Physics determinism test

[pgimeno]

Works the same in both. Interestingly, the result is different (chaotic, as expected) in versions 0.9 and 0.10, but it's unclear whether that's because of the PRNG or Box2D; using LuaJIT's math.random instead of love.math.newRandomGenerator might shed light on that. The final layout is also different in 0.9 vs 0.10.

[grump]

Works the same in both. Interestingly, the result is different (chaotic, as expected) in versions 0.9 and 0.10

Thanks for testing! I don't really care about the older versions, although it would be interesting to know what's causing the difference.

Here's another, more interesting one just for shits and giggles. I couldn't figure out a nice and easy way to draw circles in this one, so it's quads this time.

[ReFreezed]

Perfect. 10/10.

[milon]

Haha, nice!

Picture was perfectly coherent for me on Linux Mint desktop.

[knorke]

bumping this thread for a note:
I think this test is only deterministic because it updates the physics like this:

Code: Select all

local accum = 0
function love.update(dt)
	accum = accum + math.min(dt, 1 / 20)
	while accum >= 1 / 60 do
		world:update(1 / 60)
....

Note how world:update() is called with a constant as parameter. NOT with the variable dt.
(The speed is limited by accumelating dt-timesteps only it is above a threshold and then update)
If world:update(1 / 60) is replaced by world:update(dt) then the program shows a different outcome on every run. (even on the same machine)

[pgimeno]

bumping this thread for a note:
I think this test is only deterministic because it updates the physics like this:
[...]
Note how world:update() is called with a constant as parameter. NOT with the variable dt.

Duh. It's a chaotic process. Any minimal variation in any parameter causes radically different results. The test was for cross-platform reproducibility, i.e. whether identical inputs produced identical results in all platforms. Obviously you don't get identical inputs if your dt varies between frames.

I've made a modified version so that when pressing certain keys, a very slight variation is introduced in the simulation.

• Pressing space introduces a single frame with dt = 1/59.9999 instead of 1/60.
• Pressing return multiplies one of the random numbers by 0.99999 in one frame.
• Pressing an arrow key moves the obstacle 0.0001 units in the given direction.

If you press any of these keys early on, everything is different; if you press it when part of the image is already formed, anything after that is chaotic.