Angles Of Reflection (Nasty trig stuff)

It's always trigonometry where I get stuck and it's hard to avoid it in 3D!!!

I have a 2-D game in a 3-D world. Very simple the player Mesh can move in X and Z coords only. Around my SQUARE arena I have a boundary wall. I would like my Mesh to 'bounce' off this wall realistically - i.e. If it hits perpendiclar (at 90degrees), it will recoil stright backwards.

I know the angle of reflection = angle of incidence, but how can I code this?

I looked at Cosines, because the Cosine of 90degrees is 0, but it doesn't fit for other angles..

(I haven't touched trig for years and it shows :((( )

Plus I need to convert EntityYaw(Mesh) to whatever to decipher exactly which wall it is hitting, whether(for want of better descriptions) North, South, East or West...


If it's of any consequence, my walls are built from single-unit primitive cubes (made using CopyEntity from a Master)

Look for the thread "physics for stupid people like me" in this forum, I explained everything, and there's no sine/cosine involved.

A wee demo .. can't remember who did this ( maybe DarkEagle ? )

Graphics3D 640,480,16,1

Const C_BALL = 1
Const C_CUBE = 2
Const BALLS = 50
Const SPEED# = .5

Global camera,cube
Global light =CreateLight()

Type ball
	Field entity
	Field xvel#,yvel#,zvel#
End Type

Collisions C_BALL, C_BALL,1,1
Collisions C_BALL, C_CUBE,2,1

MakeStuff()

While Not KeyDown(1)

	UpdateBalls()
	UpdateWorld()
	RenderWorld()
	Flip
	
Wend

;==============================================
;==============================================
;==============================================

Function MakeStuff()
	
	;cube
	cube = CreateCube()
	ScaleMesh cube,30,30,30
	FlipMesh cube
	EntityColor cube,50,50,200
	EntityType cube,C_CUBE
	UpdateNormals cube
	
	;camera
	camera = CreateCamera()
	PositionEntity camera,0,60,0
	PointEntity camera, cube
	
	;template for balls
	temp = CreateSphere()
	EntityType temp,C_BALL
	UpdateNormals temp
	EntityShininess temp,1
	HideEntity temp
		
	;balls
	For l = 1 To BALLS
		b.ball = New ball
		b\xvel = Rnd(-SPEED,SPEED)
		b\yvel = Rnd(-SPEED,SPEED)
		b\zvel = Rnd(-SPEED,SPEED)
		b\entity = CopyEntity( temp )
		PositionEntity b\entity,Rnd(-30,30),Rnd(-30,30), Rnd(-30,30)
		EntityColor b\entity,Rand(100,255),Rand(100,255),Rand(100,255)
		radius# = Rnd( 1,4 )
		ScaleEntity b\entity,radius,radius,radius
		EntityRadius b\entity,radius
	Next

End Function

;==============================================
;==============================================
;==============================================

Function UpdateBalls()

	For b.ball = Each ball
	
		For i = 1 To CountCollisions(b\entity)
			HandleBounce(b,i)
		Next
		
		TranslateEntity b\entity,b\xvel,b\yvel,b\zvel
		
	Next

End Function

;==============================================
;==============================================
;==============================================

Function HandleBounce(b.ball,i)
	
	; Get the normal of the surface collided with. 
	Nx# = CollisionNX(b\entity, i) 
	Ny# = CollisionNY(b\entity, i) 
	Nz# = CollisionNZ(b\entity, i) 
	Mesh = CollisionEntity (b\entity,i)
	c.ball = Object.ball ( EntityName( mesh ) )
		
	; Compute the dot product of the ball's motion vector and the normal of the surface collided with. 
	VdotN# = b\xvel*Nx + b\yvel*Ny + b\zvel*Nz 
	
	; Calculate the normal force. 
	NFx# = -2.0 * Nx# * VdotN 
	NFy# = -2.0 * Ny# * VdotN 
	NFz# = -2.0 * Nz# * VdotN 
	
	; Add the normal force to the motion vector. 
	b\xvel = b\xvel + NFx
	b\yvel = b\yvel + NFy
	b\zvel = b\zvel + NFz
	
End Function

I recommend What sswift Says, as I started that Thread and it was really helpful.

Regards,
ERic