Alien#

../../../_images/alien.gif

This environment is part of the Atari environments. Please read that page first for general information.

Action Space

Discrete(18)

Observation Shape

(210, 160, 3)

Observation High

255

Observation Low

0

Import

gymnasium.make("ALE/Alien-v5")

For more Alien variants with different observation and action spaces, see the variants section.

Description#

You are stuck in a maze-like space ship with three aliens. You goal is to destroy their eggs that are scattered all over the ship while simultaneously avoiding the aliens (they are trying to kill you). You have a flamethrower that can help you turn them away in tricky situations. Moreover, you can occasionally collect a power-up (pulsar) that gives you the temporary ability to kill aliens.

For a more detailed documentation, see the AtariAge page

Actions#

Alien has the action space Discrete(18) with the table below lists the meaning of each action’s meanings. As Alien uses the full set of actions then specifying full_action_space=True will not modify the action space of the environment if passed to gymnasium.make.

Value

Meaning

Value

Meaning

Value

Meaning

0

NOOP

1

FIRE

2

UP

3

RIGHT

4

LEFT

5

DOWN

6

UPRIGHT

7

UPLEFT

8

DOWNRIGHT

9

DOWNLEFT

10

UPFIRE

11

RIGHTFIRE

12

LEFTFIRE

13

DOWNFIRE

14

UPRIGHTFIRE

15

UPLEFTFIRE

16

DOWNRIGHTFIRE

17

DOWNLEFTFIRE

Observations#

Atari environment have two possible observation types, the observation space is listed below. See variants section for the type of observation used by each environment id.

  • obs_type="rgb" -> observation_space=Box(0, 255, (210, 160, 3), np.uint8)

  • obs_type="ram" -> observation_space=Box(0, 255, (128,), np.uint8)

Additionally, obs_type="grayscale" cause the environment return a grayscale version of the rgb array for observations with the observation space being Box(0, 255, (210, 160), np.uint8)

Rewards#

You score points by destroying eggs, killing aliens, using pulsars, and collecting special prizes. When you are caught by an alien, you will lose one of your lives. The number of lives you have depends on the game flavor. For a table of scores corresponding to the different achievements, consult the AtariAge page.

Variants#

Alien has the following variants of the environment id which have the following differences in observation, the number of frame-skips and the repeat action probability.

Env-id

obs_type=

frameskip=

repeat_action_probability=

Alien-v0

"rgb"

(2, 5)

0.25

Alien-ram-v0

"ram"

(2, 5)

0.25

Alien-ramDeterministic-v0

"ram"

4

0.25

Alien-ramNoFrameskip-v0

"ram"

1

0.25

AlienDeterministic-v0

"rgb"

4

0.25

AlienNoFrameskip-v0

"rgb"

1

0.25

Alien-v4

"rgb"

(2, 5)

0.0

Alien-ram-v4

"ram"

(2, 5)

0.0

Alien-ramDeterministic-v4

"ram"

4

0.0

Alien-ramNoFrameskip-v4

"ram"

1

0.0

AlienDeterministic-v4

"rgb"

4

0.0

AlienNoFrameskip-v4

"rgb"

1

0.0

ALE/Alien-v5

"rgb"

4

0.25

ALE/Alien-ram-v5

"ram"

4

0.25

Difficulty and modes#

It is possible to specify various flavors of the environment via the keyword arguments difficulty and mode. A flavor is a combination of a game mode and a difficulty setting. The table below lists the possible difficulty and mode values along with the default values.

Available Modes

Default Mode

Available Difficulties

Default Difficulty

[0, 1, 2, 3]

0

[0, 1, 2, 3]

0

Version History#

A thorough discussion of the intricate differences between the versions and configurations can be found in the general article on Atari environments.

  • v5: Stickiness was added back and stochastic frameskipping was removed. The environments are now in the “ALE” namespace.

  • v4: Stickiness of actions was removed

  • v0: Initial versions release