How to incorporate delta time into movement?

Question

I want to use delta time in my movement code for my player, and I'm not sure how to do it. At the moment, I have this:

else if (keyboardState.IsKeyDown(Keys.D))
        {
            double moveSpeed = 0.3 * delta;

            if (velocity < 0)
            {
                double turnSpeed = 0.5 * delta;
                velocity += turnSpeed;
            }
            else
            {
                if (velocity + moveSpeed > .1)
                {
                    velocity = .1;
                }
                else
                {
                    velocity += moveSpeed;
                }
            }
        }

        position.X += (float)velocity;

Is this correct? I'm trying to make my player's movement gradually speed up until it hits the maximum velocity.

position.X is the Vector2 X of the player. delta is set to gameTime.ElapsedGameTime.TotalSeconds.

Answer 1

The code you've got there is going to produce some really small numbers.

Let's say your game is running at 60 frames per second (that's pretty typical for MonoGame). The delta variable will be about 0.016666 seconds or 16 milliseconds.

So that means after the first calculation your moveSpeed is going to be equal to 0.0005. That's probably not what you want.

The first thing you need to do is decide what your numbers actually represent. I normally think about things in "pixel" units. So in my case moveSpeed = 300 would mean "move 300 pixels per second".

Once you've come to grips with that concept it's easier to reason about what's going on in your code. For example, you're trying to calculate how far your guy will move in "one frame" if he's moving at 300 pixels per second. So..

position.X += moveSpeed * delta;

will move the guy one frame. After 60 frames (1 second) he will have moved 300 pixels.

Honestly, I don't really understand what you're trying to do in the code you posted so it's quite difficult to tell you how to fix it. My guess is you've got a little confused along the way.

My suggestion, start over, start simple and work your way up to the more complex stuff.

Answer 2

Generally the speed value is multiplied by Time.deltaTime to give a new velocity vector (Velocity is Direction + Speed) that is independent of how many frames a second any given client can handle.

This isn't always the case it depends what you're doing but in your case multiplying the velocity Vector by speed * delta below should work fine.

velocity += moveSpeed * Time.deltaTime;

Misread framework in question, How to work with delta time?

@craftworkgames Answers this question better than I.