocity of the dot
    int xVel, yVel;

    public:
    //Initializes the variables
    Dot();

    //Takes key presses and adjusts the dot's velocity
    void handle_input();

    //Moves the dot
    void move();

    //Shows the dot on the screen
    void show();
};

//The timer
class Timer
{
    private:
    //The clock time when the timer started
    int startTicks;

    //The ticks stored when the timer was paused
    int pausedTicks;

    //The timer status
    bool paused;
    bool started;

    public:
    //Initializes variables
    Timer();

    //The various clock actions
    void start();
    void stop();
    void pause();
    void unpause();

    //Gets the timer's time
    int get_ticks();

    //Checks the status of the timer
    bool is_started();
    bool is_paused();
};

SDL_Surface *load_image( std::string filename )
{
    //The image that's loaded
    SDL_Surface* loadedImage = NULL;

    //The optimized surface that will be used
    SDL_Surface* optimizedImage = NULL;

    //Load the image
    loadedImage = IMG_Load( filename.c_str() );

    //If the image loaded
    if( loadedImage != NULL )
    {
        //Create an optimized surface
        optimizedImage = SDL_DisplayFormat( loadedImage );

        //Free the old surface
        SDL_FreeSurface( loadedImage );

        //If the surface was optimized
        if( optimizedImage != NULL )
        {
            //Color key surface
            SDL_SetColorKey( optimizedImage, SDL_SRCCOLORKEY, SDL_MapRGB(     optimizedImage->format, 0, 0xFF, 0xFF ) );
        }
    }

    //Return the optimized surface
    return optimizedImage;
}

void apply_surface( int x, int y, SDL_Surface* source, SDL_Surface* destination,                 SDL_Rect* clip = NULL )
{
    //Holds offsets
    SDL_Rect offset;

    //Get offsets
    offset.x = x;
    offset.y = y;

    //Blit
    SDL_BlitSurface( source, clip, destination, &offset );
}

bool init()
{
    //Initialize all SDL subsystems
    if( SDL_Init( SDL_INIT_EVERYTHING ) == -1 )
    {
        return false;
    }

    //Set up the screen
    screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE         );

    //If there was an error in setting up the screen
    if( screen == NULL )
    {
        return false;
    }

    //Set the window caption
    SDL_WM_SetCaption( "Move the Dot", NULL );

    //If everything initialized fine
    return true;
}

bool load_files()
{
    //Load the dot image
    dot = load_image( "dot.bmp" );

    //If there was a problem in loading the dot
    if( dot == NULL )
    {
       return false;
    }

    //If everything loaded fine
    return true;
}

void clean_up()
{
    //Free the surface
    SDL_FreeSurface( dot );

    //Quit SDL
    SDL_Quit();
}

Dot::Dot()
{
    //Initialize the offsets
    x = 0;
    y = 0;

    //Initialize the velocity
    xVel = 0;
    yVel = 0;
}

void Dot::handle_input()
{
    //If a key was pressed
    if( event.type == SDL_KEYDOWN )
    {
        //Adjust the velocity
        switch( event.key.keysym.sym )
        {
            case SDLK_UP: yVel -= DOT_HEIGHT / 2; break;
            case SDLK_DOWN: yVel += DOT_HEIGHT / 2; break;
            case SDLK_LEFT: xVel -= DOT_WIDTH / 2; break;
            case SDLK_RIGHT: xVel += DOT_WIDTH / 2; break;
        }
    }
    //If a key was released
    else if( event.type == SDL_KEYUP )
    {
        //Adjust the velocity
        switch( event.key.keysym.sym )
        {
            case SDLK_UP: yVel += DOT_HEIGHT / 2; break;
           case SDLK_DOWN: yVel -= DOT_HEIGHT / 2; break;
            case SDLK_LEFT: xVel += DOT_WIDTH / 2; break;
            case SDLK_RIGHT: xVel -= DOT_WIDTH / 2; break;
        }
    }
}

void Dot::move()
{
    //Move the dot left or right
    x += xVel;

    //If the dot went too far to the left or right
    if( ( x < 0 ) || ( x + DOT_WIDTH > SCREEN_WIDTH ) )
    {
        //move back
        x -= xVel;
    }

    //Move the dot up or down
    y += yVel;

