Gym for Contra

An OpenAI Gym environment for Contra. on The Nintendo Entertainment System (NES) using the nes-py emulator.

Project address

Installation

The preferred installation of Contra is from pip:

pip install gym-contra

Usage

Python

You must import ContraEnv before trying to make an environment. This is because gym environments are registered at runtime. By default, ContraEnv use the full NES action space of 256 discrete actions. To contstrain this,ContraEnv.actions provides three actions lists (RIGHT_ONLY, SIMPLE_MOVEMENT, and COMPLEX_MOVEMENT) for the nes_py.wrappers.JoypadSpace wrapper. See Contra/actions.py for a breakdown of the legal actions in each of these three lists.

from nes_py.wrappers import JoypadSpace
import gym
from Contra.actions import SIMPLE_MOVEMENT, COMPLEX_MOVEMENT, RIGHT_ONLY

env = gym.make('Contra-v0')
env = JoypadSpace(env, RIGHT_ONLY)

print("actions", env.action_space)
print("observation_space ", env.observation_space.shape[0])

done = False
env.reset()
for step in range(5000):
    if done:
        print("Over")
        break
    state, reward, done, info = env.step(env.action_space.sample())
    env.render()

env.close()

NOTE: ContraEnv.make is just an alias to gym.make for convenience.

NOTE: remove calls to render in training code for a nontrivial speedup.

Command Line

Prepare to write please wait

NOTE: by default,-m is set to human.

Environments

These environments allow 3 attempts (lives) to play in the game. The environments only send reward-able game-play frames to agents; No cut-scenes, loading screens, etc. are sent from the NES emulator to an agent nor can an agent perform actions during these instances. If a cut-scene is not able to be skipped by hacking the NES's RAM, the environment will lock the Python process until the emulator is ready for the next action.

Step

Info about the rewards and info returned by the step method.

Reward Function

The reward function assumes the objective of the game is to move as far right as possible (increase the agent's x value), as fast as possible, without dying. To model this game, three separate variables compose the reward:

1. v: the difference in agent x values between states

• in this case this is instantaneous velocity for the given step
• v = x1 - x0
  • x0 is the x position before the step
  • x1 is the x position after the step
• moving right ⇔ v > 0
• moving left ⇔ v < 0
• not moving ⇔ v = 0

2. d: a death penalty that penalizes the agent for dying in a state
   • this penalty encourages the agent to avoid death
   • alive ⇔ d = 0
   • dead ⇔ d = -15
3. b : if the agent defeated the boss
   • this reword will encourages the agent to defeat boss as possible
   • no defeated ⇔ 0
   • defeated ⇔ 30

So the reward function is:

r = v + d + b

Note:The reward is clipped into the range (-15, 15).

info dictionary

The info dictionary returned by the step method contains the following keys:

life=self._life,
dead=self._is_dead,
done=self._get_done(),
score=self._score(),
status=self._player_state,
x_pos=self._x_position,
y_pos=self._y_position,defeated=self._get_boss_defeated,