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<article role="main">
<section id="hopper">
<h1>Hopper<a class="headerlink" href="#hopper" title="Permalink to this heading">#</a></h1>
<figure class="align-default" id="id1">
<a class="reference internal image-reference" href="../../../_images/hopper.gif"><img alt="../../../_images/hopper.gif" src="../../../_images/hopper.gif" style="width: 200px;" /></a>
</figure>
<p>This environment is part of the Mujoco environments which contains general information about the environment.</p>
<div class="table-wrapper colwidths-auto docutils container">
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p></p></th>
<th class="head"><p></p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>Action Space</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">Box(-1.0,</span> <span class="pre">1.0,</span> <span class="pre">(3,),</span> <span class="pre">float32)</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>Observation Space</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">Box(-inf,</span> <span class="pre">inf,</span> <span class="pre">(11,),</span> <span class="pre">float64)</span></code></p></td>
</tr>
<tr class="row-even"><td><p>import</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">gymnasium.make(&quot;Hopper-v4&quot;)</span></code></p></td>
</tr>
</tbody>
</table>
</div>
<section id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this heading">#</a></h2>
<p>This environment is based on the work done by Erez, Tassa, and Todorov in
<a class="reference external" href="http://www.roboticsproceedings.org/rss07/p10.pdf">“Infinite Horizon Model Predictive Control for Nonlinear Periodic Tasks”</a>. The environment aims to
increase the number of independent state and control variables as compared to
the classic control environments. The hopper is a two-dimensional
one-legged figure that consist of four main body parts - the torso at the
top, the thigh in the middle, the leg in the bottom, and a single foot on
which the entire body rests. The goal is to make hops that move in the
forward (right) direction by applying torques on the three hinges
connecting the four body parts.</p>
</section>
<section id="action-space">
<h2>Action Space<a class="headerlink" href="#action-space" title="Permalink to this heading">#</a></h2>
<p>The action space is a <code class="docutils literal notranslate"><span class="pre">Box(-1,</span> <span class="pre">1,</span> <span class="pre">(3,),</span> <span class="pre">float32)</span></code>. An action represents the torques applied between <em>links</em></p>
<div class="table-wrapper colwidths-auto docutils container">
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Num</p></th>
<th class="head"><p>Action</p></th>
<th class="head"><p>Control Min</p></th>
<th class="head"><p>Control Max</p></th>
<th class="head"><p>Name (in corresponding XML file)</p></th>
<th class="head"><p>Joint</p></th>
<th class="head"><p>Unit</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>0</p></td>
<td><p>Torque applied on the thigh rotor</p></td>
<td><p>-1</p></td>
<td><p>1</p></td>
<td><p>thigh_joint</p></td>
<td><p>hinge</p></td>
<td><p>torque (N m)</p></td>
</tr>
<tr class="row-odd"><td><p>1</p></td>
<td><p>Torque applied on the leg rotor</p></td>
<td><p>-1</p></td>
<td><p>1</p></td>
<td><p>leg_joint</p></td>
<td><p>hinge</p></td>
<td><p>torque (N m)</p></td>
</tr>
<tr class="row-even"><td><p>2</p></td>
<td><p>Torque applied on the foot rotor</p></td>
<td><p>-1</p></td>
<td><p>1</p></td>
<td><p>foot_joint</p></td>
<td><p>hinge</p></td>
<td><p>torque (N m)</p></td>
</tr>
</tbody>
</table>
</div>
</section>
<section id="observation-space">
<h2>Observation Space<a class="headerlink" href="#observation-space" title="Permalink to this heading">#</a></h2>
<p>Observations consist of positional values of different body parts of the
hopper, followed by the velocities of those individual parts
(their derivatives) with all the positions ordered before all the velocities.</p>
<p>By default, observations do not include the x-coordinate of the hopper. It may
be included by passing <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=False</span></code> during construction.
In that case, the observation space will have 12 dimensions where the first dimension
represents the x-coordinate of the hopper.