    //If the dot went too far up or down
    if( ( y < 0 ) || ( y + DOT_HEIGHT > SCREEN_HEIGHT ) )
    {
        //move back
        y -= yVel;
    }
}

void Dot::show()
{
    //Show the dot
    apply_surface( x, y, dot, screen );
}

Timer::Timer()
{
    //Initialize the variables
    startTicks = 0;
    pausedTicks = 0;
    paused = false;
    started = false;
}

void Timer::start()
{
    //Start the timer
    started = true;

    //Unpause the timer
    paused = false;

    //Get the current clock time
    startTicks = SDL_GetTicks();
}

void Timer::stop()
{
    //Stop the timer
    started = false;

    //Unpause the timer
    paused = false;
}

void Timer::pause()
{
    //If the timer is running and isn't already paused
    if( ( started == true ) && ( paused == false ) )
    {
        //Pause the timer
        paused = true;

        //Calculate the paused ticks
        pausedTicks = SDL_GetTicks() - startTicks;
    }
}

void Timer::unpause()
{
    //If the timer is paused
    if( paused == true )
    {
        //Unpause the timer
        paused = false;

        //Reset the starting ticks
        startTicks = SDL_GetTicks() - pausedTicks;

        //Reset the paused ticks
        pausedTicks = 0;
    }
}

int Timer::get_ticks()
{
    //If the timer is running
    if( started == true )
    {
        //If the timer is paused
        if( paused == true )
        {
            //Return the number of ticks when the timer was paused
            return pausedTicks;
        }
        else
        {
            //Return the current time minus the start time
            return SDL_GetTicks() - startTicks;
        }
    }

   //If the timer isn't running
    return 0;
}

bool Timer::is_started()
{
    return started;
}

bool Timer::is_paused()
{
    return paused;
}

int main( int argc, char* args[] )
{
    //Quit flag
   bool quit = false;

    //The dot that will be used
    Dot myDot;

    //The frame rate regulator
    Timer fps;

    //Initialize
    if( init() == false )
    {
        return 1;
    }    

    //Load the files
    if( load_files() == false )
    {
        return 1;
    }

    //While the user hasn't quit
    while( quit == false )
    {
        //Start the frame timer
       fps.start();

       //While there's events to handle
        while( SDL_PollEvent( &event ) )
        {
            //Handle events for the dot
            myDot.handle_input();

            //If the user has Xed out the window
            if( event.type == SDL_QUIT )
            {
                //Quit the program
                quit = true;
            }
        }

        //Move the dot
        myDot.move();

        //Fill the screen white
        SDL_FillRect( screen, &screen->clip_rect, SDL_MapRGB( screen->format, 0xFF, 0xFF, 0xFF ) );

        //Show the dot on the screen
        myDot.show();

        //Update the screen
        if( SDL_Flip( screen ) == -1 )
        {
           return 1;
        }

        //Cap the frame rate
        if( fps.get_ticks() < 1000 / FRAMES_PER_SECOND )
        {
            SDL_Delay( ( 1000 / FRAMES_PER_SECOND ) - fps.get_ticks() );
        }
    }

    //Clean up
    clean_up();

    return 0;
}

Answer 1

由于四舍五入，您损失了微秒(可能是毫秒)，这会导致奇怪的行为。 IE。您的逻辑不会以恒定速率运行。

你可以做的是类似下面的事情(考虑它是伪代码，因为我对 SDL 有点过时了):

unsigned int lag = 0;

while (running) { // main loop
    // do event handling here
    lag += passed_time;
    reset_timer();

    // This loop is a bit more complicated:
    // - Do up to 'max_runs' logic steps
    // - Do this while the remaining time that passed since last frame is bigger than the time for one frame ('frame_time')
    // - Every loop, increase the number of runs done by 1 and sub the time for one frame
    // - For a 100 Hz simulation (100 logic steps per second), 'frame_time' would be 10.
    // - For a 60 Hz simulation (60 logic steps per second), 'frame_time' would be 17.
    for (int runs = 0; runs < max_runs && lag > frame_time; ++runs, lag -= frame_time) {
        // do game logic here
    }
    // Drop additional logic steps we couldn't process
    lag = lag % frame_time;

    // do render stuff here

    sleep(1); // Sleep for 1 millisecond to avoid having a 'passed_time' of 0 on very fast systems (depends on your timer resolution)
}

