MeshCat

October 16, 2025 ยท View on GitHub

MeshCat is a remotely-controllable 3D viewer, built on top of three.js. The MeshCat viewer runs in a browser and listens for geometry commands over WebSockets. This makes it easy to create a tree of objects and transformations by sending the appropriate commands over the websocket.

The MeshCat viewer is meant to be combined with an interface in the language of your choice. Current interfaces are:

API

MeshCat can be used programmatically from JS or over a WebSocket connection.

Programmatic API

To create a new MeshCat viewer, use the Viewer constructor:

let viewer = new MeshCat.Viewer(dom_element);

where dom_element is the div in which the viewer should live. The primary interface to the viewer is the handle_command function, which maps directly to the behaviors available over the WebSocket.

Viewer.handle_command(cmd)
Handle a single command and update the viewer. cmd should be a JS object with at least the field type. Available command types are:
set_object
Set the 3D object at a given path in the scene tree from its JSON description. Any transforms previously applied to that path will be preserved and any children of that path will continue to exist. To remove transforms and delete all children from a given path, you should send a delete command first (see below).

Internally, we append a final path segment, <object>, to the provided path before creating the object. This is done because clients may disagree about what the "intrinsic" transform of a particular geometry is (for example, is a "Box" centered on the origin, or does it have one corner at the origin?). Clients can use the matrix field of the JSON object to store that intrinsic transform, and that matrix will be preserved by attaching it to the <object> path. Generally, you shouldn't need to worry about this: if you set an object at the path /meshcat/foo, then you can set the transform at /meshcat/foo and everything will just work.

Additional fields:

path
A "/"-separated string indicating the object's path in the scene tree. An object at path "/foo/bar" is a child of an object at path "/foo", so setting the transform of (or deleting) "/foo" will also affect its children.
object
The Three.js Object, with its geometry and material, in JSON form as a JS object. The nominal format accepted is anything that ObjectLoader can handle (i.e., anything you might get by calling the toJSON() method of a Three.js Object3D).

Beyond the nominal format, Meshcat also offers a few extensions for convenience:

  • Within the geometries stanza, the type field can be set to "_meshfile_geometry" to parse using a mesh file format. In this case, the geometry must also have a field named format set to one of "obj" or "dae" or "stl" and a field named "data" with the string contents of the file.
  • Within the geometries stanza, the type field can be set to "LineGeometry" to create lines with configurable width (works in all modern browsers). See the Line2 example below for details.
  • Within the materials stanza, the type field can be set to "_text" to use a string as the texture (i.e., a font rendered onto an image). In this case, the material must also have fields named font_size (in pixels), font_face (a string), and text (the words to render into a texture).
  • Within the materials stanza, the type field can be set to "LineMaterial" to create materials for Line2 objects with configurable line width. See the Line2 example below for details.
  • Within the inner object stanza (i.e., the object with a uuid, not the object argument to set_object), the type field can be set to "_meshfile_object" to parse using a mesh file format. In this case, the geometries and materials and geometry: {uuid} and material: {uuid} are all ignored, and the object must have a field named format set to one of "obj" or "dae" or "stl" and a field named "data" with the string contents of the file. When the format is obj, the object may also have a field named mtl_library with the string contents of the associated mtl file.
  • Within the inner object stanza, the type field can be set to "Line2" to render lines with configurable width using LineGeometry and LineMaterial. See the Line2 example below for details.

