07. Basis & Mount Playground

Interactively drives the device's mount transform and demonstrates how configure.basis, configure.preset, and configure.mount work together on the wire. Keeps a fixed horizontal floor so the interactive focus stays on the configure commands.

What you'll learn:

• Setting a basis permutation ("XZY" → Y-up application frame)
• Choosing a preset (arm_front) and understanding what it configures
• Overriding the mount at runtime with a rotation quaternion (keyboard-driven)
• The mutual exclusion rule: mount and preset cannot coexist in the same device configure block
• One-shot configure semantics: each key press sends exactly one configure message
• (C++ Glaze) modelling mutually-exclusive fields with std::optional

Workflow

1. On first message: send the session profile, configure.basis: "XZY", and configure.preset: arm_front. Start sending set_cursor_force for the fixed floor.
2. Each tick: read cursor Y, compute force_y = max(0, (floor_pos - y) * stiffness), send it.
3. When the user presses a mount-rotation key, flag pending_configure = true.
4. On the next tick: build a configure.mount block with a transform whose rotation is a unit quaternion (Z-then-X composition of the current pitch / yaw). Omit preset — the two are mutually exclusive on the wire.
5. Reset key (R) clears the override; next configure falls back to preset again.

Parameters

Name	Default	Purpose
BASIS	"XZY"	Axis permutation — Y-up application frame
DEVICE_PRESET / DEVICE_CONFIG_PRESET	"arm_front"	Named preset — origin at device base
FLOOR_POS_Y	0.0 m	Fixed floor plane (application-Y)
STIFFNESS	1000 N/m	Floor spring constant
MOUNT_STEP_DEG	10°	Rotation per key press
PRINT_EVERY_MS	200	Telemetry throttle

Controls

Key	Action
W / S	Rotate mount ±10° around device +X (pitch)
A / D	Rotate mount ±10° around device +Z (yaw)
R	Reset mount — revert to preset
H	Show controls
Q	Quit

mount and preset are mutually exclusive

The service rejects a device configure block containing both. Once the user overrides the mount, the tutorial omits preset from every subsequent configure. Pressing R re-enables preset on the next configure and drops mount.

Different input models

C++ variants read line-based input on a background stdin thread (press ENTER after each letter). Python uses the keyboard package for real-time key polling in the main async loop — no ENTER needed. Same keys, same commands.

State fields read

From data.inverse3[i].state:

• cursor_position.y — vec3, used to compute floor penetration
• current_cursor_force — reported for telemetry

Send / receive

The payload shape is the same across variants; the interesting differences are how each builds the mutually-exclusive mount / preset branching and how the input thread signals the WebSocket thread.

Python

Single async loop with real-time key polling via the keyboard package. pending_configure is a global flag set by the key handlers and cleared each time a configure block is sent.

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 + basis + preset
            request_msg = {
                "session": {"configure": {"profile": {"name": SLUG}}},
                "inverse3": [{
                    "device_id": device_id,
                    "configure": build_configure_block(first_handshake=True),
                    # -> {"basis": {"permutation": "XZY"},
                    #     "preset": {"preset": "arm_front"}}
                }],
            }
        else:
            handle_key_inputs()  # may set pending_configure = True (classic, not shown)

            y       = data["inverse3"][0]["state"]["cursor_position"]["y"]
            force_y = 0.0 if y > FLOOR_POS_Y else (FLOOR_POS_Y - y) * STIFFNESS

            entry = {
                "device_id": device_id,
                "commands": {"set_cursor_force":
                    {"vector": {"x": 0.0, "y": force_y, "z": 0.0}}},
            }
            if pending_configure:
                entry["configure"] = build_configure_block(first_handshake=False)
                # -> {"mount": {...}}   OR   {"preset": {...}}   (never both)
                pending_configure = False

            request_msg = {"inverse3": [entry]}

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

C++ (nlohmann)

Two-thread model: a background stdin thread reads lines and flips pending_configure (an std::atomic<bool>); the libhv I/O thread checks it each tick and emits configure when set.

