<!-- Copyright 2021 DeepMind Technologies Limited

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

         http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->

<mujoco model="2-link 6-muscle arm">
  <option timestep="0.005" iterations="50" solver="Newton" tolerance="1e-10"/>

  <size njmax="50" nconmax="10" nstack="200"/>

  <visual>
    <rgba haze=".3 .3 .3 1"/>
  </visual>

  <default>
    <joint type="hinge" pos="0 0 0" axis="0 0 1" limited="true" range="0 120" damping="0.1"/>
    <muscle ctrllimited="true" ctrlrange="0 1"/>
  </default>

  <asset>
    <texture type="skybox" builtin="gradient" rgb1="0.6 0.6 0.6" rgb2="0 0 0" width="512" height="512"/>

    <texture name="texplane" type="2d" builtin="checker" rgb1=".25 .25 .25" rgb2=".3 .3 .3" width="512" height="512" mark="cross" markrgb=".8 .8 .8"/>

    <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true"/>
  </asset>

  <worldbody>
    <geom name="floor" pos="0 0 -0.5" size="0 0 1" type="plane" material="matplane"/>

    <light directional="true" diffuse=".8 .8 .8" specular=".2 .2 .2" pos="0 0 5" dir="0 0 -1"/>

    <site name="s0" pos="-0.15 0 0" size="0.02"/>
    <site name="x0" pos="0 -0.15 0" size="0.02" rgba="0 .7 0 1" group="1"/>

    <body pos="0 0 0">
      <geom name="upper arm" type="capsule" size="0.045" fromto="0 0 0  0.5 0 0" rgba=".5 .1 .1 1"/>
      <joint name="shoulder"/>
      <geom name="shoulder" type="cylinder" pos="0 0 0" size=".1 .05" rgba=".5 .1 .8 .5" mass="0" group="1"/>

      <site name="s1" pos="0.15 0.06 0" size="0.02"/>
      <site name="s2" pos="0.15 -0.06 0" size="0.02"/>
      <site name="s3" pos="0.4 0.06 0" size="0.02"/>
      <site name="s4" pos="0.4 -0.06 0" size="0.02"/>
      <site name="s5" pos="0.25 0.1 0" size="0.02"/>
      <site name="s6" pos="0.25 -0.1 0" size="0.02"/>
      <site name="x1" pos="0.5 -0.15 0" size="0.02" rgba="0 .7 0 1" group="1"/>

      <body pos="0.5 0 0">
        <geom name="forearm" type="capsule" size="0.035" fromto="0 0 0  0.5 0 0" rgba=".5 .1 .1 1"/>
        <joint name="elbow"/>
        <geom name="elbow" type="cylinder" pos="0 0 0" size=".08 .05" rgba=".5 .1 .8 .5" mass="0" group="1"/>

        <site name="s7" pos="0.11 0.05 0" size="0.02"/>
        <site name="s8" pos="0.11 -0.05 0" size="0.02"/>
      </body>
    </body>
  </worldbody>

  <tendon>
    <spatial name="SF" width="0.01">
      <site site="s0"/>
      <geom geom="shoulder"/>
      <site site="s1"/>
    </spatial>

    <spatial name="SE" width="0.01">
      <site site="s0"/>
      <geom geom="shoulder" sidesite="x0"/>
      <site site="s2"/>
    </spatial>

    <spatial name="EF" width="0.01">
      <site site="s3"/>
      <geom geom="elbow"/>
      <site site="s7"/>
    </spatial>

    <spatial name="EE" width="0.01">
      <site site="s4"/>
      <geom geom="elbow" sidesite="x1"/>
      <site site="s8"/>
    </spatial>

    <spatial name="BF" width="0.009" rgba=".4 .6 .4 1">
      <site site="s0"/>
      <geom geom="shoulder"/>
      <site site="s5"/>
      <geom geom="elbow"/>
      <site site="s7"/>
    </spatial>

    <spatial name="BE" width="0.009" rgba=".4 .6 .4 1">
      <site site="s0"/>
      <geom geom="shoulder" sidesite="x0"/>
      <site site="s6"/>
      <geom geom="elbow" sidesite="x1"/>
      <site site="s8"/>
    </spatial>
  </tendon>

  <actuator>
    <muscle name="SF" tendon="SF"/>
    <muscle name="SE" tendon="SE"/>
    <muscle name="EF" tendon="EF"/>
    <muscle name="EE" tendon="EE"/>
    <muscle name="BF" tendon="BF"/>
    <muscle name="BE" tendon="BE"/>
  </actuator>
</mujoco>