Example (nominal format): 

{
    type: "set_object",
    path: "/meshcat/boxes/box1",
    object: {
        metadata: {version: 4.5, type: "Object"},
        geometries: [
            {
                uuid: "cef79e52-526d-4263-b595-04fa2705974e",
                type: "BoxGeometry",
                width: 1,
                height: 1,
                depth:1
            }
        ],
        materials: [
            {
                uuid: "0767ae32-eb34-450c-b65f-3ae57a1102c3",
                type: "MeshLambertMaterial",
                color: 16777215,
                emissive: 0,
                side: 2,
                depthFunc: 3,
                depthTest: true,
                depthWrite: true
            }
        ],
        object: {
            uuid: "00c2baef-9600-4c6b-b88d-7e82c40e004f",
            type: "Mesh",
            geometry: "cef79e52-526d-4263-b595-04fa2705974e",
            material: "0767ae32-eb34-450c-b65f-3ae57a1102c3"
        }
    }
}
Note the somewhat indirect way in which geometries and materials are specified. Each Three.js serialized object has a list of geometries and a list of materials, each with a UUID. The actual geometry and material for a given object are simply references to those existing UUIDs. This enables easy re-use of geometries between objects in Three.js, although we don't really rely on that in MeshCat. Some information about the JSON object format can be found on the Three.js wiki.

Example (_meshfile_geometry): 

{
    type: "set_object",
    path: "/some/file/geometry",
    object: {
        metadata: {
            version: 4.5,
            type: "Object"
        },
        geometries: [
            {
                type: "_meshfile_geometry",
                uuid: "4a08da6b-bbc6-11ee-b7a2-4b79088b524d",
                format: "obj",
                data: "v -0.06470900 ..."
            }
        ],
        images: [
            {
                uuid: "c448fc3a-bbc6-11ee-b7a2-4b79088b524d",
                url: "data:image/png;base64,iVBORw0KGgoAAA=="
            }
        ],
        textures: [
            {
                uuid: "d442ea92-bbc6-11ee-b7a2-4b79088b524d",
                wrap: [1001, 1001],
                repeat: [1, 1],
                image: "c448fc3a-bbc6-11ee-b7a2-4b79088b524d"
            }
        ],
        materials: [
            {
                uuid: "4a08da6e-bbc6-11ee-b7a2-4b79088b524d",
                type: "MeshLambertMaterial",
                color: 16777215,
                reflectivity: 0.5,
                map: "d442ea92-bbc6-11ee-b7a2-4b79088b524d"
            }
        ],
        object: {
            uuid: "4a08da6f-bbc6-11ee-b7a2-4b79088b524d",
            type: "Mesh",
            geometry: "4a08da6b-bbc6-11ee-b7a2-4b79088b524d",
            material: "4a08da6e-bbc6-11ee-b7a2-4b79088b524d"
        }
    }
}

Example (_text): 

{
    type: "set_object",
    path: "/meshcat/text",
    object: {
        metadata: {
            version: 4.5,
            type: "Object"
        },
        geometries: [
            {
                uuid: "6fe70119-bba7-11ee-b7a2-4b79088b524d",
                type: "PlaneGeometry",
                width: 8,
                height: 8,
                widthSegments: 1,
                heightSegments: 1
            }
        ],
        textures: [
            {
                uuid: "0c8c99a8-bba8-11ee-b7a2-4b79088b524d",
                type: "_text",
                text: "Hello, world!",
                font_size: 300,
                font_face: "sans-serif"
            }
        ],
        materials: [
            {
                uuid: "6fe7011b-bba7-11ee-b7a2-4b79088b524d",
                type: "MeshPhongMaterial",
                transparent: true,
                map: "0c8c99a8-bba8-11ee-b7a2-4b79088b524d",
            }
        ],
        object: {
            uuid: "6fe7011c-bba7-11ee-b7a2-4b79088b524d",
            type: "Mesh",
            geometry: "6fe70119-bba7-11ee-b7a2-4b79088b524d",
            material: "6fe7011b-bba7-11ee-b7a2-4b79088b524d",
        }
    }
}

Example (_meshfile_object): 

{
    type: "set_object",
    path: "/meshcat/wavefront_file",
    object: {
        metadata: {version: 4.5, type: "Object"},
        object: {
            uuid: "00c2baef-9600-4c6b-b88d-7e82c40e004f",
            type: "_meshfile_object",
            format: "obj",
            data: "mtllib ./cube.mtl\nusemtl material_0\nv 0.0 0.0 0.0 ...",
            mtl_library: "newmtl material_0\nKa 0.2 0.2 0.2\n ...",
            resources: {"cube.png": "data:image/png;base64,iV ..."}
        }
    }
}

Check test/meshfile_object_obj.html for the full demo.