Regardless of whether <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation</span></code> was set to true or false, the x-coordinate
will be returned in <code class="docutils literal notranslate"><span class="pre">info</span></code> with key <code class="docutils literal notranslate"><span class="pre">&quot;x_position&quot;</span></code>.</p>
<p>However, by default, the observation is a <code class="docutils literal notranslate"><span class="pre">ndarray</span></code> with shape <code class="docutils literal notranslate"><span class="pre">(11,)</span></code> where the elements
correspond to the following:</p>
<div class="table-wrapper colwidths-auto docutils container">
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Num</p></th>
<th class="head"><p>Observation</p></th>
<th class="head"><p>Min</p></th>
<th class="head"><p>Max</p></th>
<th class="head"><p>Name (in corresponding XML file)</p></th>
<th class="head"><p>Joint</p></th>
<th class="head"><p>Unit</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>0</p></td>
<td><p>z-coordinate of the top (height of hopper)</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>rootz</p></td>
<td><p>slide</p></td>
<td><p>position (m)</p></td>
</tr>
<tr class="row-odd"><td><p>1</p></td>
<td><p>angle of the top</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>rooty</p></td>
<td><p>hinge</p></td>
<td><p>angle (rad)</p></td>
</tr>
<tr class="row-even"><td><p>2</p></td>
<td><p>angle of the thigh joint</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>thigh_joint</p></td>
<td><p>hinge</p></td>
<td><p>angle (rad)</p></td>
</tr>
<tr class="row-odd"><td><p>3</p></td>
<td><p>angle of the leg joint</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>leg_joint</p></td>
<td><p>hinge</p></td>
<td><p>angle (rad)</p></td>
</tr>
<tr class="row-even"><td><p>4</p></td>
<td><p>angle of the foot joint</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>foot_joint</p></td>
<td><p>hinge</p></td>
<td><p>angle (rad)</p></td>
</tr>
<tr class="row-odd"><td><p>5</p></td>
<td><p>velocity of the x-coordinate of the top</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>rootx</p></td>
<td><p>slide</p></td>
<td><p>velocity (m/s)</p></td>
</tr>
<tr class="row-even"><td><p>6</p></td>
<td><p>velocity of the z-coordinate (height) of the top</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>rootz</p></td>
<td><p>slide</p></td>
<td><p>velocity (m/s)</p></td>
</tr>
<tr class="row-odd"><td><p>7</p></td>
<td><p>angular velocity of the angle of the top</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>rooty</p></td>
<td><p>hinge</p></td>
<td><p>angular velocity (rad/s)</p></td>
</tr>
<tr class="row-even"><td><p>8</p></td>
<td><p>angular velocity of the thigh hinge</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>thigh_joint</p></td>
<td><p>hinge</p></td>
<td><p>angular velocity (rad/s)</p></td>
</tr>
<tr class="row-odd"><td><p>9</p></td>
<td><p>angular velocity of the leg hinge</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>leg_joint</p></td>
<td><p>hinge</p></td>
<td><p>angular velocity (rad/s)</p></td>
</tr>
<tr class="row-even"><td><p>10</p></td>
<td><p>angular velocity of the foot hinge</p></td>
<td><p>-Inf</p></td>
<td><p>Inf</p></td>
<td><p>foot_joint</p></td>
<td><p>hinge</p></td>
<td><p>angular velocity (rad/s)</p></td>
</tr>
</tbody>
</table>
</div>
</section>
<section id="rewards">
<h2>Rewards<a class="headerlink" href="#rewards" title="Permalink to this heading">#</a></h2>
<p>The reward consists of three parts:</p>
<ul class="simple">
<li><p><em>healthy_reward</em>: Every timestep that the hopper is healthy (see definition in section “Episode Termination”), it gets a reward of fixed value <code class="docutils literal notranslate"><span class="pre">healthy_reward</span></code>.</p></li>
<li><p><em>forward_reward</em>: A reward of hopping forward which is measured
as <em><code class="docutils literal notranslate"><span class="pre">forward_reward_weight</span></code> * (x-coordinate before action - x-coordinate after action)/dt</em>. <em>dt</em> is
the time between actions and is dependent on the frame_skip parameter
(fixed to 4), where the frametime is 0.002 - making the
default <em>dt = 4 * 0.002 = 0.008</em>. This reward would be positive if the hopper
hops forward (positive x direction).</p></li>
<li><p><em>ctrl_cost</em>: A cost for penalising the hopper if it takes
actions that are too large. It is measured as <em><code class="docutils literal notranslate"><span class="pre">ctrl_cost_weight</span></code> *
sum(action<sup>2</sup>)</em> where <em><code class="docutils literal notranslate"><span class="pre">ctrl_cost_weight</span></code></em> is a parameter set for the
control and has a default value of 0.001</p></li>
</ul>
<p>The total reward returned is <em><strong>reward</strong></em> <em>=</em> <em>healthy_reward + forward_reward - ctrl_cost</em> and <code class="docutils literal notranslate"><span class="pre">info</span></code> will also contain the individual reward terms</p>
