卡丁车：漂移过弯快过直道

fx-kart

local Kart = {}
local KEY = { RX = 250, RY = 108, CX = 330, CY = 168 }
local state = { ang = -1.2, drift = 0 }

local function TrackPos(a)
    return KEY.CX + math.cos(a) * KEY.RX,
           KEY.CY + math.sin(a) * KEY.RY
end

local function TrackTangent(a)
    -- 参数方程的切线方向
    return math.atan2(math.cos(a) * KEY.RY, -math.sin(a) * KEY.RX)
end

function Kart.Start()
    SL:Schedule(function(dt)
        local spd = SL:GetValue("kartSpeed") or 100
        state.ang = state.ang + dt * spd * 0.011
        local x, y = TrackPos(state.ang)
        local tangent = TrackTangent(state.ang)
        -- 曲率估算: 椭圆上下弯急, 左右缓
        local curv = math.abs(math.sin(state.ang))
        -- 弹簧追赶切线, 差值即外甩角
        state.drift = state.drift * 0.9 + curv * 0.5 * 0.1 + state.drift * 0.0
        local face = tangent + state.drift * (math.cos(state.ang) > 0 and 1 or -1)
        SL:SetActorPos("kart", x, y)
        SL:SetActorDir("kart", SL:CalcDirByAngle(face))
        if curv > 0.45 and math.random() < 0.3 then
            SL:PlayFxAt("skid", x, y + 6)
        end
        -- 过计圈线
        if state.ang > 6.28 then
            state.ang = state.ang - 6.28
            SL:PlayFxAt("lap_flash", TrackPos(0))
            SL:SendEvent("kart_lap")
        end
    end, 0.016, "kartRun")
end

return Kart