How to delta time properly?

[milkbomb11]

I've got this code which controls a top-down shooter player's movement

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function player.move(speed, friction, dt)
  if love.keyboard.isDown("a") then
    player.xv = player.xv - speed*dt
  elseif love.keyboard.isDown("d") then
    player.xv = player.xv + speed*dt
  end

  if love.keyboard.isDown("w") then
    player.yv = player.yv - speed*dt
  elseif love.keyboard.isDown("s") then
    player.yv = player.yv + speed*dt
  end

  player.x = player.x + player.xv*dt
  player.y = player.y + player.yv*dt
  player.xv = player.xv * friction
  player.yv = player.yv * friction
end

Do I have to multiply dt at the last two lines too?
And when do I have to multiply stuff with dt and when do have to not?

[Rucoon]

Yes, you will want to use dt on friction too. If you don't then (lets say your game is running at 60 fps) player.xv and player.yv will be multiplied by friction 60 times a second when you only want it to be multiplied by the total of friction once a second. Delta time is the time between two frames so it will only use a part of friction each frame this code is called. Really any time you have something happen over a space of time you will want to use dt. (This can be slowly changing something like you are doing here, or a timer to have something happen.) You will also need to up the value of friction to get what you are looking for after changing this. The fps of a game can change a lot so you don't want to ever rely on it.

[keharriso]

Correct me if I'm wrong, but wouldn't one of the simple friction equations be:

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player.xv = player.xv - friction * player.xv * dt
player.yv = player.yv - friction * player.yv * dt

Also, very nit-picky, but what you call "speed" seems to be acceleration.

[ivan]

For "damping", I recommend the Box2D formula:

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velocity = velocity / (1 + damping*dt)

Where the damping coefficient is usually between 0-1
(it could be greater than 1 but then the movement looks float-y).
Further reading:
https://2dengine.com/?p=platformers#Damping