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Author Topic: A (possible) solution to rendering performance optimisation  (Read 2319 times)

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Hi guys. I have been using SFML for 2 months. As I learned the command queue system, I made a small test that you can move a tank. I decided to add accelerate each time unit, rather than keep velocity frame-independent.
The code looks like this:

void World::adaptEntityVelocity(Entity& entity, sf::Time dt)
    static constexpr float a = 500.0f;//acceleration
    static constexpr float msp = 400.0f;//max speed
    float velo = entity.getVelocity();
    //clamp velo to [-msp,msp]
    velo = std::min(msp, std::max(- msp, velo));
    if(velo > 0)
        entity.setVelocity(std::max(0.0f, velo - a * dt.asSeconds()));
    if(velo < 0)
        entity.setVelocity(std::min(0.0f, velo + a * dt.asSeconds()));
adaptEntityVelocity(mPlayer, dt);
mPlayer.move(mPlayer.getVelocity() * dt.asSeconds();

It worked just find until I use sf::RenderWindow::setFrameLimit(). It somehow makes the player moves slightly faster in 60 fps (compared to ~450 fps without setting limit) and crazily fast in 10fps. I think the problem is the time Game::update() and Game::processInput() system is also affected by setFrameLimit(). So I decided not to use setFrameLimit(), but I don’t want to let the game run as fast as possible because that hurts GPU. But if I only limit render frequency the CPU will run massively fast and consume lots of energy (same when you run an endless while() loop). Then I come up with an idea: limit Game::render() to 60 fps and game loop to 500fps. This is the code:

void Game::run()
    sf::Clock dtClock;
    sf::Time timeSinceLastFrame = sf::Time::Zero;
        sf::Time elapsed = dtClock.restart();
        //limit loops per second;
        if (elapsed < sf::seconds(0.002f))
            sf::sleep(sf::seconds(0.002f) - elapsed);
            elapsed += dtClock.restart();
        timeSinceLastFrame += elapsed;
        while(timeSinceLastFrame >= TIMEPERFRAME)
            timeSinceLastFrame -= TIMEPERFRAME;
        render(elapsed); // pass a time variable to count and set limit

Render function:

void Game::render(sf::Time dt)
    mRenderTime += dt;
    if(mRenderTime >= sf::seconds(1.0f / 60.0f))
        mRenderCount ++;
        mRenderTime -= sf::seconds(1.0f / 60.0f);




So what I want to know is would this work on any OP with various speed? Or is it just in my own laptop?

P/s: this is what I got after all: a decent render rate and a limited loops per frame


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Re: A (possible) solution to rendering performance optimisation
« Reply #1 on: July 11, 2021, 04:05:01 pm »
setFramerateLimit as well as VSync will affect your whole game loop, it can't just magically know what counts as rendering and what as game logic. ;)

The generally recommended way is to used fixed time steps for your physics and potentially run that at a higher rate than your frame rate.

This article puts it very well: https://gafferongames.com/post/fix_your_timestep/
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