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

One more question regarding the actual calculation.

The question that comes to mind is "what is the trigger that causes the
penguin to be rendered on the screen?" Is there some  internal timer?
External timer?
In either case, if we separate the drawing trigger from the actual drawing,
then the calculation becomes.

Dt = prevTime - now()
And 
X = prevX + incrementX(dt) (based on whichever factors you want to take into
account when moving, deceleration, friction, etc.).

The frame rate does not play into this calculation at all.

What am I missing?

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Esko Luontola
Sent: Monday, February 18, 2013 12:10 PM
To: [email protected]
Subject: Re: [TDD] 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-pr
ogramming/targeting-a-variation-of-dead-reckoning-r1370

--
Esko Luontola
www.orfjackal.net


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