Why are animations sometimes done using steps based on the amount of time that's passed?

Question

I've noticed that some programmers animate objects based on the difference in time. I am not sure why or even if this is logical. Does anyone know the significance?

Below is a snippet of code that explains what I mean:

var timePassed:int = getTimer()-lastTime;
lastTime += timePassed;
var newBallX = ball.x + ballDX*timePassed;
var newBallY = ball.y + ballDY*timePassed;

Answer 1

When you animate based on time, you make yourself independent of the framerate. No matter how many frames have passed, your ball will move the same distance in a given amount of time. Compare that to depending on the framerate, which is dependent on many variables, like how much processing power is available to do the animation.

This is a common game-physics issue -- check out Glenn Fiedler's excellent "Fix Your Timestep!" article for a more detailed take on this. (Doing it right is slightly more complicated than just multiplying your direction vectors by the timestep.)

Answer 2

The logic is simple.

BallDX => Ball Delta X => The distance the ball can move on the x coordinate in one second

timepassed => amount of time passed

if OldBallX = 0
if BallDX = 10
if TimePassed = 1 sec
Then NewBallX = OldBallX + (BallDX*TimePassed)

Which means

NewBallX = 0 + (10 * 1) = 10 pixels

In that case

if TimePassed = 0.5 sec (half a second)

Then

NewBallX = 0 + (10 * 0.5) = 5 pixels 

Logical?

Answer 3

Why NOT do it that way? As opposed to doing what? It is a simple linear motion right? Here is a thought: this allows for the ball to catch up with its intended position in the case other programs are slowing down the computer.