<mujoco>
    <asset>
        <!--grid ground texture definition: type="2d", builtin="checker"-->
        <texture name="texplane" type="2d" builtin="checker" rgb1=".2 .3 .4" rgb2=".1 0.15 0.2" width="512" height="512"/>
        <!--sky background definition: type="skybox"-->
        <texture builtin="gradient" height="256" rgb1=".9 .9 1." rgb2=".3 .7 .9" type="skybox" width="256"/>
        <!--grid ground material definition: Mirror Geom（reflectance：0.3）-->
        <material name="MatGnd" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true"/>
    </asset>

   <worldbody>
      <light diffuse=".5 .5 .5" pos="0 0 3" dir="0 0 -1"/>    
      <geom name="ground" pos="0 0 0" size="5 5 10" material="MatGnd" type="plane" contype="1" conaffinity="1"/>
      <body pos="0 0 2">
         <joint type="free"/>
         <geom type="sphere" size="0.2" rgba="0 .9 0 1" contype="1" conaffinity="1" solref="-10000 -6"/>
      </body>
   </worldbody>
</mujoco>       