---
title: Defender
---

# Defender

```{figure} ../../_static/videos/atari/defender.gif
:width: 120px
:name: Defender
```

This environment is part of the <a href='..'>Atari environments</a>. Please read that page first for general information.

## Description

Aliens attack the earth. You control a spaceship and must defend humanity by destroying alien ships and rescuing humanoids.You have three lives and three smart bombs. You lose a live when you are shot down by an alien spaceship.Points are scored by destroying enemies and retrieving humans that are being abducted. You have an unlimited number of laser missiles.

For a more detailed documentation, see [the AtariAge page](https://atariage.com/manual_html_page.php?SoftwareLabelID=128)

## Actions

Defender has the action space `Discrete(18)` with the table below lists the meaning of each action's meanings.
As Defender 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         |
|---------|-----------------|
| `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 receive points for destroying enemies, rescuing abducted humans and keeping humans alive.
For a more detailed documentation, see [the AtariAge page](https://atariage.com/manual_html_page.php?SoftwareLabelID=128).

## Variants

Defender 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=   |
|------------------------------|-------------|--------------|------------------------------|
| Defender-v0                  | `"rgb"`     | `(2, 5)`     | `0.25`                       |
| Defender-ram-v0              | `"ram"`     | `(2, 5)`     | `0.25`                       |
| Defender-ramDeterministic-v0 | `"ram"`     | `4`          | `0.25`                       |
| Defender-ramNoFrameskip-v0   | `"ram"`     | `1`          | `0.25`                       |
| DefenderDeterministic-v0     | `"rgb"`     | `4`          | `0.25`                       |
| DefenderNoFrameskip-v0       | `"rgb"`     | `1`          | `0.25`                       |
| Defender-v4                  | `"rgb"`     | `(2, 5)`     | `0.0`                        |
| Defender-ram-v4              | `"ram"`     | `(2, 5)`     | `0.0`                        |
| Defender-ramDeterministic-v4 | `"ram"`     | `4`          | `0.0`                        |
| Defender-ramNoFrameskip-v4   | `"ram"`     | `1`          | `0.0`                        |
| DefenderDeterministic-v4     | `"rgb"`     | `4`          | `0.0`                        |
| DefenderNoFrameskip-v4       | `"rgb"`     | `1`          | `0.0`                        |
| ALE/Defender-v5              | `"rgb"`     | `4`          | `0.25`                       |
| ALE/Defender-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   |
|-------------------|----------------|--------------------------|----------------------|
| `[1, ..., 9, 16]` | `1`            | `[0, 1]`                 | `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