std::atomic<bool> pending_configure{false};

ws.onmessage = [&](const std::string &msg) {
    const json data = json::parse(msg);
    if (!data.contains("inverse3") || data["inverse3"].empty()) return;

    const bool do_handshake = first_message;
    if (first_message) first_message = false;
    const bool do_configure = do_handshake || pending_configure.exchange(false);

    json request = {};
    if (do_handshake) {
        request["session"] = {{"configure", {{"profile",
            {{"name", "co.haply.inverse.tutorials:basis-and-mount"}}}}}};
    }

    request["inverse3"] = json::array();
    for (auto &el : data["inverse3"].items()) {
        json dev_cmd = {{"device_id", el.value()["device_id"]}};

        if (do_configure) {
            json cfg = {};
            if (do_handshake) cfg["basis"] = {{"permutation", BASIS}};
            if (mount_overridden) {
                cfg["mount"] = {{"transform", {
                    {"position", {{"x", 0.0}, {"y", 0.0}, {"z", 0.0}}},
                    {"rotation", quat_from_xz_deg(mount_angle_x_deg, mount_angle_z_deg)},
                    {"scale",    {{"x", 1.0}, {"y", 1.0}, {"z", 1.0}}},
                }}};
            } else {
                cfg["preset"] = {{"preset", DEVICE_CONFIG_PRESET}};
            }
            dev_cmd["configure"] = cfg;
        }

        const float y = el.value()["state"]["cursor_position"]["y"].get<float>();
        const float force_y = y > FLOOR_POS_Y ? 0.0f : (FLOOR_POS_Y - y) * STIFFNESS;
        dev_cmd["commands"] = {{"set_cursor_force",
            {{"vector", {{"x", 0.0}, {"y", force_y}, {"z", 0.0}}}}}};

        request["inverse3"].push_back(dev_cmd);
    }
    ws.send(request.dump());
};
std::thread input_thr(input_thread_func);  // stdin reader — flips pending_configure
ws.open("ws://localhost:10001");
while (running.load()) std::this_thread::sleep_for(50ms);

C++ (Glaze)

The mutual-exclusion rule maps naturally onto std::optional<preset_cfg> and std::optional<mount_cfg>: only the populated one appears in the serialized JSON. The preset name is modelled as an enum class with a glz::meta specialization that maps it to the string the service expects.

// The preset set modelled as an enum
enum class device_preset {
    defaults, arm_front, arm_front_centered,
    led_front, led_front_centered, custom,
};

// Glaze meta — serialize the enum as the JSON string the service expects
template <> struct glz::meta<device_preset> {
    using enum device_preset;
    static constexpr auto value =
        enumerate(defaults, arm_front, arm_front_centered,
                  led_front, led_front_centered, custom);
};

// Transform + the configure block
struct vec3 { float x{}, y{}, z{}; };
struct quat { float w{1.0f}, x{}, y{}, z{}; };
struct transform_t { vec3 position{}; quat rotation{}; vec3 scale{1,1,1}; };

struct preset_cfg { device_preset preset; };
struct basis_cfg  { std::string permutation; };
struct mount_cfg  { transform_t transform; };
struct device_configure {
    std::optional<preset_cfg> preset;  // mutually exclusive with mount
    std::optional<basis_cfg>  basis;
    std::optional<mount_cfg>  mount;   // mutually exclusive with preset
};

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

    const bool do_handshake = first_message.exchange(false);
    const bool do_configure = do_handshake || pending_configure.exchange(false);

    commands_message out_cmds{};
    if (do_handshake) {
        out_cmds.session = session_cmd{ /* profile = basis-and-mount */ };
    }

    for (const auto &dev : data.inverse3) {
        device_commands dc{ .device_id = dev.device_id };

        if (do_configure) {
            device_configure cfg{};
            if (do_handshake) cfg.basis = basis_cfg{BASIS};
            if (mount_overridden) {
                cfg.mount = mount_cfg{ .transform = transform_t{
                    .rotation = quat_from_xz_deg(mount_angle_x_deg, mount_angle_z_deg)}};
            } else {
                cfg.preset = preset_cfg{DEVICE_CONFIG_PRESET};
            }
            dc.configure = std::move(cfg);
        }

        const float y = dev.state.cursor_position.y;
        const float force_y = y > FLOOR_POS_Y ? 0.0f : (FLOOR_POS_Y - y) * STIFFNESS;
        dc.commands.set_cursor_force = set_cursor_force_cmd{{0.0f, force_y, 0.0f}};
        out_cmds.inverse3.push_back(std::move(dc));
    }

    std::string out_json;
    (void)glz::write_json(out_cmds, out_json);
    ws.send(out_json);
};
std::thread input_thr(input_thread_func);
ws.open("ws://localhost:10001");
while (running.load()) std::this_thread::sleep_for(50ms);

Source: Python · C++ · C++ Glaze

Related: Basis Permutation · Mount & Workspace · Device Configuration · Control Commands (set_cursor_force) · Types (transform) · Tutorial 04 (Hello Floor)