您的计时器分辨率越高，您的游戏运行就越流畅。尽管这种逻辑应该将“滞后”保持在最低限度。以这种方式使用计时，您还可以节省“等待”以获得特定帧率的时间。您仍然可以限制它，但您也可以只启用 VSync，这是更好的方法。

编辑: 这段代码实际上做的事情非常简单:

• 首先，它将自上次迭代以来耗时添加到我们在上次迭代中忽略的时间。
• 这为您提供了自上次逻辑步骤以来的时间窗口(逻辑步骤可以是与游戏逻辑相关的任何内容，例如移动玩家、移动子弹、添加或删除对象等)。
• 在此时间范围内每隔(例如)10 毫秒，运行一次逻辑步骤。如果 50 毫秒过去了，您将运行游戏逻辑 5 次以 catch 进度。如果过去的时间少于 10 毫秒，则您什么都不做。
• 为了避免锁定无法 catch 的较慢的计算机(无法处理每秒给定的迭代次数)，有一个最大的迭代次数可以 catch 。几行之后的模运算符 (%) 消除了被丢弃的开销。
• 为避免速度太快的计算机什么都不做(由于四舍五入，永远不会累加时间)整个循环等待 1 毫秒，因此总是至少有 1 毫秒要添加。

---

获取耗时(以毫秒为单位):

unsigned int timestamp = SDL_GetTicks(); // on startup or right before entering your main loop

// when you need the time passed:
unsigned int time_now = SDL_GetTicks(); // get current time
unsigned int time_passed = time_now - timestamp; // get time difference/time passed
timestamp = time_now; // essentially reset the timer

那样你可能仍然会损失一些微秒，但你应该没问题(总比等待好)。

编辑:我还没有看过新的 SDL，所以您可能不得不使用一些方法来代替 C 风格的函数。只需查看文档即可。

---

这是一个不需要任何额外库的示例(它仅适用于 Windows，不确定您正在使用哪个系统)。请记住，这只是我放在一起的一些快速代码(也可能使用 MinGW、MSVC 编译得很好)。

#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <cstdio>

__int64 frequency = 0;
__int64 last_time = 0;

double lag = 0;
double stat_lag = 0;

unsigned int render_steps = 0;
unsigned int logic_steps = 0;

const unsigned char target_updates_per_second = 100;
const unsigned char target_frames_per_second = 60;

// reset or initialize the timer
void reset_timer(__int64 *timer) {
    QueryPerformanceFrequency((PLARGE_INTEGER)&frequency);
    QueryPerformanceCounter((PLARGE_INTEGER)timer);
}

// get the time since last query and reset the timer for next iteration
double get_delta(__int64 *timer) {
    __int64 temp; // stores the new timestamp
    __int64 delta; // stores the difference
    QueryPerformanceCounter((PLARGE_INTEGER)&temp);
    delta = temp - *timer;
    *timer = temp;
    return (double)delta / frequency;
}

int main(int argc, char *argv[]) {
    reset_timer(&last_time);

    const unsigned int logic_frame_time = 1000 / target_updates_per_second;
    const unsigned int render_frame_time = 1000 / target_frames_per_second;

    while (1) {
        double delta = 1000 * get_delta(&last_time); // get the time that passed since last iteration (multiplied with 1000 for milliseconds)

        lag += delta; // here we add the time passed to the lag counter/timer
        stat_lag += delta; // here we do the same for the fps/ups counter

        for (char runs = 0; runs < 10 && lag > logic_frame_time; ++runs, lag -= logic_frame_time) {
            ++logic_steps; // here game logic would happen

            Sleep(0); // increase this number to simulate more load during logic steps
        }

        // lag = lag % logic_frame_time;
        // i made it the manual way to keep the double value, which is more precise here
        while (lag > logic_frame_time)
            lag -= logic_frame_time;

        if (stat_lag > 1000) { // if more than 1 second passed
            printf("updates per second: %u\nframes per second: %u\n\n", logic_steps, render_steps);

            // reset the stats
            logic_steps = 0;
            render_steps = 0;

            // stat_lag = stat_lag % 1000;
            // as above, doing it the manual way for double
            while (stat_lag > 1000)
                stat_lag -= 1000;
        }

        ++render_steps; // here rendering would happen

        // here, vsync would slow down processing, lowering the number of frames per second!
        //Sleep(...);

        Sleep(1); // avoid 0 milliseconds passing; this also causes rendering to be limited to 1000 fps!
    }
}