; Transparent Sprite demo by Dave Kirk (GFK)
; Use cursor keys to move sprite, ESCAPE to quit.
; Not really fast enough for lots of (or big) sprites, but still pretty effective.

AppTitle "Transparent Sprite Demo"

Graphics 800,600,16,1

SetBuffer BackBuffer()

Global FPS = 50

Global BKG = LoadImage("Background.bmp")
Global IMG = LoadImage("sprite.bmp")

While KeyDown(1) = 0
   t = MilliSecs()
   
   If KeyDown(203) Then X = X - 2
   If KeyDown(205) Then X = X + 2
   If KeyDown(200) Then Y = Y - 2
   If KeyDown(208) Then Y = Y + 2
   
   DrawBlock BKG,0,0
   
   AlphaSprite IMG,X,Y
   
   While MilliSecs()-t < 1000/FPS : Wend
   Flip
Wend

End

Function AlphaSprite(Image%,XP%,YP%)
   AlphaImage = CopyImage(Image)
   W = ImageWidth(AlphaImage)
   H = ImageHeight(AlphaImage)
   
   LockBuffer ImageBuffer(AlphaImage)
   LockBuffer ImageBuffer(BKG)
   For X = 0 To W - 1
      For Y = 0 To H - 1
         CB = ReadPixelFast(XP+X,YP+Y,ImageBuffer(BKG))
         CI = ReadPixelFast(X,Y,ImageBuffer(AlphaImage))
         Col# = Abs(CB - CI)
         Col = col /2
         If CB < CI
            Col = CB + Col
         Else
            Col = CI + Col
         EndIf
         WritePixelFast X,Y,Col,ImageBuffer(AlphaImage)
      Next
   Next
   
   UnlockBuffer ImageBuffer(AlphaImage)
   UnlockBuffer ImageBuffer(BKG)
   
   DrawImage AlphaImage,XP,YP
   FreeImage alphaimage
End Function