04. Hello Floor

Your first haptic effect: a virtual horizontal floor that pushes back when the cursor presses into it. The force is a simple penalty spring — stiffness × penetration_depth — applied along Z via set_cursor_force.

What you'll learn:

• Using set_cursor_force to apply a penalty-spring force
• Reading cursor_position and computing force in real time
• (Python) Setting a workspace preset (arm_front_centered) so the origin sits at workspace centre
• (Python) Interactive keyboard tuning of floor height and stiffness via the keyboard package

Workflow

1. Open a WebSocket to ws://localhost:10001 and wait for the first state frame.
2. On the first frame: register the session profile. The Python variant additionally sends configure.preset: arm_front_centered so the origin sits at the middle of the workspace; the C++ variants use whatever configuration is already active on the device.
3. On every frame: read cursor_position.z, compute force_z = max(0, (floor_pos - z) * stiffness), and send it back as a set_cursor_force command.
4. Subsequent ticks send only the force command — the session profile is a one-shot handshake.
5. (Python) Each tick also polls keyboard arrows and updates floor_pos / stiffness live.

Parameters

Name	Default	Purpose
floor_pos	0.10 m	Z-coordinate of the virtual floor plane
stiffness	1000 N/m	Spring constant (1 mm penetration → 1 N)
PRINT_EVERY_MS	100–200	Telemetry throttle
Session profile name	co.haply.inverse.tutorials:hello-floor	Identifies this simulation in Haply Hub

Interactive (Python only)

The Python variant uses the keyboard package (elevated privileges required on Linux):

• ↑ / ↓ — raise / lower the floor plane
• ← / → — decrease / increase stiffness
• R — reset to defaults

Extending the force

Forces are additive on the service tick — summed across sources before being sent to the device. A tutorial like this can coexist with other force producers without either blocking the other.

State fields read

From data.inverse3[i].state:

• cursor_position.z — vec3, used to compute penetration depth
• current_cursor_force — reported for telemetry

Send / receive

Each tick: read the cursor's Z, compute force_z = max(0, (floor_pos - z) * stiffness), and send a set_cursor_force. The first outgoing message also carries the session profile (all variants) and, for Python, configure.preset: arm_front_centered.

Python

Single async loop. Handshake on first frame carries profile + configure.preset so floor_pos = 0.1 aligns with workspace-centre coordinates.

async with websockets.connect(URI) as websocket:
    while True:
        msg  = await websocket.recv()
        data = json.loads(msg)

        if first_message:
            first_message = False
            device_id = data["inverse3"][0]["device_id"]
            # Handshake: profile + preset (one-shot)
            request_msg = {
                "session": {"configure": {"profile": {"name": "co.haply.inverse.tutorials:hello-floor"}}},
                "inverse3": [{
                    "device_id": device_id,
                    "configure": {"preset": {"preset": "arm_front_centered"}}
                }]
            }
        else:
            # Per tick: compute force along Z, send set_cursor_force
            z = data["inverse3"][0]["state"]["cursor_position"]["z"]
            force_z = 0.0 if z > floor_pos else (floor_pos - z) * stiffness
            request_msg = {
                "inverse3": [{
                    "device_id": device_id,
                    "commands": {"set_cursor_force":
                        {"vector": {"x": 0.0, "y": 0.0, "z": force_z}}}
                }]
            }

        await websocket.send(json.dumps(request_msg))

C++ (nlohmann)

libhv callback model — onmessage runs on the WebSocket I/O thread; the main thread blocks on ENTER. C++ variants ship the minimal handshake (session profile only, no preset).

ws.onmessage = [&](const std::string &msg) {
    const json data = json::parse(msg);

    // If no Inverse3 yet, ask the service to re-send the full state
    if (!data.contains("inverse3") || data["inverse3"].empty()) {
        ws.send(R"({"session":{"force_render_full_state":{}}})");
        return;
    }

    json request = {};
    if (first_message) {
        first_message = false;
        request["session"] = {{"configure", {{"profile",
            {{"name", "co.haply.inverse.tutorials:hello-floor"}}}}}};
    }

    request["inverse3"] = json::array();
    for (auto &el : data["inverse3"].items()) {
        const float z = el.value()["state"]["cursor_position"]["z"].get<float>();
        const float force_z = z > floor_pos ? 0.0f : (floor_pos - z) * stiffness;
        request["inverse3"].push_back({
            {"device_id", el.value()["device_id"]},
            {"commands", {{"set_cursor_force",
                {{"vector", {{"x", 0.0}, {"y", 0.0}, {"z", force_z}}}}}}},
        });
    }
    ws.send(request.dump());
};
ws.open("ws://localhost:10001");
while (std::cin.get() != '\n') {}  // block main thread

C++ (Glaze)

Same libhv callback model — only the body changes. Typed structs for both state and commands. std::optional<session_cmd> holds the one-shot profile — Glaze omits it from the serialized JSON when unset.

// Struct models
struct vec3 { float x{}, y{}, z{}; };
struct inverse_state { vec3 cursor_position{}, current_cursor_force{}; };
struct inverse_device { std::string device_id; inverse_state state; };
struct devices_message { std::vector<inverse_device> inverse3; };

struct set_cursor_force_cmd { vec3 vector; };
struct commands_message {
    std::optional<session_cmd> session;
    std::vector<device_commands> inverse3;
};

// Send / receive
ws.onmessage = [&](const std::string &msg) {
    devices_message data{};
    if (glz::read<glz_settings>(data, msg)) return;
    if (data.inverse3.empty()) {
        ws.send(R"({"session":{"force_render_full_state":{}}})");
        return;
    }

    commands_message out_cmds{};
    if (first_message) {
        first_message = false;
        out_cmds.session = session_cmd{ /* profile = hello-floor */ };
    }
    for (const auto &dev : data.inverse3) {
        const float z = dev.state.cursor_position.z;
        const float force_z = z > floor_pos ? 0.0f : (floor_pos - z) * stiffness;
        device_commands dc{ .device_id = dev.device_id };
        dc.commands.set_cursor_force = set_cursor_force_cmd{{0.0f, 0.0f, force_z}};
        out_cmds.inverse3.push_back(std::move(dc));
    }

    std::string out_json;
    (void)glz::write_json(out_cmds, out_json);
    ws.send(out_json);
};
ws.open("ws://localhost:10001");
while (std::cin.get() != '\n') {}  // block main thread

Source: Python · C++ · C++ Glaze

Related: Control Commands (set_cursor_force) · Mount & Workspace (presets) · Types (vec3) · Sessions · Tutorial 07 (Basis & Mount)