fix(assets): hand_track prop physics — mass refit on 4 props + tennis-ball restitution

Five prop MJCFs, measured standalone against the shipped contact profile with
`RoboVerse/scripts/audit_asset_physics.py` (MuJoCo 3.8.0, each prop compiled alone over a ground
plane). **Only densities and one `solref` change — no geometry moves**, so collision shapes, grasp
affordances and resting heights are untouched.

## Mass refit

| prop | before | after | target | change | source of the error |
|---|---|---|---|---|---|
| `cups_a` | 0.029 kg | **0.013 kg** | 0.013 kg | density 700 → 314 | absent from `build_ycb_assets.py`'s `REAL_MASS_KG`, so it took the 700 kg/m³ fallback |
| `cups_f` | 0.064 kg | **0.021 kg** | 0.021 kg | density 700 → 231 | same fallback |
| `feast_scented_candle` | 0.039 kg | **0.400 kg** | 0.400 kg | density 120 → 1219 | a scented candle in a glass jar weighing 39 g |
| `meshycup` | 0.677 kg | **0.300 kg** | 0.300 kg | visual geom tagged `density="0"`; collision 250 → 1964 | see below |

`meshycup` is the substantive one. `meshycup_visual` carried no `density="0"`, so MuJoCo weighed
the render mesh at its default 1000 kg/m³ — **0.639 kg of the compiled 0.677 kg, 94 % of the mass
and the inertia, came from a geom nothing in the scene can touch.** Its sibling
`meshybottle_visual` was already tagged; this one was missed. Tagging it leaves only the shell, so
the collision density is refit to land a 0.30 kg glazed ceramic cup over the 0.153 L shell.

What a 10× mass error means in practice, measured: given the *same* 0.12 N·s impulse, the 39 g
candle slides **579 mm**; the 400 g candle slides **14 mm**.

## Tennis-ball restitution

`solref` `0.01 1.0` → `0.006 0.30`. Critical damping made the one prop here that is *supposed* to
bounce perfectly inelastic — measured **e = 0.000** against the ITF spec e = 0.73 (1.35 m rebound
from 2.54 m). The new value gives **e = 0.458 over 4 bounces**, settling in 0.76 s.

0.73 is **not reachable** in this contact model at dt = 0.002. Sweeping the damping ratio down, the
ball starts *gaining* energy between bounces below 0.28 — measured bounce-to-bounce e = 1.32 and
1.63, which is numerical, not a springier ball. 0.30 is the stiffest setting whose successive
bounces stay physically consistent (0.40, 0.46).

**This one does not take effect on its own.** `HAND_TRACK_SIM_PARAMS` overwrites every geom's
`solref` in the assembled scene, so the authored value is clobbered at load — re-measured with the
override on, e is back to 0.000. It needs a backend change (skip geoms that author their own
`solref`, or an explicit per-asset opt-out). The asset is correct now; the pipeline still discards
it. Landing this file is still right — it stops the asset from being wrong once the backend allows
it through.

## Land together with the code side

`cups_a` / `cups_f` must be added to `REAL_MASS_KG` in `RoboVerse/scripts/build_ycb_assets.py`, or
the next rebuild silently restores the 700 kg/m³ fallback and reverts both files. That is a
separate change in the code repo and should land alongside this PR.

## Caveats for reviewers

- **Every mass change is a re-collection event.** Demos on disk were recorded under the old masses
and PD action-replay reproduces recorded *torques*, so the replayed trajectory changes with the
mass. `feast_scented_candle`'s density of 120 was explicitly copied from the Parrot scene so
replay reproduced the recorded contact dynamics — this **does** shift `pack_box` and
`place_candle_in_box` action→motion. Snapped-state replay is unaffected. Re-run the grasp
regression on `nest_cups`, `pour_bottle_cup`, `pack_box`, `place_candle_in_box`, `grasp_candle`
before trusting any success rate measured against these props.
- **`feast_soda_can` is NOT in this PR** and carries the identical defect — the same density 120,
compiling to 0.042 kg for a full can. It was left out because it was not on the fix list; flagging
in case that was an oversight.
- **`meshybottle` is deliberately unchanged.** An earlier pass called it 2.2× too heavy by judging
1.100 kg against a 0.5 kg PET water bottle. The MJCF documents a deliberate 2026-07-05 rebalance
to a partially-full *wine* bottle — explicit `<inertial>`, COM dropped for the liquid. 750 ml of
wine is ~0.75 kg before the glass, so 1.1 kg is right. The reference was wrong, not the asset.
- Reference masses for `cups_a` / `cups_f` are the least certain numbers here (the YCB 065 stacking
cups) — worth a check against the published listing before merge.
- `meshycup.xml` ships CRLF line endings; they are preserved.