</section>
<section id="starting-state">
<h2>Starting State<a class="headerlink" href="#starting-state" title="Permalink to this heading">#</a></h2>
<p>All observations start in state
(0.0, 1.25, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0) with a uniform noise
in the range of [-<code class="docutils literal notranslate"><span class="pre">reset_noise_scale</span></code>, <code class="docutils literal notranslate"><span class="pre">reset_noise_scale</span></code>] added to the values for stochasticity.</p>
</section>
<section id="episode-end">
<h2>Episode End<a class="headerlink" href="#episode-end" title="Permalink to this heading">#</a></h2>
<p>The hopper is said to be unhealthy if any of the following happens:</p>
<ol class="arabic simple">
<li><p>An element of <code class="docutils literal notranslate"><span class="pre">observation[1:]</span></code> (if <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=True</span></code>, else <code class="docutils literal notranslate"><span class="pre">observation[2:]</span></code>) is no longer contained in the closed interval specified by the argument <code class="docutils literal notranslate"><span class="pre">healthy_state_range</span></code></p></li>
<li><p>The height of the hopper (<code class="docutils literal notranslate"><span class="pre">observation[0]</span></code> if <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=True</span></code>, else <code class="docutils literal notranslate"><span class="pre">observation[1]</span></code>) is no longer contained in the closed interval specified by the argument <code class="docutils literal notranslate"><span class="pre">healthy_z_range</span></code> (usually meaning that it has fallen)</p></li>
<li><p>The angle (<code class="docutils literal notranslate"><span class="pre">observation[1]</span></code> if <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=True</span></code>, else <code class="docutils literal notranslate"><span class="pre">observation[2]</span></code>) is no longer contained in the closed interval specified by the argument <code class="docutils literal notranslate"><span class="pre">healthy_angle_range</span></code></p></li>
</ol>
<p>If <code class="docutils literal notranslate"><span class="pre">terminate_when_unhealthy=True</span></code> is passed during construction (which is the default),
the episode ends when any of the following happens:</p>
<ol class="arabic simple">
<li><p>Truncation: The episode duration reaches a 1000 timesteps</p></li>
<li><p>Termination: The hopper is unhealthy</p></li>
</ol>
<p>If <code class="docutils literal notranslate"><span class="pre">terminate_when_unhealthy=False</span></code> is passed, the episode is ended only when 1000 timesteps are exceeded.</p>
</section>
<section id="arguments">
<h2>Arguments<a class="headerlink" href="#arguments" title="Permalink to this heading">#</a></h2>
<p>No additional arguments are currently supported in v2 and lower.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gymnasium</span> <span class="k">as</span> <span class="nn">gym</span>
<span class="n">env</span> <span class="o">=</span> <span class="n">gym</span><span class="o">.</span><span class="n">make</span><span class="p">(</span><span class="s1">&#39;Hopper-v2&#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>v3 and v4 take <code class="docutils literal notranslate"><span class="pre">gymnasium.make</span></code> kwargs such as <code class="docutils literal notranslate"><span class="pre">xml_file</span></code>, <code class="docutils literal notranslate"><span class="pre">ctrl_cost_weight</span></code>, <code class="docutils literal notranslate"><span class="pre">reset_noise_scale</span></code>, etc.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gymnasium</span> <span class="k">as</span> <span class="nn">gym</span>
<span class="n">env</span> <span class="o">=</span> <span class="n">gym</span><span class="o">.</span><span class="n">make</span><span class="p">(</span><span class="s1">&#39;Hopper-v4&#39;</span><span class="p">,</span> <span class="n">ctrl_cost_weight</span><span class="o">=</span><span class="mf">0.1</span><span class="p">,</span> <span class="o">....</span><span class="p">)</span>
</pre></div>
</div>
<div class="table-wrapper colwidths-auto docutils container">
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Parameter</p></th>
<th class="head"><p>Type</p></th>
<th class="head"><p>Default</p></th>
<th class="head"><p>Description</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">xml_file</span></code></p></td>
<td><p><strong>str</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">&quot;hopper.xml&quot;</span></code></p></td>
<td><p>Path to a MuJoCo model</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">forward_reward_weight</span></code></p></td>
<td><p><strong>float</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">1.0</span></code></p></td>
