> ## Documentation Index
> Fetch the complete documentation index at: https://dadd.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Graspable Objects

> Configure objects that can be picked up by grippers

By default, MuJoCo contact parameters are tuned for collision, not grasping. Objects will slide out of grippers when lifted unless you increase friction and tighten the contact solver.

## Required Contact Parameters

Add these to your `SceneObject` definitions:

```tsx theme={null}
const config: SceneConfig = {
  src: "https://raw.githubusercontent.com/google-deepmind/mujoco_menagerie/main/franka_emika_panda/",
  sceneFile: "scene.xml",
  sceneObjects: [{
    name: "cube",
    geomName: "cube_geom",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.4, 0, 0.025],
    rgba: [0.9, 0.2, 0.15, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  }],
};
```

`geomName` is optional and defaults to `${name}_geom` for generated
`SceneObject` entries. Set it explicitly when you need stable contact telemetry
or camera-capture filters such as `hiddenGeomNames`.

## What Each Parameter Does

### `friction: '1.5 0.3 0.1'`

MuJoCo friction has three components: `[sliding, torsional, rolling]`. High sliding friction (1.5+) prevents the object from slipping through the gripper fingers.

### `condim: 4`

Contact dimensionality. The default (3) uses a friction pyramid, but `condim: 4` enables full elliptic friction cone with torsional friction — critical for maintaining grip when the object is rotated or lifted.

### `solref: '0.01 1'`

Solver reference parameters `[timeconst, dampratio]`. A short time constant (0.01) makes contacts stiff — the object doesn't slowly push through the gripper.

### `solimp: '0.95 0.99 0.001 0.5 2'`

Solver impedance `[dmin, dmax, width, midpoint, power]`. Tight values (0.95–0.99) mean the constraint is enforced aggressively with minimal penetration.

### `mass: 0.05`

Keep mass low. Heavy objects need proportionally stronger grip forces, which may exceed what position-controlled grippers can provide.

### `freejoint: true`

Required for the object to move freely in the scene. Without this, the object is welded to the world body.

## Without These Parameters

If you omit `condim: 4` and friction settings:

* The gripper can close around the object
* Lifting causes the object to slide out immediately
* The object may tunnel through the gripper fingers

## Full Example

```tsx theme={null}
const graspableObjects: SceneObject[] = [
  {
    name: "red_cube",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.4, 0.0, 0.025],
    rgba: [0.9, 0.2, 0.15, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  },
  {
    name: "green_cube",
    type: "box",
    size: [0.025, 0.025, 0.025],
    position: [0.35, 0.1, 0.025],
    rgba: [0.15, 0.8, 0.3, 1],
    mass: 0.05,
    freejoint: true,
    friction: "1.5 0.3 0.1",
    solref: "0.01 1",
    solimp: "0.95 0.99 0.001 0.5 2",
    condim: 4,
  },
];

const config: SceneConfig = {
  src: "https://raw.githubusercontent.com/google-deepmind/mujoco_menagerie/main/franka_emika_panda/",
  sceneFile: "scene.xml",
  gripperActuatorName: "gripper",
  sceneObjects: graspableObjects,
};
```

## Tips

* **Smaller objects** are easier to grasp — they fit better between gripper fingers
* **Lower mass** reduces the force needed to hold the object
* **Spheres** are harder to grasp than cubes — cubes have flat faces that create better contact patches
* Use `<ContactMarkers />` or `<Debug showContacts />` to visualize contacts and verify the gripper is making good contact