assets/box_task/feast_scented_candle/feast_scented_candle.xml

@@ -4,8 +4,14 @@
 
   Aligned with Parrot/parrot/assets/openarm_wuji_pack_box.xml so that
   action-replay through RoboVerse reproduces the recorded contact dynamics:
-    - cylinder collision size 0.032 x 0.05 (radius x half-height), density 120
-    - 8 rim capsules near the top (z=+0.045), radius 0.003, density 120
+    - cylinder collision size 0.032 x 0.05 (radius x half-height), density 1219
+
+  2026-08-13 mass fix: density 120 compiled to 0.039 kg -- a scented candle in a glass jar
+  weighing 39 g. Refit to 1219 kg/m^3 -> 0.40 kg over the 0.328 L collision volume.
+  NOTE: 120 was copied from the Parrot scene so action-replay reproduced the recorded contact
+  dynamics. This changes those dynamics; pack_box / place_candle_in_box demos recorded before
+  this date replay from snapped state, but action->motion differs across the boundary.
+    - 8 rim capsules near the top (z=+0.045), radius 0.003, density 1219
     - inline friction "3.0 0.15 0.01" + solref "0.01 1" + priority 2 on every
       collision geom
 -->
@@ -26,42 +32,42 @@
             material="feast_scented_candle_mat" group="2"
             contype="0" conaffinity="0" density="0" />
       <geom name="feast_scented_candle_collision" type="cylinder"
-            size="0.032 0.05" density="120"
+            size="0.032 0.05" density="1219"
             friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <!-- Top rim ring: 8 capsule segments at z = +half_h - 5mm = +0.045.
            Centerline radius 0.032; capsule radius 3mm. -->
       <geom name="feast_scented_candle_rim_0" type="capsule"
             fromto="0.032 0 0.045   0.02263 0.02263 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_1" type="capsule"
             fromto="0.02263 0.02263 0.045   0 0.032 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_2" type="capsule"
             fromto="0 0.032 0.045   -0.02263 0.02263 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_3" type="capsule"
             fromto="-0.02263 0.02263 0.045   -0.032 0 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_4" type="capsule"
             fromto="-0.032 0 0.045   -0.02263 -0.02263 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_5" type="capsule"
             fromto="-0.02263 -0.02263 0.045   0 -0.032 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_6" type="capsule"
             fromto="0 -0.032 0.045   0.02263 -0.02263 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="feast_scented_candle_rim_7" type="capsule"
             fromto="0.02263 -0.02263 0.045   0.032 0 0.045" size="0.003"
-            density="120" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
+            density="1219" friction="3.0 0.15 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
     </body>
   </worldbody>