Example (Line2 with LineGeometry and LineMaterial):

Modern browsers don't support the GL_LINEWIDTH parameter, so standard Line objects won't show varying line widths. MeshCat supports Line2 objects with LineGeometry and LineMaterial for lines with configurable properties.

{
    type: "set_object",
    path: "/meshcat/fatline",
    object: {
        metadata: {version: 4.5, type: "Object"},
        geometries: [
            {
                uuid: "6fe70119-bba7-11ee-b7a2-4b79088b524d",
                type: "LineGeometry",
                position: {
                    array: new Float32Array([0, 0, 0, 1, 0, 0, 1, 1, 0])
                },
                color: {
                    array: new Float32Array([1, 0, 0, 0, 1, 0, 0, 0, 1])
                }
            }
        ],
        materials: [
            {
                uuid: "6fe7011b-bba7-11ee-b7a2-4b79088b524d",
                type: "LineMaterial",
                color: 16777215,
                linewidth: 5.0,
                vertexColors: true,
                dashed: false,
                worldUnits: false
            }
        ],
        object: {
            uuid: "6fe7011c-bba7-11ee-b7a2-4b79088b524d",
            type: "Line2",
            geometry: "6fe70119-bba7-11ee-b7a2-4b79088b524d",
            material: "6fe7011b-bba7-11ee-b7a2-4b79088b524d"
        }
    }
}

LineGeometry supports these attributes:

  • position: Float32Array of vertex positions (x, y, z, x, y, z, ...)
  • color: (optional) Float32Array of per-vertex RGB colors (r, g, b, r, g, b, ...)

LineMaterial supports the exact same attributes as documented in the Three.js LineMaterial docs.

Check test/fatlines.html for a full demo.

set_transform
Set the homogeneous transform for a given path in the scene tree. An object's pose is the concatenation of all of the transforms along its path, so setting the transform of "/foo" will move the objects at "/foo/box1" and "/foo/robots/HAL9000".

Additional fields:

path
A "/"-separated string indicating the object's path in the scene tree. An object at path "/foo/bar" is a child of an object at path "/foo", so setting the transform of (or deleting) "/foo" will also affect its children.
matrix
The homogeneous transformation matrix, given as a 16-element Float32Array in column-major order.
Example: 
{
    type: "set_transform",
    path: "/meshcat/boxes",
    matrix: new Float32Array([1, 0, 0, 0,
                            0, 1, 0, 0,
                            0, 0, 1, 0,
                            0.5, 0.0, 0.5, 1])
}
delete
Delete the object at the given path as well as all of its children.

Additional fields:

path
A "/"-separated string indicating the object's path in the scene tree. An object at path "/foo/bar" is a child of an object at path "/foo", so setting the transform of (or deleting) "/foo" will also affect its children.
Example: 
{
    type: "delete",
    path: "/meshcat/boxes"
}
set_property
Set a single named property of the object at the given path. If no object exists at that path, an empty one is automatically created.

Note: as we append an extra path element with the name <object> to every item created with set_object, if you want to modify a property of the object itself, rather than the group containing it, you should ensure that your path is of the form /meshcat/foo/<object>

Additional fields:

property
The name of the property to set, as a string. The following properties are convenience properties. Meshcat provides a mapping from these *names* to various properties contained throughout its scene graph:
  • visible: bool
  • position: number[3]
  • quaternion: number[4]
  • scale: number[3]
  • color: number[4]
  • opacity: number (this is the same as the 4th element of color)
  • modulated_opacity: number
  • top_color: number[3] (only for the Background)
  • bottom_color: number[3] (only for the Background)
Properties not on the above list will be set directly on the THREE.Object3D object. This provides a powerful capability to customize the scene, but should be considered an advanced usage -- you're on your own to avoid any unwanted side-effects.

