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MarkusH82

Speeding up a game question

11 posts in this topic

Hi,

 

I am currently developing a simple Tower Defense game and would like to implement the functionality for the player to speed up the game.

 

First, I was thinking it is easy to solve by speeding up the the enemies and towers, but a simple equation showed me it is not that easy.

 

For example my enemy is standing at x=0m its speed is 10m/s and the time per frame is 1s (to make the calculation easier).

 

So after one frame my enemy is at x=10m   (pos = speed * time)

 

To double the game speed I would double the enemy speed, and would get the enemy position at x=20m after one frame.

 

Now, if I have a tower at x=10m with a range of 5m in both directions, then the enemy in normal speed would be in range. The enemy with double speed would not be in range. So, this shows me that just increasing the speed is not the solution for a time based game.

 

Does anybody has experience with that and could help me?

 

Best regards,

 

Markus

Edited by MarkusH82
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I was also thinking about making the game framerate based, but is there not also a solution for time based games? Is a fixed timestep not a problem if one computer runs faster then another?

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I was also thinking about making the game framerate based, but is there not also a solution for time based games?

 

not a good one since it won't be deterministic. you can multiply the elapsed time by x to speed everything up but it will change the simulation. (Fixed timestep does not mean that the framerate is fixed, only that you update the game state at a fixed rate)

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The solution to this varies greatly depending on your implementation details. If you're using a grid, you can just force units through every square/hex of the grid even if they're moving 2 squares in one unit of time. This assumes you trigger from that in some way, shape, or form. If you're using geometric shapes and effectively no grid, you'd check for intersection between the line segment of the unit's movement and range of the tower. How long the tower fires (how much damage it does) needs to be calculated, but your existing code for that should require minimal changes to handle whatever solution you choose.

 

I have to ask, are you really writing a tower defense game where units move so fast that they pass an entire tower's range within 1 frame (~16ms at 60fps)? I cannot fathom a game where I'm expected to see action that fast. Usually, you cap the speedup a player can use well, well, well below that point.

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@SimonForsman: Thanks for that article, just finished reading it. I will try the last solution with a fixed time step.

 

@richardurich: Of course no enemy will be fast enough to skip a tower range. My calculation was really just an extreme example, but it shows that even if the time per frame is small enough the gameresults can variate with different speed values. This is maybe just a small difference, but I would like to avoid it.

Edited by MarkusH82
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If you could grab the delta time between frames and multiply that by 2, it would give you a 2x speed. I guess it does depend on how your code structure is setup though

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@emcconnell: it's still the same problem. Doubling the delta time is the same effect then doubling the speed.

@Spiro: I don't think I did a poor job in describing my problem. It is simple math. And yes you are wrong. If I double the speed and the enemy skips the tower range within one frame then even doubling the bullet speed doesn't help, if there is no collision.

Anyway, I reached my goal with the article that Simon recommented.
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Anyway, I reached my goal with the article that Simon recommented.

It is impossible to say that I was wrong and that you reached your goal with that article.  These are mutually exclusive statements.

 

If you believe you have reached your goal with that article, you have a fundamental misunderstanding of the problem and the math behind it.  You just confirmed that by stating that doubling enemy speed has anything to do with frame counts.  What does a “frame” mean to a game?  What length of time is that?

16 milliseconds?  2 minutes?  32.3 days?

 

Of course you need a fixed time-step.  That’s true for anything you make.  That link alone is not the answer to your problem in itself.  It’s about how you speed up and slow down the game and how you manipulate the fixed time-step.  Read my reply again until it makes sense.

 

 

L. Spiro

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His original description sounds simple. Doubling delta time will effectively just double the speed, will double the distance traveled in one frame and will fail horribly if things are determined on a "snapshot" base. If enemy x starts the movement outside a towers range and ends the movement behind its range, the tower won't fire at it in the first place. Bullet speed never comes into play.

 

Keeping the simple approach can work, as long as all speeds and sizes/ranges are carefully designed and in combination with fixed time steps. It will still be inconsistent. Assuming the time step allows an enemy to move half a tower radius, it might start shooting anywhere from the enemy just barely having entered and the enemy already being halfway through. Not even fixing time steps can help with this (though aligning everything in a grid of carefully selected size could).

 

Personally I'd prefer to go with the extra math: you don't just check "is the enemy in range now", but "did it move through my range this update, at which point did it enter, when did the tower start shooting, where exactly did bullets hit the enemy during its movement, etc.". I like to believe that treating movement as stuff actually moving, rather than just teleporting around is somewhat important to have "proper" physics.

 

Of course, sometimes "simple and kind of good enough" is all you got time for...

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I'd say there's something fundamentally wrong and game breaking if units are fast enough to pass right next to the tower and avoid it's firing range in one frame. Either the time step is way too large, or the metrics for unit speeds/tower ranges are designed wrong.

Having a 16ms timestep, units should move a fraction of a meter, and not 40 meters (assuming a tower has a range of 20 meters). Maybe the design should be changed a bit so units don't move fast enough to skip towers?

Edited by Strewya
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