assets/local_meshy_cup_bottle/meshycup/mjcf/meshycup.xml

@@ -1,6 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
 <mujoco model="meshycup">
-  <!-- Aligned with Parrot feast scene: density-only inertial, prop_collision (2026-05-15). -->
+  <!-- Aligned with Parrot feast scene: density-only inertial, prop_collision (2026-05-15).
+       2026-08-13 mass fix: meshycup_visual carried NO density="0", so MuJoCo weighed the render
+       mesh at its default 1000 kg/m^3 -- 0.639 kg of the compiled 0.677 kg, i.e. 94% of the mass
+       and the inertia came from a geom nothing can touch (its sibling meshybottle_visual was
+       already tagged; this one was missed). Tagging it leaves only the shell, so the collision
+       density goes 250 -> 1964 to land a 0.30 kg glazed ceramic cup over the 0.153 L shell. -->
   <default>
     <default class="prop_collision">
       <geom priority="2" friction="1.5 0.05 0.002" solref="0.01 1" />
@@ -14,26 +19,26 @@
   <worldbody>
     <body name="meshycup">
       <joint type="free" />
-      <geom name="meshycup_visual" type="mesh" mesh="meshycup_mesh" material="meshycup_mat" group="2" contype="0" conaffinity="0" quat="0.7071067811865476 0.7071067811865475 0 0" />
-      <geom name="meshycup_collision_bowl_bottom" type="cylinder" pos="0 0 -0.006" size="0.016 0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_0" type="capsule" fromto="0.016 0 -0.004 0.031 0 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_1" type="capsule" fromto="0.011314 0.011314 -0.004 0.021920 0.021920 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_2" type="capsule" fromto="0 0.016 -0.004 0 0.031 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_3" type="capsule" fromto="-0.011314 0.011314 -0.004 -0.021920 0.021920 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_4" type="capsule" fromto="-0.016 0 -0.004 -0.031 0 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_5" type="capsule" fromto="-0.011314 -0.011314 -0.004 -0.021920 -0.021920 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_6" type="capsule" fromto="0 -0.016 -0.004 0 -0.031 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_wall_7" type="capsule" fromto="0.011314 -0.011314 -0.004 0.021920 -0.021920 0.092" size="0.0055" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_0" type="capsule" fromto="0.031 0 0.094 0.021920 0.021920 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_1" type="capsule" fromto="0.021920 0.021920 0.094 0 0.031 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_2" type="capsule" fromto="0 0.031 0.094 -0.021920 0.021920 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_3" type="capsule" fromto="-0.021920 0.021920 0.094 -0.031 0 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_4" type="capsule" fromto="-0.031 0 0.094 -0.021920 -0.021920 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_5" type="capsule" fromto="-0.021920 -0.021920 0.094 0 -0.031 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_6" type="capsule" fromto="0 -0.031 0.094 0.021920 -0.021920 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_rim_7" type="capsule" fromto="0.021920 -0.021920 0.094 0.031 0 0.094" size="0.004" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_stem" type="cylinder" pos="0 0 -0.037" size="0.006 0.044" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
-      <geom name="meshycup_collision_base" type="cylinder" pos="0 0 -0.091" size="0.03 0.008" density="250" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_visual" type="mesh" mesh="meshycup_mesh" material="meshycup_mat" density="0" group="2" contype="0" conaffinity="0" quat="0.7071067811865476 0.7071067811865475 0 0" />
+      <geom name="meshycup_collision_bowl_bottom" type="cylinder" pos="0 0 -0.006" size="0.016 0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_0" type="capsule" fromto="0.016 0 -0.004 0.031 0 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_1" type="capsule" fromto="0.011314 0.011314 -0.004 0.021920 0.021920 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_2" type="capsule" fromto="0 0.016 -0.004 0 0.031 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_3" type="capsule" fromto="-0.011314 0.011314 -0.004 -0.021920 0.021920 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_4" type="capsule" fromto="-0.016 0 -0.004 -0.031 0 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_5" type="capsule" fromto="-0.011314 -0.011314 -0.004 -0.021920 -0.021920 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_6" type="capsule" fromto="0 -0.016 -0.004 0 -0.031 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_wall_7" type="capsule" fromto="0.011314 -0.011314 -0.004 0.021920 -0.021920 0.092" size="0.0055" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_0" type="capsule" fromto="0.031 0 0.094 0.021920 0.021920 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_1" type="capsule" fromto="0.021920 0.021920 0.094 0 0.031 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_2" type="capsule" fromto="0 0.031 0.094 -0.021920 0.021920 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_3" type="capsule" fromto="-0.021920 0.021920 0.094 -0.031 0 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_4" type="capsule" fromto="-0.031 0 0.094 -0.021920 -0.021920 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_5" type="capsule" fromto="-0.021920 -0.021920 0.094 0 -0.031 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_6" type="capsule" fromto="0 -0.031 0.094 0.021920 -0.021920 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_rim_7" type="capsule" fromto="0.021920 -0.021920 0.094 0.031 0 0.094" size="0.004" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_stem" type="cylinder" pos="0 0 -0.037" size="0.006 0.044" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
+      <geom name="meshycup_collision_base" type="cylinder" pos="0 0 -0.091" size="0.03 0.008" density="1964" group="3" class="prop_collision" rgba="0 0 0 0" />
     </body>
   </worldbody>
 </mujoco>

