LÖVE

Posted: **Mon Aug 04, 2014 8:53 am**

So I made a little prototype to test out some Gravity related code, along with a few other things, and then it kinda got a little carried away with unneeded options and stuff. So yeah, what the hell may as well share it.
Have fun, all instructions are shown when you run the file.

Gravity.love: Gravity, "Now with bug fixes!"; (3.58 KiB) Downloaded 159 times

p.s. Any and all feedback as to code conventions, tips or anything else is highly welcome.

Posted: **Mon Aug 04, 2014 12:32 pm**

Backspace causes a crash, sim.lua line 84: attempt to get lenght of 'varPlanets' (nil).

Do you use Euler integration to calculate the orbits?
If so, and you're still interested in playing around with gravity I can recommend having a look at fourth order Runge-Kutta integration:
http://en.wikipedia.org/wiki/RK4

I just found a nice explaination (far better than I could ever write it probably) on why it is superior here:
http://gamedev.stackexchange.com/questi ... nt-gravity

I implemented it a while back, this might be easier to read than the pure mathematical approach:

Code: Select all

local function rk4( object, gameSpeed )
	local k1, l1, k2, l2, k3, l3, k4, l4
	local _mass, _position, _speed, _toOrigin = object.mass, object.position, object.speed, object.toOrigin

	k1 = gameSpeed * _speed
	l1 = gameSpeed * accelerationToOrigin( _toOrigin        , _mass)

	k2 = gameSpeed * (_speed + l1/2)
	l2 = gameSpeed * accelerationToOrigin( _toOrigin + k1/2 , _mass)

	k3 = gameSpeed * (_speed + l2/2)
	l3 = gameSpeed * accelerationToOrigin( _toOrigin + k2/2 , _mass)

	k4 = gameSpeed * (_speed + l3)
	l4 = gameSpeed * accelerationToOrigin( _toOrigin + k3   , _mass)

	local position = _position + k1/6 + k2/3 + k3/3 + k4/6
	local speed    = _speed    + l1/6 + l2/3 + l3/3 + l4/6

	return position, speed
end

The local variables k1, l1, and so on only serve readability. You could also do it all in one big ugly term.

Posted: **Mon Aug 04, 2014 1:27 pm**

1. Bug is fixed. Thanks, thought I had fixed that.

2. I implemented the gravity using this line of code:

Code: Select all

varBalls[w][1]:applyForce((varPlanets[q][1]:getX()-varBalls[w][1]:getX())*(varPlanets[q][2]:getRadius()/10), (varPlanets[q][1]:getY()-varBalls[w][1]:getY())*(varPlanets[q][2]:getRadius()/10))

or simply:
Force to apply on the x axis is equal to the distance between the ball (on the x)and the planet multiplied by the strength of the gravity.
Same for the Y axis but obviously y instead of x.
Or even simpler using Newton's law of universal gravitation;
http://en.wikipedia.org/wiki/Newton's_l ... ravitation

I'm not using the other implementations I have seen that are using (I think) Euler integration like you talked about. However that function will come in very hand for something along the same lines that I am making in a different language.

LÖVE

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