Surface Normal

--By xXxMoNkEyMaNxXx
--[[Example usage:
local Part,Point=workspace:FindPartOnRay(ray)
local Normal=NormalVector(Part,Point)
yay now you have the normal vector too
--]]
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