Leap Motion C API  4.1.0
The API to the LeapC library.
Using LeapC

The typical tasks a client application performs to get tracking and status information include:

  • Setting up a message pump to handle tracking and status events
  • Requesting images.
  • Getting and setting policy settings.
  • Getting and setting configuration settings.
  • Synchronizing app and Leap Motion clocks and getting interpolated frames.
  • Closing the connection.

Getting Tracking Frames and Other Event Messages

You can use a message pump to efficiently monitor the LeapC message queue. The message queue provides tracking data and status information from the Leap Motion service.

A typical message pump is a function that runs in its own thread and calls LeapPollConnection() in a tight loop:

void serviceMessageLoop(){
eLeapRS result;
unsigned int timeout = 1000;
result = LeapPollConnection(connectionHandle, timeout, &msg);
//Handle message

The types of messages that can be returned by LeapPollConnection() are defined in the eLeapEventType enumeration. Your application should handle each event type appropriately and synchronize any data that needs to be used by other threads. LeapPollConnection() blocks until a message is available or until the specified timeout period has elapsed.

To start the message pump, you must first open a connection and start the message pump thread:

LEAP_CONNECTION* OpenConnection(){
eLeapRS result = LeapCreateConnection(NULL, &connectionHandle);
if(result == eLeapRS_Success){
result = LeapOpenConnection(connectionHandle);
if(result == eLeapRS_Success){
_isRunning = true;
pthread_create(&pollingThread, NULL, serviceMessageLoop, NULL);
return &connectionHandle;

The LEAP_CONNECTION struct returned by LeapCreateConnection() becomes your handle for referencing this connection in future LeapC API calls. (You can maintain more than one connection, but typically there is no reason to do so.) Pass a NULL value for the LEAP_CONNECTION_CONFIG struct. There aren't currently any config options to set.

LeapOpenConnection() performs client-side verification and starts any operations needed to connect to the service. However, any operations that may block are deferred until the first call to LeapPollConnection().

Note: you do not need to open a device to get tracking data. The device-related functions are only needed to get device properties or to change device state. See Reading Device Properties.

See the Callback Example.

Interpolating Frames

Instead of taking frames at measured at whatever rate the Leap Motion device happens to be producing them, you can request each frame at a particular time. LeapC will then interpolate the nearest measured frames to synthesize a frame at the desired time. In general, requesting frames further in the past will provide improve the perception of smoothness in the tracking data, but at a cost of increased latency. Conversely, requesting frames with a shorter delay will improve perceived responsiveness, but at a cost of increased jitteryness. You should tune the value as best suits your application; if you use the same code on desktop and mobile platforms (when mobile is supported), you will probably need different delay values for each.

A timestamp that is more recent than the timestamp most recently returned by LeapPollConnection() (e.g. for a tracking or image complete event) is considered to be in the future. If an interpolated frame is requested for a timestamp in the future, LeapC will use extrapolation instead of interpolation. If the same call was delayed so that the same timestamp value is no longer in the future, LeapC will choose to interpolate instead of extrapolate.

The LeapGetNow() function returns the current value of the clock used to generate frame timestamps. On Windows, this value is equal to QueryPerformanceCounter divided by QueryPerformanceFrequency, represented in microseconds. On all other platforms, it is the time since epoch of std::chrono::high_resolution_clock::now(), in microseconds. If your application time is derived from the same source as LeapGetNow(), you can calculate the desired frame timestamps directly and do not need to use the LEAP_CLOCK_REBASER object.

To get interpolated frames:

  1. Open the connection.
  2. Call LeapPollConnection() at least once to complete opening the connection.
  3. Create a LEAP_CLOCK_REBASER struct.
  4. Pass this struct to LeapCreateClockRebaser(), which initializes the struct.
  5. Every time you render a frame of graphics, call LeapUpdateRebase() to synchronize the Leap Motion and application clocks. The clocks will drift apart rapidly, otherwise.
  6. Translate the target timestamp of the interpolated frame by passing the application time to LeapRebaseClock(). LeapC writes the corresponding Leap clock time to the pLeapClock parameter.
  7. Call LeapGetFrameSize() to calculate the size of the buffer needed to require the tracking data at the desired point in time. (Frame size varies with the number of hands in view.)
  8. Allocate a buffer of the required size.
  9. Call LeapInterpolateFrame(), passing in the frame time and your buffer. LeapC writes the frame data to your buffer. This buffer can be treated as a LEAP_TRACKING_EVENT object to access the tracking data.
  10. Continue calling LeapUpdateRebase() and LeapInterpolateFrame() at your application frame rate.

Note that if you have an alternative means of converting your local timestamp units into the same clock domain as that used by LeapGetNow(), you do not need to use the clock rebasing functions and can call LeapInterpolateFrame() using the converted timestamp directly.

When getting interpolated frames, you can safely ignore tracking messages from LeapPollConnection() – but you should still service the message loop to get status and image completion messages.

See the Interpolated Frames Example.

