# DeltaTime and acceleration

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Hello,

I've a little problem of logic in a game i'm currently working on.

I've vX += acceleration * dT which represent the velocity in X

and x += vX * dT;

I've done so because of the reading of book. But after thinking about it and tying with values It seams not really accurate:

imagine acceleration = 1 and two computers A with dT = 1s and B with dt = 2s.

With A:

at the beginning vX = x = 0;

vX += 1 * 1; // = 1

x += 1 * 1; // = 1

vX += 1 * 1; // = 2

x += 2 * 1; // = 3

vX += 1 * 1; // = 3

x += 6 * 1;  // = 6

so at 1s x = 1 px

at 2s x = 3 px

at 3s x = 6 px

at 4s x = 10 px

With B:

vX += 1 * 2; // = 2

x += 2 * 2; // = 4

vX += 1 * 2; // = 4

x += 4 * 2; // = 12

so at 2s x = 4 px

at 4s x = 12 px

So here we clearly see that at 4s both x values are not at the same position, wich is clearly not ideal for my gameplay.

Any Idea on what should be really done ?

Thx

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Hum ok, i'll see. And if I don't wan to fix the timestep, wich is the case in many games ?

Edited by panicq

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You can keep track of the elapsed time and instead of doing:

v += a*dt;
x += v*dt;


you can do:

elapsedTime += dt;
v = v0 + a*elapsedTime;
x = x0 + v*elapsedTime;


where v0 and x0 are the initial values of velocity and speed when the object starts moving.

If the object can change it's direction (meaning a different acceleration), you set a to the new value, v0 to the velocity at that moment and x0 to the position at that moment.

Edited by DiegoSLTS

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Had you used correct formula you'd have identical result.

// dT = 1

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 0.5
vX += acceleration * dT; // == 1

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 2
vX += acceleration * dT; // == 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 4.5
vX += acceleration * dT; // == 3

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 8
vX += acceleration * dT; // == 4
// dT = 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 2
vX += acceleration * dT; // == 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 8
vX += acceleration * dT; // == 4

However as other's mentioned this isn't the only place that gets undetermined without fixed timestep.

Edited by Zaoshi Kaba

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Had you used correct formula you'd have identical result.

You've got a typo, 5.5 should be 4.5.

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Apart from that, one of the reasons you (panicq) should use a fixed timestep is related to collision checking/handling. You can actually see it from Zaoshi Kaba's example; what if there's something you'd collide with at x = 4.5 (after 3 seconds), but which you would NOT collide with at x = 8 (after 4 seconds)?

How would you solve this issue? Now imagine having to deal with/special case all other issues like this. OR you could fix your timestep (a 1 time thing), and have the results deterministic regardless of what framerate the game is running at.

Note that even though the logical simulation (handing movement, collision, etc.) should have a fixed timestep, the render part does not need to have a fixed timestep. If you wanted (for some reason), you could force rendering to occur every 3 seconds, without impacting any of the logical/gameplay stuff.

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Can you give an example of why a game would not want to have a fixed timestep? You claim this is the case for many games, but I can't think of a single game for which this is true.

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Hum ok, i'll see. And if I don't wan to fix the timestep, wich is the case in many games ?

This was the case in at least one mainstream game I heard of (I think it was Quake 2) and as a result, there became a known exploit that players on faster computers could jump higher and further, which became an issue in multiplayer.

I'm not sure why you think a fixed time step is not the case in "many games"? Where are you getting that from? This isn't something you can tell from playing a game (other than deducing its absence as the cause of bugs).

Fix your physics timestep. Seriously. There are no good reasons not to and many good reasons to do so.

Edited by Aardvajk

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I'm not sure why you think a fixed time step is not the case in "many games"? Where are you getting that from? This isn't something you can tell from playing a game (other than deducing its absence as the cause of bugs).

Indeed, the common physics middleware most games use all but mandate a fixed timestep.

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Ok, thank for advises, I'll fix my frame rate for physics and let a free frame rate for my logic.

Have a nice day.

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