need to rotate a pixmap with smooth filtering

Here is the BlitzMax version of the rotozoomer demo. However, it is not working right. I get what seems to be different colors in X but in Y the same color is repeated down the screen. I cannot figure out where the bug is. Can anyone help?

OEJ: Should be easy enough to adapt this to what you want, once it's working, it handles alphablending, rotation, zoom, a background image, and the rotation angle can be a Float.

Anyone see the bug? (My images are just 640x480 images) I am quite certain it has nothing to do with the alphablending. Even with bare minimum of two lines - WritePixel(ReadPixel()) it is still wrong.

'Software rotation demo

Strict
SetGraphicsDriver(GLMax2DDriver())
Graphics 640,480,0

'Prepare pixmaps
Local BackgroundPixmap:TPixmap=LoadPixmap("Background.jpg")
If PixmapFormat(BackgroundPixmap)<>PF_RGBA8888 Then BackgroundPixmap=ConvertPixmap(BackgroundPixmap,PF_RGBA8888)
Local SourcePixmap:TPixmap=LoadPixmap("RotoZoom.jpg")
If PixmapFormat(SourcePixmap)<>PF_RGBA8888 Then SourcePixmap=ConvertPixmap(SourcePixmap,PF_RGBA8888)
Local DestPixmap:TPixmap=CreatePixmap(640,480,PF_RGBA8888,4)
Local SourcePixelsPerRow=PixmapWidth(SourcePixmap)
Local DestPixelsPerRow=PixmapWidth(DestPixmap)
Local OpWidth:Int=128'PixmapWidth(SourcePixmap) 'Width of operation
Local OpHeight:Int=128'PixmapHeight(SourcePixmap) 'Height of operation
Local OpWidth2:Int=OpWidth/2
Local OpHeight2:Int=OpHeight/2
Local XCenter:Int'=OpWidth/2
Local YCenter:Int'=OpHeight/2

'Dummy alpha values
For Local aa:Int=0 To OpHeight-1
	For Local bb:Int=0 To OpWidth-1
		WritePixel(SourcePixmap,bb,aa,ReadPixel(SourcePixmap,bb,aa)+Max(aa+bb,256)) 'Make alpha to see effect if image doesn't have one
	Next
Next

'Set up variables
Local Angle:Float=0
Local Magnification:Float=1.0
Local Y_UAdd,Y_VAdd,X_UAdd,X_VAdd:Float
Local Y_U,Y_V,X_U,X_V:Float
Local SourceBase:Int Ptr=Int Ptr(PixmapPixelPtr(SourcePixmap,0,0))
Local DestBase:Int Ptr=Int Ptr(PixmapPixelPtr(DestPixmap,0,0))
Local SourcePixel,DestPixel:Int
Local Red,Green,Blue:Int
Local Alpha:Float

'Demo
Repeat
	'Interaction/autorotate
	If MouseDown(1) Then Magnification:+0.01
	If MouseDown(2) Then Magnification:-0.01
	Angle:+1
	If Angle>=360 Then Angle:-360
	If Angle<0 Then Angle:+360

	'Setup
	Y_UAdd=Cos(Angle+90)*Magnification
	Y_VAdd=Sin(Angle+90)*Magnification
	X_UAdd=Cos(Angle)*Magnification
	X_VAdd=Sin(Angle)*Magnification
	XCenter=MouseX()
	YCenter=MouseY()
	Y_U=-(Y_UAdd*OpHeight2)-(X_UAdd*OpWidth2)
	Y_V=-(Y_VAdd*OpHeight2)-(X_VAdd*OpWidth2)
	
	'Draw it
	DestPixmap.Paste(BackgroundPixmap,0,0) 'Draw background
	For Local Y:Int=0 To OpHeight-1
		X_U=Y_U
		X_V=Y_V
		For Local X:Int=0 To OpWidth-1
			SourcePixel=Int Ptr(SourceBase + Min(Max(0,XCenter+X_U),OpWidth) + ((Min(Max(0,YCenter+X_V),OpHeight) * SourcePixelsPerRow)))[0] 'Read full pixel, clipped
			'SourcePixel=ReadPixel(SourcePixmap,XCenter+X_U,YCenter+X_V)
			Alpha=(SourcePixel Shr 24)/256.0
			Red=((SourcePixel Shl 8) Shr 24) * Alpha
			Green=((SourcePixel Shl 16) Shr 24) * Alpha
			Blue=(SourcePixel & $FF) * Alpha
			Alpha=1.0-Alpha
			DestPixel=(DestBase+X+(Y*OpWidth))[0] 'Read full pixel, clipped
			'DestPixel=ReadPixel(DestPixmap,X,Y)
			Red:+(((DestPixel Shl 8) Shr 24) * Alpha)
			Green:+(((DestPixel Shl 16) Shr 24) * Alpha)
			Blue:+((DestPixel & $FF) * Alpha)
			(DestBase+X+(Y*DestPixelsPerRow))[0]=(Red Shl 24) | (Green Shl 16) | (Blue Shl 8) 'Plot RGBA
			'WritePixel(DestPixmap,X,Y,(Red Shl 24)|(Green Shl 16)|(Blue Shl 8)) 'Write RGBA
			'WritePixel DestPixmap,X,Y,SourcePixel
			X_U:+X_UAdd
			X_V:+X_VAdd
		Next
		Y_U:+Y_UAdd
		Y_V:+Y_VAdd
	Next
	DrawPixmap DestPixmap,0,0
	Flip 1
Until KeyHit(KEY_ESCAPE)