SetViewPort Problem

I have written a little test app, and it seems to work fine. Only one small mistake. If the Viewport starts at (100,100) and I draw the image at (120,120) the lase pixel row and colum (above and left from the image) will be repeated.

Here's the code. You can hold down "Q" to swap between BlitzMax SetViewport and the overwritten SetViewport-Command

SuperStrict

Graphics 640,480,0

'Viewport coords
Global vx:Float, vy:Float, vw:Float, vh:Float

Local t:TImage = LoadImage ("gfx/mainmenu.png")
Local a:Int = 0
While(Not KeyDown(KEY_ESCAPE))
	a:+1
	Cls
	If KeyDown(KEY_Q)
		SetViewportC 200+(Sin(a)*100),200+(Cos(a)*100),100,100
		DrawImageC t,210,210
	Else
		SetViewport 200+(Sin(a)*100),200+(Cos(a)*100),100,100
		DrawImage t,210,210
		SetViewport 0,0,640,480
	End If
	Flip
Wend

Function SetViewportC( x:Float, y:Float, width:Float, height:Float ) 
	vx = x
	vy = y
	vw = width
	vh = height
End Function

Function DrawImageC(image:TImage, x:Float, y:Float, frame:Int=0)
	
        'Calculate new coords, I don't know how to calculate them

	Local rx:Float = vx
	Local ry:Float = vy
	Local rw:Float = vw
	Local rh:Float = vh
	
	rx:-x
	ry:-y

	x = vx 
	y = vy 
	


		
	ccDrawImageArea(image,x,y,rx,ry,rw,rh,frame)
		
End Function

Function ccDrawImageArea(image:TImage, x#, y#, rx#, ry#, rw#, rh#, frame:Int=0)
  Local origin_x#, origin_y# ; GetOrigin (origin_x, origin_y)
  Local tw:Int = Pow2Size(image.width)
  Local th:Int = Pow2Size(image.height)
  Local rw1#  = rx + rw
  Local rh1#  = ry + rh
  Local x0# = -image.handle_x, x1# = x0 + rw
  Local y0# = -image.handle_y, y1# = y0 + rh
  
  If rw1 > image.width
    x1 = x0 + rw + image.width - rw1
    rw1 = image.width
  EndIf
   
  If rh1 > image.height
    y1 = y0 + rh + image.height - rh1
    rh1 = image.height
  EndIf
?Win32
  If TD3D7ImageFrame(image.frame(frame))
    Local frame:TD3D7ImageFrame = TD3D7ImageFrame(image.frame(frame))
    
    frame.setUV(rx / tw, ry / th, rw1 / tw, rh1 / th)
    frame.Draw x0, y0, x1, y1, x + origin_x, y + origin_y
    frame.setUV(0, 0, image.width / Float(tw), image.height / Float(th))
  Else
?
    Local frame:TGLImageFrame = TGLImageFrame (image.frame(frame))
                
    frame.u0 = rx / tw
    frame.v0 = ry / th
    frame.u1 = rw1 / tw
    frame.v1 = rh1 / th
    
    frame.Draw x0, y0, x1, y1, x + origin_x, y + origin_y
    
    frame.u0 = 0
    frame.v0 = 0
    frame.u1 = image.width / Float(tw)
    frame.v1 = image.height / Float(th)
?Win32
  EndIf
?
  
  Function Pow2Size:Int(n:Int)
    Local ry:Int = 1
    
    While ry:Int < n
      ry :* 2
    Wend
    
    Return ry
  End Function
End Function