Re: How do you write tests if you aren't sure what the result should be?

Ron Jeffries wrote on 17.2.2013 13:39:
> OK, well, he's moving 15 per tick. But now on the new iPad I get 60 ticks and he should move about half that.
> I should generalize this.
> Hm, well, I want him to go 900 pixels in 2 seconds. That's 450 pixels per second.
> Um, maybe if I just read out the actual time since last time I can use that.
> So I'll save time in timeThen and read time now and difference them.

If we happen to have some domain knowledge of game development and 
common patterns in game design, we might decide to decouple the physics 
time step from the frame rate [1] and use a fixed time step for the 
physics, to keep the physics calculations deterministic. For example the 
game Supreme Commander (2007) does its physics calculations at 10 fps, 
as said in [2]:

"Take a look at the video if you haven’t already. What frame rate do you 
think the game is running at? The correct answer is 10 frames per 
second. Wait, what? It looks far smoother than 10 fps you say! It is and 
it isn’t. The game is actually running at two separate frame rates."

To make the visuals update at a smoother pace, the game state is 
interpolated to match the frame rate. There are various techniques for 
interpolating/dead reckoning [3][4], but that's a whole nother story.


[1] http://gafferongames.com/game-physics/fix-your-timestep/
[2] 
http://www.altdevblogaday.com/2011/07/09/synchronous-rts-engines-and-a-tale-of-desyncs/
[3] 
http://www.gamasutra.com/view/feature/3230/dead_reckoning_latency_hiding_for_.php
[4] 
http://www.gamedev.net/page/resources/_/technical/multiplayer-and-network-programming/targeting-a-variation-of-dead-reckoning-r1370

-- 
Esko Luontola
www.orfjackal.net


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