Limiting the crosshair

:) funny ..

	;setup graphics
	Graphics3D 800, 600, 0, 2
	SetBuffer BackBuffer()
	
	;create camera
	cam = CreateCamera()
	
	;position camera
	MoveEntity cam, 0, 0, -15
	
	;create cube
	cube = CreateCube(cam)
	PositionEntity cube, 0, 0, 0, 1
	ScaleEntity cube, 1, 1, 0.01
	
	;init bounds
	GetLimits(cam, cube, 100, 100, 700, 500)
	EntityOrder cube, -1
			
	z# = 0
	
	For i = 0 To 100
		c = CreateCube()
		ScaleEntity c, 0.1, 0.1, 0.1
		EntityColor c,Rand(255),Rand(255),Rand(255)
		PositionEntity c, Rnd(-10, 10), Rnd(-10, 10), Rnd(-10, 10) - 15
   Next
	
	;main loop
	Repeat
	
		TurnEntity cam, 0, 1, 0
	
		;move cube	
		x# = x# + (MouseXSpeed() * 0.01)
		y# = y# - (MouseYSpeed() * 0.01)
		PositionEntity cube, x, y, EntityZ(cube)
		MoveMouse 400, 300
	
		;limit cube's position
		LimitPosition cam, cube
	
		RenderWorld()
	
		;draw bounds (100,100)-(700,500)
		Color 0, 0, 255
		Rect 100, 100, 600, 400, 0
			
		Flip
	
	Until KeyHit(1)
	
	End

Global minx#, miny#, maxx#, maxy#
Function GetLimits(cam, ent, x1, y1, x2, y2)

	;determine distance entity->camera
	TFormPoint EntityX(ent, 1), EntityY(ent, 1), EntityZ(ent, 1), 0, cam

	;create temp. plane	
	plane = CreatePlane(1, cam)
	;move away from camera
	PositionEntity plane, 0, 0, TFormedZ()
	;turn towards camera
	RotateEntity plane, -90, 0, 0

	;make plane pickable	
	EntityPickMode plane, 2
	
	;pick upperleft coords
	CameraPick cam, x1, y1
	TFormPoint PickedX(), PickedY(), PickedZ(), 0, cam

	;store upperleft coords	
	minx# = TFormedX()
	miny# = TFormedY()

	;pick bottomright coords
	CameraPick cam, x2, y2
	TFormPoint PickedX(), PickedY(), PickedZ(), 0, cam
	
	;store bottomright coords
	maxx# = TFormedX()
	maxy# = TFormedY()	

	;free temp. plane	
	FreeEntity plane

	;swap coords if needed	
	If maxx < minx Then 
		t# = maxx
		maxx = minx
		minx = t#
	End If
	
	;swap coords if needed	
	If maxy < miny Then
		t# = maxy
		maxy = miny
		miny = t
	End If
	
End Function

;apply limits
Function LimitPosition(cam, ent)

	;convert current coords to camera coords
	TFormPoint EntityX(ent, 1), EntityY(ent, 1), EntityZ(ent, 1), 0, cam
	x# = TFormedX()
	y# = TFormedY()
	z# = TFormedZ()

	;limit coords	
	If x < minx Then x = minx
	If x > maxx Then x = maxx
	If y < miny Then y = miny
	If y > maxy Then y = maxy

	;convert coords back to world coords	
	TFormPoint x, y, z, cam, 0
	;reposition object
	PositionEntity ent, TFormedX(), TFormedY(), TFormedZ(), 1

End Function

However, instead of using MouseXSpeed/MouseYSpeed, you could also create the plane, make it invisible using EntityAlpha and pickable, and use CameraPick to determine where the crosshair should be:

	;setup graphics
	Graphics3D 800, 600, 0, 2
	SetBuffer BackBuffer()
	
	;create camera
	cam = CreateCamera()
	
	;position camera
	MoveEntity cam, 0, 0, -15
	
	;create cube
	cube = CreateCube()
	ScaleEntity cube, 1, 1, 0.01
	
	plane = CreatePlane()
	TurnEntity plane, -90, 0, 0
	EntityPickMode plane, 2
	EntityAlpha plane, 0.1
	
	;main loop
	Repeat
		
		mx = mx + MouseXSpeed()
		my = my + MouseYSpeed()
		MoveMouse 400, 300
		If mx < 0 Then mx = 0
		If my < 0 Then my = 0
		If mx > 800 Then mx = 800
		If my > 600 Then my = 600
		
		CameraPick cam, mx, my
		PositionEntity cube, PickedX(), PickedY(), PickedZ()	
	
		RenderWorld()
				
		Flip
	
	Until KeyHit(1)
	
	End