Reading Device Properties

To read the properties of a device, you must get the list of known devices from LeapC using LeapGetDeviceList(). You can then get the specific properties of the devices in the returned list using LeapOpenDevice() and LeapGetDeviceInfo().

you must first open it with LeapOpenDevice(). For that you need a LEAP_DEVICE_REF struct, which you can get from LeapGetDeviceList(). However, you must pass an array to LeapGetDeviceList() to receive the device reference structs. So first, you must get the number of attached devices. You get the device count by calling LeapGetDeviceList() passing in a NULL pointer in place of the device array.

To get device information:

  1. Open the connection.
  2. Call LeapPollConnection() at least once to complete opening the connection.
  3. Get the device count by calling LeapGetDeviceList(), passing a NULL pointer instead of an array of device reference structs. LeapC writes the device count to the pnArray parameter.
  4. Create an array of LEAP_DEVICE_REF structs large enough to hold the number of attached devices.
  5. Call LeapGetDeviceList() again, passing in the correct size array and setting pnArray to the number of elements in your array. LeapC fills in the device reference struct for each attached device and updates pnArray to the number of records it actually wrote.

    Because LeapGetDeviceList() does not block, the returned list contains references for any devices known to LeapC at the time of the call. This local list can be different than the list of devices know to the Leap Motion service.

Once you have a LEAP_DEVICE_REF struct for a device, you can read its properties:

  1. Create a LEAP_DEVICE struct, which will become your device handle.
  2. Call LeapOpenDevice(), passing in the LEAP_DEVICE_REF and LEAP_DEVICE structs.
  3. Create a LEAP_DEVICE_INFO struct.
  4. Allocate a buffer large enough to hold the serial number string.
  5. Call LeapGetDeviceInfo() to fill in the device info struct.
  6. If the serial number buffer is too small, then LeapGetDeviceInfo() returns a value of eLeapRS_InsufficientBuffer. It also sets the LEAP_DEVICE_INFO::serial_length field to the required buffer lenght. You can use this value to allocate a properly sized buffer and call the function again.
  7. Call LeapCloseDevice() when done.
LEAP_DEVICE_INFO deviceProperties;
deviceProperties.serial_length = 1;
deviceProperties.serial = malloc(deviceProperties.serial_length);
result = LeapGetDeviceInfo(deviceHandle, &deviceProperties);
deviceProperties.serial = malloc(deviceProperties.serial_length);
result = LeapGetDeviceInfo(deviceHandle, &deviceProperties);

Controlling Policies

Policies control whether certain features should be used by an application. Generally, these features affect general usability or change service behavior that will affect all applications on a system.

See Policy for a description of the current policies.

Reading and writing policies

To get and set policies:

  1. Open the connection.
  2. Call LeapPollConnection() at least once.
  3. Combine the flags that you want to set by bitwise ANDing elements of the eLeapPolicyFlag enumeration.
  4. Likewise, combine the flags that you want to clear by bitwise ANDing elements of the eLeapPolicyFlag enumeration.
  5. Call LeapSetPolicyFlags(), passing in your set and clear policy combinations.
  6. When the policy change is processed by the service, LeapC adds a LEAP_POLICY_EVENT message to the queue. You can compare LEAP_POLICY_EVENT::current_policy with the desired policy to verify that the policy change was "approved."

To read the current policies without making changes, set both the set and the clear parameters to 0 when calling LeapSetPolicyFlags(). This will generate a LEAP_POLICY_EVENT message containing the current policies.

Note that the other Leap Motion language bindings support a slightly different set of polcies and are based on libraries that enforce these policies differently. For more informations, see https://developer.leapmotion.com/documentation/cpp/devguide/Leap_Policies.html.

Getting and Setting Configuration Values

The Leap Motion service has several configuration settings. A few are useful to individual applications. For a description of these settings, see Configuration.

Reading a confg setting

To read a config value:

  1. Open the connection.
  2. Call LeapPollConnection() at least once.
  3. Call LeapRequestConfigValue(), providing the name of the desired config value as a string (char*). LeapC provides a request ID value that you can use to correlate the response to the request.
  4. Continue calling LeapPollConnection() to service the message loop.
  5. When available, a LEAP_CONFIG_RESPONSE_EVENT message is returned from the message loop. The value is returned in a variant structure that defines a union of the different supported data types. Use the LEAP_VARIANT::type field to decide how to interpret the data.

Reading a configuration value requires a round-trip to the Leap Motion service. The routine used to initiate this round-trip does not block, but it also cannot return the current value in the same call. The requested value will be passed back to your application as an event in LeapPollConnection.

Writing a config setting

To write a config value:

  1. Open the connection.
  2. Call LeapPollConnection() at least once.
  3. Create a LEAP_VARIANT struct, specifying the data type and a value.
  4. Call LeapSaveConfigValue(), providing the config setting name key and variant structure. LeapC provides a request ID that you can use to correltae the response to the request.
  5. Continue calling LeapPollConnection() to service the message loop.
  6. When available, a LEAP_CONFIG_CHANGE_EVENT message is returned from the message loop. The LEAP_CONFIG_CHANGE_EVENT::status field indicates whether the change request was successful.