Properties can be *chained*. For example, for an object with a phong material (MeshPhongMaterial), we may want to tweak its shininess, making it duller. Shininess is not a property of the object itself, but the object's material. There is no *path* to that material, but the property name can include a property name chain, e.g., `material.shininess`. While setting the property, the chained properties will be evaluated in sequence, such that the final name in the chain is the property that receives the `value`.

As noted, specifying a `path` that doesn't exist creates that path. However, specifying a property that doesn't exist does *not* create that property. If a name in the property chain is missing, an error message will be printed to the console and no value will be assigned. This is not a no-op per se. If the `path` led to the implicit creation of a new folder and object, that pair will still be in place.

More subtly, if the property name chain has an interior name (e.g., the `foo` in `material.foo.color`) that exists but is not an object and does not have properties (such as if `foo` were a `Number`), then, again, an error gets written to the console and no value will be assigned.

Finally, property chains can include arrays, such as `"children[1].material.specular"`. The index will be evaluated as a property (with all of the potential consequences as outlined above). In error messages, it may be reported as `children.1` instead of `children[1]`.

value
The new value.
Example 1: 
{
    type: "set_property",
    path: "/Cameras/default/rotated/<object>",
    property: "zoom",
    value: 2.0
}
Example 2: 
{
    type: "set_property",
    path: "/Lights/DirectionalLight/<object>",
    property: "intensity",
    value: 1.0
}
Example 3 (chained properties): 
{
    type: "set_property",
    path: "/Lights/SpotLight/<object>",
    property: "shadow.radius",
    value: 1.0
}
set_animation
Create an animation of any number of properties on any number of objects in the scene tree, and optionally start playing that animation.

Additional fields:

animations
A list of objects, each with two fields:
path
The path to the object whose property is begin animated. As with set_property above, you will need to append <object> to the path to set an object's intrinsic property.
clip
A Three.js AnimationClip in JSON form. The clip in turn has the following fields:
fps
The frame rate of the clip
name
A name for this clip. Not currently used.
tracks
The tracks (i.e. the properties to animate for this particular object. In Three.js, it is possible for a track to specify the name of the object it is attached to, and Three.js will automatically perform a depth-first search for a child object with that name. We choose to ignore that feature, since MeshCat already has unambiguous paths. So each track should just specify a property in its name field, with a single "." before that property name to signify that it applies to exactly the object given by the path above.

Each track has the following fields:

name
The property to be animated, with a leading "." (e.g. ".position")
type
The Three.js data type of the property being animated (e.g. "vector3" for the position property)
keys
The keyframes of the animation. The format is a list of objects, each with a field time (in frames) and value indicating the value of the animated property at that time.
options
Additional options controlling the animation. Currently supported values are:
play
Boolean [true]. Controls whether the animation should play immediately.
repetitions
Integer [1]. Controls the number of repetitions of the animation each time you play it.
Example: 
{
    type: "set_animation",
    animations: [{
            path: "/Cameras/default",
            clip: {
                fps: 30,
                name: "default",
                tracks: [{
                    name: ".position"
                    type: "vector3",
                    keys: [{
                        time: 0,
                        value: [0, 1, .3]
                    },{
                        time: 80,
                        value: [0, 1, 2]
                    }],
                }]
            }
        },{
            path: "/meshcat/boxes",
            clip: {
                fps: 30,
                name: "default",
                tracks: [{
                    name: ".position"
                    type: "vector3",
                    keys: [{
                        time: 0,
                        value: [0, 1, 0]
                    },{
                        time: 80,
                        value: [0, -1, 0]
                    }],
                }]
            }
        }],
    options: {
        play: true,
        repetitions: 1
    }
}
set_target
Set the target of the 3D camera, around which it rotates. This is expressed in a left-handed coordinate system where y is up.

