what's the difference between the same game with different version : v0,...,v4

[initial-h]

i find there are many versions of the same atari game,like PongNoFrameskip-v4 and so on.

what's the defference and where i can find the introduction?

[pzhokhov]

Very good question (although more gym than baselines), had it myself more than once :) @christopherhesse @gdb @joschu who'd be the best person to route this to?

[christopherhesse]

I don't know the answer to this, but there are some notes in the gym changelog for a couple of the v* suffixes: https://github.com/openai/gym#whats-new

[gdb]

We bump the version whenever there is a semantic change to the environment, no matter how slight. That means you can always trust that PongNoFrameskip-v3 has exactly the same semantics as the version used by some paper. That being said, you should consider all older versions to be deprecated and just use the most recent one — that is, you should think of older versions like a history that is accessible but not intended for normal use. Accordingly, we don't always document the difference between the versions, as you should always be using the most recent one except when reproing a historical result.

[KristianHolsheimer]

For Pong there's a nice video that makes the difference immediately clear: https://www.youtube.com/watch?v=zp72VTr730s

[rfali]

Breakout-v0 vs Breakout-v4
v0: repeat_action_probability of 0.25 (meaning 25% of the time the previous action will be used instead of the new action)
v4: repeat_action_probability of 0 (always follow issued action)

Breakout-v4 vs BreakoutDeterministic-v4 vs BreakoutNoFrameskip-v4
game-vX: frameskip is sampled from (2,5), meaning either 2, 3 or 4 frames are skipped [low: inclusive, high: exclusive]
game-Deterministic-vX: a fixed frame skip of 4
game-NoFrameskip-vX: with no frame skip

Breakout-v4 vs Breakout-ram-v4
game-vX: Observation Space (128,). Observation is given as a vector of size 128.
game-ram-vX: Observation Space (210,160,3). Observation is given as pixels of Width:210, Height:160, Depth:3

The atari environment source code has been removed from Gym [AFAIK] and you can see it on the ALE's GitHub. See source code here

Other useful references:
This video
This GitHub Issue
ALE v0.7 Blog