SteamVR Skeletal Input
The Skeletal Input system allows drivers for each type of controller to provide applications with an animated skeleton of the user's hand to the best level of fidelity that its sensors are able to detect. Developers can then reduce their need to create custom hand animations for each controller they plan to support, and can instead use the animation provided by the current driver in conjunction with their own game-specific animations. Skeletal data can be bound to input actions, just like other input. The animation can be requested to be as accurate as possible (which will often mean that the range of motion of the hand will conform to the shape of the controller), or to represent the full range of motion of an empty human hand; in some cases these two options will effectively be the same. The animation can also be provided as an additive offset from the tightest grip the user is able to make when grasping the controller, so that it can be layered on top of animations that the developer has made. Finally, functionally it provided to compress and decompress the skeletal pose for the current frame to allow for more efficient transfer to remote clients in a networked multi-user application.
To use the Skeletal Input API, first add a "skeletal action" entry for each hand to the actions section of your application's action manifest file. Make sure that the 'type' is skeleton, and that it has an additional 'skeleton' field to define which skeleton the action should use. The choices for skeleton are either "/skeleton/hand/left" or "/skeleton/hand/right". For example:
{
"name": "/actions/demo/in/lefthand_anim",
"type": "skeleton",
"skeleton": "/skeleton/hand/left"
}Next, you need to add a binding for this action to each controller. You can do this through the SteamVR binding UI (preferred).
Call vr::VRInput()->GetActionSetHandle() as described in the SteamVR Input documentation to get a handle to the skeletal action that you defined in the manifest. This will later be used to retrieve the skeletal pose associated with the action.
Each frame your application should already be calling vr::VRInput()->UpdateActionState() to update the status of all the actions that have been defined in your action manifest. This will update the skeletal input too. To retrieve the pose, first call vr::VRInput()->GetSkeletalActionData() with the 'VRActionHandle_t' for your skeletal action and a pointer to a InputSkeletalActionData_t object. If InputSkeletalActionData_t.bActive is true, that means there is bone transform data that is ready. InputSkeletalActionData_t.boneCount will contain the number of bones in the skeleton; you will need an array of at least this many vr::VRBoneTransform_t to hold the bone transforms.
To retrieve the bone transforms, call vr::VRInput()->GetSkeletalBoneData() with the options for the format you want to receive the data in and buffer to place the transform in.
Instead of using vr::VRInput()->GetSkeletalBoneData() to retrieve the full bone transforms, can instead call vr::VRInput()->GetSkeletalBoneDataCompressed() to retrieve a compressed binary blob suitable for networking. Note that variable compression is used, so the size of the buffer will fluctuate as the hand pose changes. Once you have transmitted the compressed buffer to a remote client, the bone transforms can be retrieve by passing the buffer to vr::VRInput()->DecompressSkeletalBoneData(). Note that the remote client may not have the same type of controller as the client that sent the buffer, so the compression and decompression is designed to be used independ of any particular driver.
The root bone transform provided by the API will always be in relation to an offset an the input device, and not a positioned in the VR space. To get the location in VR space to place the skeleton so that it matches the location of the user's hand, you must retrieve it from the API the same way that you retrieve the locations of the controllers themselves, using vr::VRInput()->GetPoseActionData(), and passing in the value InputSkeletalActionData_t.activeOrigin that you received from calling GetSkeletalActionData().
Detailed documentation on the Skeletal Input API can be found with the SteamVR Input API documentation here.
Detailed information on the skeleton used by the Skeletal Input system can be found here: Hand Skeleton