How to know if a triangle is facing the camera?

ok, nearly got it :)

;setup graphics
Graphics3D 1024, 768, 0, 2
SetBuffer BackBuffer()

;enable wireframe	
WireFrame 1

;create camera	
Global main_cam = CreateCamera()
MoveEntity main_cam, 0, 0, -5

;create cube	
Global cube = CreateSphere(4)

Global surfaces,tris,verts

Function plotvertex(x,y,col)
	If col=1 Then 
		Color 0,255,0
	Else
		Color 255,0,0
	EndIf
	Oval x,y,3,3
End Function


Function Hidden(mesh)
	; Calculate the normal which indicates the direction the camera is pointing.	
	;count the surfaces of the mesh
	surfaces=CountSurfaces(mesh)
	
	;loop through all the surfaces
	For surfcount=1 To surfaces
		;count the number of triangles on the surfaces
		s=GetSurface(mesh,surfcount)
		tris=CountTriangles(s)
		verts=CountVertices(s)
		For t=0 To tris-1
			
			;get the corners of each triangle
			corner1=TriangleVertex(s,t,0)
			corner2=TriangleVertex(s,t,1)
			corner3=TriangleVertex(s,t,2)
			
			; get vertex information for corner 1
			TFormPoint VertexX(s,corner1),VertexY(s,corner1),VertexZ(s,corner1),mesh,0
			ax#=TFormedX()
			ay#=TFormedY()
			az#=TFormedZ()
			CameraProject main_cam,ax,ay,az
			vax#=ProjectedX()	
			vay#=ProjectedY()
			
			; get vertex information for corner 2
			TFormPoint VertexX(s,corner2),VertexY(s,corner2),VertexZ(s,corner2),mesh,0
			bx#=TFormedX()
			by#=TFormedY()
			bz#=TFormedZ()
			CameraProject main_cam,bx,by,bz
			vbx#=ProjectedX()	
			vby#=ProjectedY()
			
			; get vertex information for corner 3
			TFormPoint VertexX(s,corner3),VertexY(s,corner3),VertexZ(s,corner3),mesh,0
			cx#=TFormedX()
			cy#=TFormedY()
			cz#=TFormedZ()
			CameraProject main_cam,cx,cy,cz
			vbx#=ProjectedX()	
			vby#=ProjectedY()
			
			; find mid point of triangle and plot it
			mx# = (ax+bx+cx) / 3
			my# = (ay+by+cy) / 3
			mz# = (az+bz+cz) / 3
			
			; Triangle 1
			; Get the vectors for two edges of the triangle.
			Px# = Ax#-Bx#
			Py# = Ay#-By#
			Pz# = Az#-Bz#
			
			Qx# = Bx#-Cx#
			Qy# = By#-Cy#
			Qz# = Bz#-Cz#
			
			; Compute their cross product to get the face normal.
			FNx# = Py#*Qz# - Pz#*Qy#
			FNy# = Pz#*Qx# - Px#*Qz#
			FNz# = Px#*Qy# - Py#*Qx#
			
			PositionEntity campivot,EntityX(main_cam,1),EntityY(main_cam,1),EntityZ(main_cam,1)
			PositionEntity temppivot,mx,my,mz
			PointEntity campivot,temppivot
			TFormNormal 0,0,1,campivot,0
			cnx#=TFormedX()
			cny#=TFormedY()
			cnz#=TFormedZ()
			
			; Compute the dot product of the face and camera normal.
			DP# = FNx#*CNx# + FNy#*CNy# + FNz#*CNz#
			
			; Is this face facing towards the camera, if so then draw its verts?
			If DP# <0
				plotvertex(vax,vay,1)
				plotvertex(vbx,vby,1)
				plotvertex(vcx,vcy,1)
			EndIf	
		Next
	Next
End Function

Global campivot=CreatePivot()
Global temppivot=CreatePivot()
fps=0
oldfps=0
fpstime=MilliSecs()+1000

While Not KeyDown(1)
	; fps stuff
	If MilliSecs()>fpstime Then
		fps=oldfps
		oldfps=0
		fpstime=MilliSecs()+1000
	Else
		oldfps=oldfps+1
	EndIf
	; main stuff
	If MouseDown(1) Then TurnEntity cube,0,0.4,0
	If MouseDown(2) Then TurnEntity cube,0.4,0,0
	If KeyDown(200) Then TranslateEntity main_cam,0,0.1,0
	If KeyDown(205) Then TranslateEntity main_cam,0.1,0,0
	If KeyDown(203) Then TranslateEntity main_cam,-0.1,0,0
	If KeyDown(208) Then TranslateEntity main_cam,0,-0.1,0
	RenderWorld()
	Hidden(cube)
	Color 255,255,255
	Text 10,10,"Surfaces : "+surfaces
	Text 10,25,"Triangles : "+tris
	Text 10,40,"Vertices : "+verts
	Text 930,10,"FPS : "+fps
	Text 10,750,"Left / Right Mouse Button to rotate, and cursors to move. "
	Flip 
	
Wend

There are now only a couple of point which should be highlight, which aren't. Not sure why. Anyone got an ideas? Im hoping its something stupid that I have forgotten to do.

Cheers :)