Legacy SKEL File Format
SKEL is DART’s legacy XML format for describing worlds, skeletons, robots, and environments. Its structure is based on SDFormat with a few DART-specific extensions.
DART 7 is removing SKEL support instead of redesigning SKEL as YAML. Use URDF,
SDFormat, or MJCF for new models, and migrate existing SKEL assets before
depending on DART 7. Use a release-6.* branch when you need compatibility with
old SKEL assets.
Legacy loading (DART 6 / migration only)
On DART 6, dart::io::readSkeleton could load a SKEL file from a file path.
Whole-world SKEL loading through the old public DART 6 API remains available on
release-6.* branches for parity work.
#include <dart/io/Read.hpp>
auto skeleton = dart::io::readSkeleton("path/to/legacy.skel");
Document structure
A SKEL document has a <skel> root containing one <world>. A world holds
<physics> settings and one or more <skeleton> elements; each skeleton is a
tree of <body> elements connected by <joint> elements.
<skel version="1.0">
<world>
<physics> ... </physics>
<skeleton>
<body> ... </body>
<joint> ... </joint>
</skeleton>
</world>
</skel>
Physics
<physics> configures the world:
<time_step>— integration step in seconds (e.g.0.001).<gravity>— gravity vector asx y z(e.g.0 -9.81 0).<collision_detector>— collision backend, e.g.dartorfcl.
Skeletons, bodies, and shapes
Each <skeleton name="..."> contains bodies and joints. A <body name="...">
may specify:
<transformation>— pose as three translation values followed by XYZ Euler angles (six numbers); defaults to the identity.<inertia>—<mass>, an inertial<offset>, and an optional<moment_of_inertia>.<visualization_shape>and<collision_shape>— each wraps a<geometry>(plus an optional<transformation>and, for visuals,<color>).
<geometry> supports box, sphere, ellipsoid, cylinder, capsule,
cone, pyramid, plane, multi_sphere, and mesh. Meshes are imported with
Assimp, so any Assimp-supported mesh format can be
referenced.
Joints
A <joint type="..." name="..."> connects a <parent> and a <child> body. Use
world as the parent name to attach a body to the world frame.
Supported type values are weld, revolute, prismatic, screw,
universal, ball, euler, translational, planar, and free.
For joints with movable axes, <axis> (and <axis2>, <axis3> for
multi-DOF joints) carry:
<xyz>— the axis direction.<dynamics>—<damping>,<friction>,<spring_stiffness>, and<spring_rest_position>.<limit>—<lower>and<upper>bounds.
The joint’s actuator attribute may be force (the default), passive,
servo, acceleration, velocity, or locked. Initial state can be set with
<init_pos> and <init_vel>.
A minimal example
A single free-floating box falling under gravity:
<?xml version="1.0" ?>
<skel version="1.0">
<world name="my world">
<physics>
<time_step>0.001</time_step>
<gravity>0 -9.81 0</gravity>
<collision_detector>dart</collision_detector>
</physics>
<skeleton name="box skeleton">
<body name="box">
<inertia>
<mass>1</mass>
</inertia>
<visualization_shape>
<geometry>
<box><size>0.1 0.05 0.1</size></box>
</geometry>
<color>0.8 0.3 0.3</color>
</visualization_shape>
<collision_shape>
<geometry>
<box><size>0.1 0.05 0.1</size></box>
</geometry>
</collision_shape>
</body>
<joint type="free" name="joint 1">
<parent>world</parent>
<child>box</child>
</joint>
</skeleton>
</world>
</skel>