Surface Normal Shower

--By xXxMoNkEyMaNxXx
local Terrain=workspace.Terrain
local GetCell=Terrain.GetCell
local CellCenterToWorld=Terrain.CellCenterToWorld
local WorldToCellPreferSolid=Terrain.WorldToCellPreferSolid

local vec3=Vector3.new
local IdentityVector=vec3()
local dot=IdentityVector.Dot
local cross=IdentityVector.Cross

local mat3=CFrame.new
local IdentityCFrame=mat3()
local ptos=IdentityCFrame.pointToObjectSpace
local vtws=IdentityCFrame.vectorToWorldSpace

--Returns:
--Index of closest plane to p
--Distance to surface from p
local function ClosestNormalVector(p,planes)
    local best_d=-math.huge
    local best_i
    for i=1,#planes do
        local plane=planes[i]
        local d=dot(plane[1],p-plane[2])
        if d>best_d then
            best_i,best_d=i,d
        end
    end
    return best_i,best_d
end

--Part geometry data
local UnitaryConvexPlaneMeshes={--I realized that I could make each component of the normal vector dependent on every component of the size using matrices (genius!)
    WedgePart={{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,1),vec3(0,0,0.5)},{mat3(0,0,0, 0,0,0, 0,0,1, 0,-1,0),vec3(0,0,0)},{vec3(1,0,0),vec3(0.5,0,0)},{vec3(-1,0,0),vec3(-0.5,0,0)}},
    CornerWedgePart={{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,0,-1),vec3(0,0,-0.5)},{mat3(0,0,0, 0,-1,0, 1,0,0, 0,0,0),vec3(0,0,0)},{mat3(0,0,0, 0,0,0, 0,0,1, 0,1,0),vec3(0,0,0)}},
    Part={{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,1,0),vec3(0,0.5,0)},{vec3(0,0,1),vec3(0,0,0.5)},{vec3(-1,0,0),vec3(-0.5,0,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)}}
}

--Terrain geometry data
local TerrainCellSize=vec3(4,4,4)--Support arbitrary stuff BECAUSE I CAN
local TerrainCellOrientations={
    [0]=mat3(0,0,0, 1,0,0, 0,1,0, 0,0,1),
    mat3(0,0,0, 0,0,1, 0,1,0, -1,0,0),
    mat3(0,0,0, -1,0,0, 0,1,0, 0,0,-1),
    mat3(0,0,0, 0,0,-1, 0,1,0, 1,0,0)
}
local TerrainCellBlockUnitaryConvexPlaneMeshes={
    [0]={{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,1,0),vec3(0,0.5,0)},{vec3(0,0,1),vec3(0,0,0.5)},{vec3(-1,0,0),vec3(-0.5,0,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)}},
    {{vec3(1,0,0),vec3(0.5,0,0)},{vec3(-1,0,0),vec3(-0.5,0,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)},{mat3(0,0,0, 0,0,0, 0,0,1, 0,1,0),vec3(0,0,0)}},
    {{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)},{mat3(0,0,0, 0,-1,-1, 1,0,1, 1,1,0),vec3(0.5,-0.5,-0.5)/3}},
    {{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,1,0),vec3(0,0.5,0)},{vec3(0,0,1),vec3(0,0,0.5)},{vec3(-1,0,0),vec3(-0.5,0,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)},{mat3(0,0,0, 0,-1,-1, 1,0,1, 1,1,0),vec3(-0.5,0.5,0.5)/3}},
    {{vec3(1,0,0),vec3(0.5,0,0)},{vec3(0,1,0),vec3(0,0.5,0)},{vec3(0,-1,0),vec3(0,-0.5,0)},{vec3(0,0,-1),vec3(0,0,-0.5)},{mat3(0,0,0, 0,0,-1, 0,0,0, 1,0,0),vec3(0,0,0)}}
}

local function NormalVector(part,point)
    if part.ClassName=="Part" and (part.Shape==Enum.PartType.Ball or part.Shape==Enum.PartType.Cylinder) then
        return vtws(part.CFrame,ptos(part.CFrame,point).unit)--A bit simpler than the other ones.  Just a bit.
    else
        local partCFrame,partSize=part.CFrame,part.Size
        local UCPM
        if part.ClassName=="Terrain" then
            local CellGridLocation=WorldToCellPreferSolid(part,vec3(point.x,point.y-1e-5,point.z))--Ugly floating point fix.  Alternatively, one could check the distance to the surrounding cells' CPM, and use the closest one, but I don't feel like it.
            local CellMaterial,CellBlock,CellOrientation=GetCell(part,CellGridLocation.x,CellGridLocation.y,CellGridLocation.z)
            partCFrame=TerrainCellOrientations[CellOrientation.Value]+CellCenterToWorld(part,CellGridLocation.x,CellGridLocation.y,CellGridLocation.z)
            partSize=TerrainCellSize
            UCPM=TerrainCellBlockUnitaryConvexPlaneMeshes[CellBlock.Value]
        else
            UCPM=UnitaryConvexPlaneMeshes[part.ClassName] or UnitaryConvexPlaneMeshes.Part--Trusses, SpawnLocations, etc.
        end
        local CPM={}
        for i=1,#UCPM do
            local plane=UCPM[i]
            CPM[i]={(plane[1]*partSize).unit,plane[2]*partSize}
        end
        local PlaneIndex,DistanceToSurface=ClosestNormalVector(ptos(partCFrame,point),CPM)
        if PlaneIndex then
            return vtws(partCFrame,CPM[PlaneIndex][1])
        else
            return IdentityVector--Dead code unless the tables are tampered with
        end
    end
end

--NormalShower part--
local Player=Game:GetService'Players'.LocalPlayer
local Mouse=Player:GetMouse()

repeat wait() until Player.Character
local NormalShower=Player.Character:FindFirstChild'NormalShower' or Instance.new("Part",Player.Character)
NormalShower.Name="NormalShower"
NormalShower.FormFactor=3
NormalShower.Size=Vector3.new(0.2,0.2,4)
NormalShower.TopSurface=0
NormalShower.BottomSurface=0
NormalShower.CanCollide=false
NormalShower.Anchored=true

local tick=tick
local sqrt=math.sqrt
local data={}
local N=1e3
local dN=1e3
local function Update()
    local MouseTarget=Mouse.Target
    if MouseTarget then
        local HitPoint=Mouse.hit.p
        local t0=tick()
        local Normal=NormalVector(MouseTarget,HitPoint)--Math time!
        local t1=tick()
        data[#data+1]=t1-t0
        NormalShower.Transparency=0
        NormalShower.CFrame=mat3(HitPoint,HitPoint+Normal)*mat3(0,0,-NormalShower.Size.z/2)
    else
        NormalShower.Transparency=1
    end
    if #data>N then
        N=N+dN
        local biggest=0
        local smallest=math.huge
        local sum=0
        for i=1,#data do
            local di=data[i]
            sum=sum+di
            if di>biggest then
                biggest=di
            end
            if di<smallest then
                smallest=di
            end
        end
        local avg=sum/#data
        local var=0
        for i=1,#data do
            local d=data[i]-avg
            var=var+d*d
        end
        print("Calls per second: max="..1/smallest.." min="..1/biggest.." avg="..1/avg.."+-"..sqrt(var/(#data*(#data-1)))/(avg*avg))
    end
end

Game:GetService'RunService'.RenderStepped:connect(Update)
Mouse.Move:connect(Update)