Example:

{
    "type": "set_target",
    value: [0., 1., 0.]
}
This sets the camera target to `(0, 1, 0)`
capture_image
Capture an image from the viewport. At the moment it will return the image at a provided resolution (by default 1920x1080). 
{
    "type": "capture_image",
    "xres": 1920,
    "yres": 1080
}
This sets the camera target to `(0, 1, 0)`
set_render_callback
Each render loop updates the camera, renders the scene and updates animation. Between updating the camera and rendering the scene, the Viewer invokes a user-configurable callback. This callback can be used to execute arbitrary code per rendered frame.

The callback is evaluated in a scope such that the Viewer instance is available as `this`. In declaring the command, the callback should be a *string* that gets evaluated into a function. Passing the string "null" will restore the render callback to being its default no-op function.

For example, the following is an example callback that dispatches the camera's pose in the world to the open websocket connection. 
{
    "type": "set_render_callback",
    "callback": `() => {
        if (this.is_perspective()) {
            if (this.connection.readyState == 1 /* OPEN */) {
                this.connection.send(msgpack.encode({
                    'type': 'camera_pose',
                    'camera_pose': this.camera.matrixWorld.elements
                }));
            }
        }
    }`
}
This dispatches the camera's pose in the world to the websocket connection.

WebSocket API

Viewer.connect(url)
Set up a web socket connection to a server at the given URL. The viewer will listen for messages on the socket as binary MsgPack blobs. Each message will be decoded using msgpack.decode() from msgpack-javascript and the resulting object will be passed directly to Viewer.handle_command() as documented above.

Note that we do support the MsgPack extension types listed in msgpack-javascript#extension-types, with additional support for the Float32Array type which is particularly useful for efficiently sending point data and for Uint32Array, Uint8Array, and Int32Array.

Setting opacity

Objects can have their opacity changed in the obvious way, i.e.:

{
    type: "set_property",
    path: "/path/to/my/geometry",
    property: "opacity",
    value: 0.5
}

This would assign the opacity value 0.5 to all of the materials found rooted at "/path/to/my/geometry". (That means using the path "/path" could affect many geometries.) However, this will overwrite whatever opacity the geometry had inherently (i.e., from the material defined in the mesh file); a transparent object could become more opaque.

Meshcat offers a pseudo property "modulated_opacity". Meshcat remembers an object's inherent opacity and, by setting this value, sets the rendered opacity to be the product of the inherent and modulated opacity value. The corresponding command would be:

{
    type: "set_property",
    path: "/path/to/my/geometry",
    property: "modulated_opacity",
    value: 0.5
}

Setting "modulated_opacity" to 1 will restore the geometry's original opacity. This does not introduce a queryable modulated_opacity property on any material. This is what makes it a "pseudo" property. The same tree-based scope of effect applies to "modulated_opacity" as with "opacity" (and also the "color" property).

Meshcat always remembers the inherent opacity value. So, if you've overwritten the value (via setting "opacity" or "color"), you can restore it by setting the "modulated_opacity" value to 1.0.

Useful Paths

The default MeshCat scene comes with a few objects at pre-set paths. You can replace, delete, or transform these objects just like anything else in the scene.

/Lights/DirectionalLight
The single directional light in the scene.
/Lights/AmbientLight
The ambient light in the scene.
/Grid
The square grid in the x-y plane, with 0.5-unit spacing.
/Axes
The red, green, and blue XYZ triad at the origin of the scene (invisible by default, click on "open controls" in the upper right to toggle its visibility).
/Cameras
The camera from which the scene is rendered (see below for details)
/Background
The background texture, with properties for "top_color" and "bottom_color" as well as a boolean "visible".
/Render Settings/
Contains properties for how the overall scene renders in your current session. This is not a 3D element in the scene, but provides parameters for configuring the renderer.

Camera Control

The camera is just another object in the MeshCat scene, so you can move it around with set_transform commands like any other object. You can also use set_target to change the camera's target. Please note that replacing the camera with set_object is not currently supported (but we expect to implement this in the future).

