I'm new to game development and I'm trying to build my first Pong game on the Godot engine. the tutorial I'm following is given below.

Simple Pong Game

What I don't understand is the line:

ball_pos += direction * ball_speed * delta

How is the new ball position being calculated?

As far as I know, delta is the time elapsed in seconds (float) since the last _process() call.

Also, why is the direction vector set to (1.0, 0.0)?

I'm looking for a clear explanation since the tutorial doesn't explain much. Thank you.

  • \$\begingroup\$ Direction * Speed * DeltaTime = Velocity * DeltaTime = DeltaPosition \$\endgroup\$ – S. Tarık Çetin Jul 10 '17 at 19:08

The += operator adds the right hand operand to the left hand operand. So the line is equivalent to

ball_pos = ball_pos + direction*ball_speed*delta

So direction*ball_speed*delta is the offset of the new position compared to the old position. It's the distance the ball travelled during this frame. direction*ball_speed is the speed of the ball combined with its direction, so it's a vector. Direction is a so called unit vector, so its length is one. This represents the velocity of the ball in (some distance-unit)/second. If you multiply this by the time that passed since the last update you have the change in position in this frame.

To learn more about this I recommend researching the basics of linear algebra.


Mostly assumptions based off what I've seen for similar variables and formulas, but ball_pos is probably a vector (so it has a direction, hence multiplying by direction to get it's direction). ball_speed is likely velocity, so you need to know how far it travels/elapsed time, and delta is likely the elapsed time between calculations.

That with the += basically says where was I, what direction did I go in and how fast did I go over the previous time frame, now where did that get me to.

Direction is set to (#, 0) because presumably it's not an x, y, z direction but just x, basically is it facing left or right (or up and down depending on what movement you're looking for).


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