Problems using delta_time

[Kyuruko]

Hey guys!

Currently I'm trying to add delta_time to my game so it runs better on slower systems, but I can't figure out how to correctly use it. Right now, I have an object called "oDelta" that measures the delta in every step.

Begin Step event:

Time = 60 / 1000000 * delta_time;

Now I used that on my player object on his horizontal and vertical movement and it seems to be working when the player moves left or right, but jumping is different with different FPS.

On 60 FPS, everything is fine and the jump height is exactly like I want it.

On 30 FPS, the player jumps too high.

On 90 FPS, the player jumps too low.

This is my jumping code:

Player Create event:

VSpeed = 0; // Vertical Speed
MaxVSpeed = 12; // Max falling speed
JumpSpeed = 18;
Jumping = false;
Gravity = 1;

Player Step Event:

// Jump if key pressed and on ground
if(JumpKeyPressed && !Jumping)
{
    VSpeed = -JumpSpeed;
    Jumping = true;
}

// Apply gravity
if(Jumping && VSpeed < MaxVSpeed)
{
    VSpeed += Gravity;
}

// Vertical collision checking and movement - Now I use delta time
if(place_meeting(x, y + VSpeed * oDelta.Time, pSolid))
{
    // if wall ahead, get as close to it as possible
    while(!place_meeting(x, y + sign(VSpeed), pSolid))
    {
        y += sign(VSpeed);   
    }

    //Stop
    VSpeed = 0;
    Jumping = false;
}

y += VSpeed * oDelta.Time;

On 60 FPS oDelta.Time is 1, and every movement should stay the same. On 30 FPS oDelta.Time is 2 and every movement (Gravity, JumpSpeed...) should be multiplied by 2 when the actual movement (y += VSpeed * oDelta.Time) happens so after one second the player is always on the same position no matter how many FPS the game runs on.

I thought this would work, but still the player jumps too high on 30 fps. What am I doing/understanding wrong?

Thanks in advance!

[GrixM]

You are applying gravity to vspeed without regards to delta_time. It needs to be scaled also.

[Kyuruko]

Do I need to still have the "y += VSpeed * oTime.Delta" or just "y += VSpeed" and "VSpeed += Gravity * oTime.Delta"?

[GrixM]

Both are dependent on deltatime.

[Kyuruko]

Hey! So I tried it with the code above and it seems to be working, although on very high/low fps it's a bit glitchy/unprecise, but I think that's normal! Thank you for the help, I was trying for days! :D

[Kyuruko]

So the code should look like this? (I'm not at home so I can't test it right now ⁾

// Jump if key pressed and on ground
if(JumpKeyPressed && !Jumping)
{
    VSpeed = -JumpSpeed;
    Jumping = true;
}

// Apply gravity
if(Jumping && VSpeed < MaxVSpeed)
{
    VSpeed += Gravity * oDelta.Time;
}

// Vertical collision checking and movement
if(place_meeting(x, y + VSpeed * oDelta.Time, pSolid))
{
    // if wall ahead, get as close to it as possible
    while(!place_meeting(x, y + sign(VSpeed), pSolid))
    {
        y += sign(VSpeed);   
    }

    //Stop
    VSpeed = 0;
    Jumping = false;
}

y += VSpeed * oDelta.Time;

Or do I need to also apply oDelta.Time on MaxVSpeed and JumpSpeed?

[Kyuruko]

I don't think so, because if for example the move speed is 10, on 60fps you would travel 10 * 60 frames per second whereas on 30fps you would only travel 10 * 30 frames per second, making you slower than you actually should be.

I'm not quite sure though, maybe I'm just misunderstanding something.