Controlling the camera is slightly more complicated than moving a single object because the camera actually has two important poses: the origin about which the camera orbits when you click-and-drag with the mouse, and the position of the camera itself. In addition, cameras and controls in Three.js assume a coordinate system in which the Y axis is upward. In robotics, we typically have the Z axis pointing up, and that's what's done in MeshCat. To account for this, the actual camera lives inside a few path elements:

/Cameras/default/rotated/<object>

The /rotated path element exists to remind users that its transform has been rotated to a Y-up coordinate system for the camera inside.

There is one additional complication: the built-in orbit and pan controls (which allow the user to move the view with their mouse) use the translation of only the intrinsic transform of the camera object itself to determine the radius of the orbit. That means that, in practice, you can allow the user to orbit by setting the position property at the path /Cameras/default/rotated/<object> to a nonzero value like [2, 0, 0], or you can lock the orbit controls by setting the position property at that path to [0, 0, 0]. Remember that whatever translation you choose is in the rotated, Y-up coordinate system that the Three.js camera expects. We're sorry.

Examples

To move the camera's center of attention to the point [1, 2, 3], while still allowing the user to orbit and pan manually, we suggest setting the transform of the /Cameras/default path to whatever center point you want and setting the position property of /Cameras/default/rotated/<object> to [2, 0, 0]. That means sending two commands:

{
    type: "set_transform",
    path: "/Cameras/default",
    matrix: new Float32Array([1, 0, 0, 0,
                              0, 1, 0, 0,
                              0, 0, 1, 0,
                              1, 2, 3, 1])  // the translation here is the camera's
                                            // center of attention
},
{
    type: "set_property",
    path: "/Cameras/default/rotated/<object>",
    property: "position",
    value: [2, 0, 0] // the offset of the camera about its point of rotation
}

To move the camera itself to the point [1, 2, 3] and lock its controls, we suggest setting the transform of /Cameras/default to the exact camera pose and setting the position property of /Cameras/default/rotated/<object> to [0, 0, 0]:

{
    type: "set_transform",
    path: "/Cameras/default",
    matrix: new Float32Array([1, 0, 0, 0,
                              0, 1, 0, 0,
                              0, 0, 1, 0,
                              1, 2, 3, 1])  // the translation here is now the camera's
                                            // exact pose
},
{
    type: "set_property",
    path: "/Cameras/default/rotated/<object>",
    property: "position",
    value: [0, 0, 0] // set to zero to lock the camera controls
}

Developing MeshCat

The MeshCat javascript sources live in src/index.js. We use webpack to bundle up MeshCat with its Three.js dependencies into a single javascript bundle. If you want to edit the MeshCat source, you'll need to regenerate that bundle. Fortunately, it's pretty easy:

Option 1: Use Docker

If you have Docker + BuildKit installed, then you can run rebuild/rebuild.sh to hermetically regenerate the dist files. The very first time you run it, it might take a little time to download all of the relevant files. Subsequent runs should be much faster (~5-10 seconds).

This option offers the easiest way to rebuild, with the drawback that "watch" mode of webpack (to automatically rebuild every time you save) is unavailable.

To install Docker + BuildKit on Ubuntu, run:

sudo apt install docker.io docker-buildx

To run as non-root, you will probably also need to update the docker group permissions:

https://docs.docker.com/engine/install/linux-postinstall/

Option 2: Install Yarn + NPM tools

This option offers a convenient "watch" mode during an edit-test development cycle, but if you're not careful when installing the yarn+node+npm tools you might damage other software on your computer.

  1. Install yarn. This should also install node and npm. Try running yarn -v and npm -v to make sure those programs are installed.
  2. Run yarn
    • This will read the project.json file and install all the necessary javascript dependencies.
  3. Run yarn build
    • This will run webpack and create the bundled output in dist/main.min.js. The build script will also watch for changes to the MeshCat source files and regenerate the bundle whenever those source files change.
  4. Try it out! You can load the bundled main.min.js in your own application, or you can open up dist/index.html in your browser.

Note that due to caching, you may need to do a hard refresh (shift+F5 or ctrl+shift+R) in your browser to reload the updated javascript bundle.