How can I get my objects to move at the same speed regardless of frame rate?

Question

I tried using the methods on lazyfoo's site for frame rate independence. One problem I'm having though is that my game is running at different speeds on different computers. The differences are drastic. For example, on a Core 2 Duo 2.4 ghz 2gb ram PC, the ball will move extremely fast, but the paddle moves slowly. On an Intel Centrino laptop running Win7 w/plenty of ram, the ball moves slowly, but the paddle moves fast. On older computers like a Pentium 4 2.4 ghz, 768 ram computer, the ball moves realllllllllyyyyyyy sllllooowwww. Yes, that slow.

What might be some reasons this is happening, and how can I fix it? I can only assume that the code I wrote that deals with frame rate independence is wrong. Here are the relevant parts of the code. (btw, it's a breakout clone).

Code to move the ball

bool Ball::MoveBall( Uint32 deltaTicks ) 
{ 
    // X-COORDINATE
    xOffset += cos( radians ) * ( ballVelocity / 1000.f ) * flipx;
    ballPosition.x = (Sint16)xOffset;

        .......

    // Y-COORDINATE
    yOffset += sin( radians ) * ( ballVelocity / 1000.f ) * flipy;
    ballPosition.y = (Sint16)yOffset;

    .....
}

Code to move the paddle

void Paddle::Move( Uint32 deltaTicks ) 
{
    xOffset += xVel * ( deltaTicks / 1000.f );
    paddlePosition.x = (Sint16)xOffset;
}

Main loop

    while ( SDL_PollEvent( &gEvent ) ) 
    {
        handle.PaddleInput( &gEvent, stick );

        if ( gEvent.type == SDL_QUIT || gEvent.key.keysym.sym == SDLK_ESCAPE )
            quit = true;
    }
    stick.Move( time.GetTicks() );

    if ( ball.MoveBall( time.GetTicks() ) )
        wallSound.Play();

    if ( ball.CheckPaddleCollision( stick ) )
        paddleSound.Play();

    if ( ball.CheckBrickCollision( brickmap ) )
        brickSound.Play();

    if ( ball.IsBallDead() ) {
        ball.ResetBallPosition();
        stick.ResetPaddlePosition();
        stick.SetPaddleState( Paddle::RESET );
        launched = false;
        draw.GameObjects( lvl, brickmap, stick, ball );
    }

    draw.GameObjects( lvl, brickmap, stick, ball );
    time.Start();
    draw.Refresh();

Answer 1

The movement of your ball varies from platform to platform because you're not using the delta-time at all for your movement calculation.

First of all you should convert your ticks to seconds in your update loop and then call the Move functions with the delta-time in seconds.. otherwise you'll have to convert to seconds in every Move function.

All your speed definitions should be in units per second. Eg. ballVelocity would be 100 if your ball should move at a rate of 100 units (pixels) per second.

Then a simple Move method could look like this:

bool Ball::MoveBall(float dt) 
{ 
    ballPosition.x += cos(radians) * ballVelocity * dt;
    ballPosition.y += sin(radians) * ballVelocity * dt;
}

A further optimization would be to not always calculate cos/sin, but rather have a direction vector that you only recalculate whenever the direction changes... Then it would turn into: ballPosition.x += direction.x * ballVelocity * dt;