<td><p>Weight for <em>forward_reward</em> term (see section on reward)</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">ctrl_cost_weight</span></code></p></td>
<td><p><strong>float</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">0.001</span></code></p></td>
<td><p>Weight for <em>ctrl_cost</em> reward (see section on reward)</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">healthy_reward</span></code></p></td>
<td><p><strong>float</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">1</span></code></p></td>
<td><p>Constant reward given if the ant is “healthy” after timestep</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">terminate_when_unhealthy</span></code></p></td>
<td><p><strong>bool</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">True</span></code></p></td>
<td><p>If true, issue a done signal if the hopper is no longer healthy</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">healthy_state_range</span></code></p></td>
<td><p><strong>tuple</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">(-100,</span> <span class="pre">100)</span></code></p></td>
<td><p>The elements of <code class="docutils literal notranslate"><span class="pre">observation[1:]</span></code> (if <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=True</span></code>, else <code class="docutils literal notranslate"><span class="pre">observation[2:]</span></code>) must be in this range for the hopper to be considered healthy</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">healthy_z_range</span></code></p></td>
<td><p><strong>tuple</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">(0.7,</span> <span class="pre">float(&quot;inf&quot;))</span></code></p></td>
<td><p>The z-coordinate must be in this range for the hopper to be considered healthy</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">healthy_angle_range</span></code></p></td>
<td><p><strong>tuple</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">(-0.2,</span> <span class="pre">0.2)</span></code></p></td>
<td><p>The angle given by <code class="docutils literal notranslate"><span class="pre">observation[1]</span></code> (if <code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation=True</span></code>, else <code class="docutils literal notranslate"><span class="pre">observation[2]</span></code>) must be in this range for the hopper to be considered healthy</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">reset_noise_scale</span></code></p></td>
<td><p><strong>float</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">5e-3</span></code></p></td>
<td><p>Scale of random perturbations of initial position and velocity (see section on Starting State)</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">exclude_current_positions_from_observation</span></code></p></td>
<td><p><strong>bool</strong></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">True</span></code></p></td>
<td><p>Whether or not to omit the x-coordinate from observations. Excluding the position can serve as an inductive bias to induce position-agnostic behavior in policies</p></td>
</tr>
</tbody>
</table>
</div>
</section>
<section id="version-history">
<h2>Version History<a class="headerlink" href="#version-history" title="Permalink to this heading">#</a></h2>
<ul class="simple">
<li><p>v4: All MuJoCo environments now use the MuJoCo bindings in mujoco &gt;= 2.1.3</p></li>
<li><p>v3: Support for <code class="docutils literal notranslate"><span class="pre">gymnasium.make</span></code> kwargs such as <code class="docutils literal notranslate"><span class="pre">xml_file</span></code>, <code class="docutils literal notranslate"><span class="pre">ctrl_cost_weight</span></code>, <code class="docutils literal notranslate"><span class="pre">reset_noise_scale</span></code>, etc. rgb rendering comes from tracking camera (so agent does not run away from screen)</p></li>
<li><p>v2: All continuous control environments now use mujoco-py &gt;= 1.50</p></li>
<li><p>v1: max_time_steps raised to 1000 for robot based tasks. Added reward_threshold to environments.</p></li>
<li><p>v0: Initial versions release (1.0.0)</p></li>
</ul>
</section>
</section>
</article>
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<li><a class="reference internal" href="#">Hopper</a><ul>
<li><a class="reference internal" href="#description">Description</a></li>
<li><a class="reference internal" href="#action-space">Action Space</a></li>
<li><a class="reference internal" href="#observation-space">Observation Space</a></li>
<li><a class="reference internal" href="#rewards">Rewards</a></li>
<li><a class="reference internal" href="#starting-state">Starting State</a></li>
<li><a class="reference internal" href="#episode-end">Episode End</a></li>
<li><a class="reference internal" href="#arguments">Arguments</a></li>
<li><a class="reference internal" href="#version-history">Version History</a></li>
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