assets/ycb/cups_a/cups_a.xml

@@ -5,7 +5,9 @@
   textured.obj -> trimesh recenter (xy centered on AABB, base at z=0) -> visual mesh
   + CoACD convex-decomposed collision (36 parts, threshold=0.1) so the
   concave shape is collision-true (cavity preserved / tool silhouette kept).
-  Density 700.0 kg/m^3 over the summed part volume 4.1e-05 m^3 -> mass 0.029 kg.
+  Density 314.0 kg/m^3 over the summed part volume 4.1e-05 m^3 -> mass 0.013 kg.
+  2026-08-13 mass fit: the 700 kg/m^3 fallback (this cup is absent from build_ycb_assets.py's
+  REAL_MASS_KG) put it at 2.2x the YCB 065-a stacking cup it scans. Refit to the real mass.
   Recentered AABB (m): size=[0.0571, 0.0574, 0.0615].
   License: YCB CC BY 4.0.
 -->
@@ -57,112 +59,112 @@
       <geom name="cups_a_visual" type="mesh" mesh="cups_a_mesh" material="cups_a_mat"
             group="2" contype="0" conaffinity="0" density="0" />
       <geom name="cups_a_collision0" type="mesh" mesh="cups_a_col0"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision1" type="mesh" mesh="cups_a_col1"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision2" type="mesh" mesh="cups_a_col2"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision3" type="mesh" mesh="cups_a_col3"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision4" type="mesh" mesh="cups_a_col4"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision5" type="mesh" mesh="cups_a_col5"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision6" type="mesh" mesh="cups_a_col6"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision7" type="mesh" mesh="cups_a_col7"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision8" type="mesh" mesh="cups_a_col8"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision9" type="mesh" mesh="cups_a_col9"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision10" type="mesh" mesh="cups_a_col10"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision11" type="mesh" mesh="cups_a_col11"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision12" type="mesh" mesh="cups_a_col12"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision13" type="mesh" mesh="cups_a_col13"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision14" type="mesh" mesh="cups_a_col14"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision15" type="mesh" mesh="cups_a_col15"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision16" type="mesh" mesh="cups_a_col16"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision17" type="mesh" mesh="cups_a_col17"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision18" type="mesh" mesh="cups_a_col18"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision19" type="mesh" mesh="cups_a_col19"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision20" type="mesh" mesh="cups_a_col20"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision21" type="mesh" mesh="cups_a_col21"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision22" type="mesh" mesh="cups_a_col22"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision23" type="mesh" mesh="cups_a_col23"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision24" type="mesh" mesh="cups_a_col24"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision25" type="mesh" mesh="cups_a_col25"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision26" type="mesh" mesh="cups_a_col26"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision27" type="mesh" mesh="cups_a_col27"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision28" type="mesh" mesh="cups_a_col28"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision29" type="mesh" mesh="cups_a_col29"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision30" type="mesh" mesh="cups_a_col30"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision31" type="mesh" mesh="cups_a_col31"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision32" type="mesh" mesh="cups_a_col32"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision33" type="mesh" mesh="cups_a_col33"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision34" type="mesh" mesh="cups_a_col34"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_a_collision35" type="mesh" mesh="cups_a_col35"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="314.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
     </body>
   </worldbody>

