Attaching objects to tabbers and updating.

you only really need one canvas. I hope this helps you understand it better:

' GUI application to adjust RGB levels of a photograph
Strict

Global start_x:Float = 320
Global start_y:Float = 240

' a simple spline type
Function spline_f:Float(p:Float, p1:Float, p2:Float, t:Float) 
	Local a:Float = (1 - t) 
	Return (a ^ 2.0 * p) + (2.0 * t * a * p1) + (t ^ 2 * p2) 
End Function

Function draw_spline(x:Float, y:Float, x2:Float, y2:Float, x3:Float, y3:Float, divisions:Float) 
	
End Function

' create the window and tabbers
Local MyWindow:TGadget = CreateWindow("Color Curves", 320, 320, 312, 324, Null, 1) 
Local Tabber:TGadget = CreateTabber(0, 0, 312, 300, MyWindow)
' make some canvases (canvi?) to house the Red,Green, and Blue curve drawings respectivly
Local canvas:TGadget = CreateCanvas(12, 12, 255, 255, tabber) 
' create tabs
AddGadgetItem(Tabber, "Red Curve", 0, 0, "Adjust red levels.") 
AddGadgetItem (Tabber, "Green Curve", 0, 0, "Adjust green levels.") 
AddGadgetItem (Tabber, "Blue Curve", 0, 0, "Adjust blue levels.")
SelectGadgetItem tabber,0 
Global grfx:tgraphics = CanvasGraphics(canvas)
SetGraphics grfx 
Global text$ = "Red Canvas"
Global r% = 255,g%=0,b%=0
CreateTimer 60
Global index% = 0

Repeat
	WaitEvent() 

	Select EventID() 
		Case EVENT_WINDOWCLOSE
     	End
	
		Case EVENT_TIMERTICK
			RedrawGadget canvas
		Case EVENT_GADGETPAINT
			Cls
			SetColor r,g,b
			DrawText text,0,0
			Flip()
		Case EVENT_GADGETACTION
			Select EventData()
    			Case 0
					r = 255
					g = 0
					b = 0 
    				text = "Red Canvas" 
				Case 1
					r = 0
					g = 255
					b = 0 
    				text = "Green Canvas"
				Case 2
					r = 0
					g = 0
					b = 255 
    				text = "Blue Canvas"
			End Select
   End Select
Forever

having more than one canvas is a waste of resources. Or maybe it's cause I don't know how to do it that way :-).