mujoco

<mujoco model="Stone arch">
  <visual>
    <global azimuth="140" elevation="-20"/>
  </visual>

  <!-- Delicate frictional structure, held up by the elliptic cone, contact
       damping (solref dampratio 2) and one noslip pass, NOT by high impratio
       (>~15 suffices; higher only slows the solver, which drops the frame rate
       when a launched object lands and its cold-start solve is dear). A low
       impratio keeps real-time headroom, so an impact costs one hitchy frame
       rather than a sustained slowdown. The discrete integrator treats the
       contact rows implicitly, so the authored contact stiffness is honored
       at 6 ms; the keystone still creeps down ~1 cm per 5 minutes, half the
       implicitfast rate at 1 ms. -->
  <option timestep="6e-3" integrator="discrete" cone="elliptic" impratio="20" noslip_iterations="1"/>

  <!-- 27 voussoirs spanning 180 degrees: two static springers, 6 replicated
       pairs per flank, and a protruding keystone. Wedge faces are shrunk so
       neighboring voussoirs sit 0.05mm apart. The base poses and the replicate
       step are the arch as settled under load (the exact 180/13.5 degree
       pattern with its joint gaps closed, keystone 7 mm down), so it loads at
       rest. -->
  <asset>
    <texture name="ibl" type="cube" builtin="gradient" rgb1=".25 .3 .45" rgb2=".55 .5 .45" width="32" height="32"/>
    <texture type="skybox" builtin="gradient" rgb1=".12 .16 .28" rgb2=".5 .45 .4" width="32" height="512"/>
    <texture name="grid" type="2d" builtin="checker" rgb1=".1 .2 .3" rgb2=".2 .3 .4" width="100" height="100"/>
    <material name="grid" texture="grid" texrepeat="8 8" texuniform="true" reflectance="0.06"/>

    <mesh name="voussoir" vertex="-0.15 -0.0508548 -0.1264791
                                  -0.15  0.0508548 -0.1264791
                                  -0.15 -0.0653848  0.1230983
                                  -0.15  0.0653848  0.1230983
                                   0.15 -0.0508548 -0.1264791
                                   0.15  0.0508548 -0.1264791
                                   0.15 -0.0653848  0.1230983
                                   0.15  0.0653848  0.1230983"/>
    <mesh name="keystone" vertex="-0.15 -0.0473676 -0.1863777
                                  -0.15  0.0473676 -0.1863777
                                  -0.15 -0.0688720  0.1829969
                                  -0.15  0.0688720  0.1829969
                                   0.15 -0.0473676 -0.1863777
                                   0.15  0.0473676 -0.1863777
                                   0.15 -0.0688720  0.1829969
                                   0.15  0.0688720  0.1829969"/>
  </asset>

  <default>
    <geom type="mesh" density="2500" solref="0.005 2" solimp="0.95 0.99 1e-4"/>
  </default>

  <worldbody>
    <light name="key" pos="-1.35 -2.7 5.4" dir=".3 .6 -1" cutoff="25" softness="1" range="30" ambient=".15 .15 .15" specular=".15 .15 .15" intensity="3000000"/>
    <light type="image" texture="ibl" intensity="7500"/>
    <camera name="front" pos="3.2 0 0.9" xyaxes="0 1 0 0 0 1"/>

    <geom name="floor" type="plane" size="4 4 .01" material="grid"/>

    <geom name="springer_right" mesh="voussoir" pos="0 0.9983082 0.0581448" euler="-86.66667 0 0" rgba=".45 .44 .46 1"/>
    <geom name="springer_left" mesh="voussoir" pos="0 -0.9983082 0.0581448" euler="86.66667 0 0" rgba=".45 .44 .46 1"/>

    <replicate count="6" euler="13.30626 0 0">
      <body name="right_a" pos="0 0.9859367 0.1744001" euler="-80.44336 0 0">
        <freejoint/>
        <geom mesh="voussoir" rgba=".71 .71 .73 1"/>
      </body>
      <body name="right_b" pos="0 0.9585519 0.2873076" euler="-73.76479 0 0">
        <freejoint/>
        <geom mesh="voussoir" rgba=".80 .80 .82 1"/>
      </body>
    </replicate>

    <replicate count="6" euler="-13.30626 0 0">
      <body name="left_a" pos="0 -0.9859367 0.1744001" euler="80.44336 0 0">
        <freejoint/>
        <geom mesh="voussoir" rgba=".71 .71 .73 1"/>
      </body>
      <body name="left_b" pos="0 -0.9585519 0.2873076" euler="73.76479 0 0">
        <freejoint/>
        <geom mesh="voussoir" rgba=".80 .80 .82 1"/>
      </body>
    </replicate>

    <body name="keystone" pos="0 0 0.992797">
      <freejoint/>
      <geom mesh="keystone" rgba=".87 .79 .63 1"/>
    </body>
  </worldbody>
</mujoco>