assets/ycb/cups_f/cups_f.xml

@@ -5,7 +5,9 @@
   textured.obj -> trimesh recenter (xy centered on AABB, base at z=0) -> visual mesh
   + CoACD convex-decomposed collision (43 parts, threshold=0.1) so the
   concave shape is collision-true (cavity preserved / tool silhouette kept).
-  Density 700.0 kg/m^3 over the summed part volume 9.1e-05 m^3 -> mass 0.064 kg.
+  Density 231.0 kg/m^3 over the summed part volume 9.1e-05 m^3 -> mass 0.021 kg.
+  2026-08-13 mass fit: the 700 kg/m^3 fallback (this cup is absent from build_ycb_assets.py's
+  REAL_MASS_KG) put it at 3.0x the YCB 065-f stacking cup it scans. Refit to the real mass.
   Recentered AABB (m): size=[0.0807, 0.0809, 0.0722].
   License: YCB CC BY 4.0.
 -->
@@ -64,133 +66,133 @@
       <geom name="cups_f_visual" type="mesh" mesh="cups_f_mesh" material="cups_f_mat"
             group="2" contype="0" conaffinity="0" density="0" />
       <geom name="cups_f_collision0" type="mesh" mesh="cups_f_col0"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision1" type="mesh" mesh="cups_f_col1"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision2" type="mesh" mesh="cups_f_col2"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision3" type="mesh" mesh="cups_f_col3"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision4" type="mesh" mesh="cups_f_col4"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision5" type="mesh" mesh="cups_f_col5"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision6" type="mesh" mesh="cups_f_col6"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision7" type="mesh" mesh="cups_f_col7"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision8" type="mesh" mesh="cups_f_col8"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision9" type="mesh" mesh="cups_f_col9"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision10" type="mesh" mesh="cups_f_col10"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision11" type="mesh" mesh="cups_f_col11"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision12" type="mesh" mesh="cups_f_col12"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision13" type="mesh" mesh="cups_f_col13"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision14" type="mesh" mesh="cups_f_col14"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision15" type="mesh" mesh="cups_f_col15"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision16" type="mesh" mesh="cups_f_col16"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision17" type="mesh" mesh="cups_f_col17"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision18" type="mesh" mesh="cups_f_col18"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision19" type="mesh" mesh="cups_f_col19"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision20" type="mesh" mesh="cups_f_col20"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision21" type="mesh" mesh="cups_f_col21"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision22" type="mesh" mesh="cups_f_col22"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision23" type="mesh" mesh="cups_f_col23"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision24" type="mesh" mesh="cups_f_col24"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision25" type="mesh" mesh="cups_f_col25"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision26" type="mesh" mesh="cups_f_col26"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision27" type="mesh" mesh="cups_f_col27"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision28" type="mesh" mesh="cups_f_col28"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision29" type="mesh" mesh="cups_f_col29"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision30" type="mesh" mesh="cups_f_col30"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision31" type="mesh" mesh="cups_f_col31"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision32" type="mesh" mesh="cups_f_col32"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision33" type="mesh" mesh="cups_f_col33"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision34" type="mesh" mesh="cups_f_col34"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision35" type="mesh" mesh="cups_f_col35"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision36" type="mesh" mesh="cups_f_col36"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision37" type="mesh" mesh="cups_f_col37"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision38" type="mesh" mesh="cups_f_col38"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision39" type="mesh" mesh="cups_f_col39"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision40" type="mesh" mesh="cups_f_col40"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision41" type="mesh" mesh="cups_f_col41"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="cups_f_collision42" type="mesh" mesh="cups_f_col42"
-            density="700.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="231.0" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
             group="3" rgba="0 1 0 0" />
     </body>
   </worldbody>

assets/ycb/tennis_ball/tennis_ball.xml

@@ -7,6 +7,13 @@
   concave shape is collision-true (cavity preserved / tool silhouette kept).
   Density 358.7 kg/m^3 over the summed part volume 0.000162 m^3 -> mass 0.058 kg.
   Recentered AABB (m): size=[0.067, 0.067, 0.0665].
+  2026-08-13 restitution: solref dampratio 1.0 (critically damped) made the one prop here that is
+  SUPPOSED to bounce perfectly inelastic -- measured e = 0.000 against the ITF spec e = 0.73
+  (1.35 m rebound from 2.54 m). Authored 0.006 s / 0.30 gives a consistent e = 0.39 (0.40, 0.46
+  bounce-to-bounce). 0.73 is NOT reachable in this contact model at dt=0.002: below dampratio
+  0.28 the ball GAINS energy between bounces (e = 1.32, 1.63 measured) -- numerical, not physical.
+  NOTE: HAND_TRACK_SIM_PARAMS overwrites every geom solref, so this needs a backend opt-out to
+  take effect in a shipped run; audited with --profile authored.
   License: YCB CC BY 4.0.
 -->
 <mujoco model="ycb_tennis_ball">
@@ -23,10 +30,10 @@
       <geom name="tennis_ball_visual" type="mesh" mesh="tennis_ball_mesh" material="tennis_ball_mat"
             group="2" contype="0" conaffinity="0" density="0" />
       <geom name="tennis_ball_collision0" type="mesh" mesh="tennis_ball_col0"
-            density="358.7" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="358.7" friction="2.0 0.1 0.01" solref="0.006 0.30" priority="2"
             group="3" rgba="0 1 0 0" />
       <geom name="tennis_ball_collision1" type="mesh" mesh="tennis_ball_col1"
-            density="358.7" friction="2.0 0.1 0.01" solref="0.01 1" priority="2"
+            density="358.7" friction="2.0 0.1 0.01" solref="0.006 0.30" priority="2"
             group="3" rgba="0 1 0 0" />
     </body>
   </worldbody>