combining 2 pixmaps, including alpha

The finished product. I found a working formula on the apple site.

http://www.devworld.apple.com/documentation/Performance/Conceptual/vImage/Chapter8/chapter_8_section_3.html

SuperStrict

Framework BRL.LinkedList
Import BRL.Pixmap
Import BRL.PNGLoader
Import BRL.StandardIO

'output.png 46 46 tile.png 0 0 letter.png 0 0 score.png 92 0
'output.png 30 30 test1.png 0 0 test2.png 0 0

Type ttilemaker
	Field pixmap:tpixmap
	Field output:String
	Field width:Int
	Field height:Int
	Field layers:tlist = CreateList()
	
	Method Save()
		Local array:Int[,,]
		Local layer:tlayer
		
		For layer = EachIn layers
			If array = Null
				array = PixmapToPixelArray(layer.pixmap,layer.x,layer.y)
			Else
				CombinePixelArrays(array,PixmapToPixelArray(layer.pixmap,layer.x,layer.y))
			End If
		Next

		SavePixelArray(array,output)
	End Method
	
	Method PixmapToPixelArray:Int[,,](pixmap:tpixmap,offsetx:Int=0,offsety:Int=0)
		Local x:Int
		Local y:Int
		Local array:Int[width,height,4]
		Local pixel:Int
		
		For y = 0 Until height
			For x = 0 Until width
				pixel = ReadPixel(pixmap,x+offsetx,y+offsety)
				
				array[x,y,0] = GetA(pixel)
				array[x,y,1] = GetR(pixel)
				array[x,y,2] = GetG(pixel)
				array[x,y,3] = GetB(pixel)
			Next
		Next
		
		Return array
	End Method
	
	Method CombinePixelArrays:Int[,,](bottom:Int[,,],top:Int[,,])
		'this function will take 2 pixel arrays [x,y,rgba] and combine them.
		Local x:Int
		Local y:Int
		Local a:Float
		Local r:Float
		Local g:Float
		Local b:Float
		
		For y = 0 Until height
			For x = 0 Until width
				If top[x,y,0] = 255
					a = 255
					r = top[x,y,1]
					g = top[x,y,2]
					b = top[x,y,3]
				ElseIf top[x,y,0] = 0
					a = bottom[x,y,0]
					r = bottom[x,y,1]
					g = bottom[x,y,2]
					b = bottom[x,y,3]
				Else
					a = (top[x,y,0] * 255 + (255 - top[x,y,0]) * bottom[x,y,0] + 127) / 255
					r = (top[x,y,0] * top[x,y,1] + (((255 - top[x,y,0]) * bottom[x,y,0] + 127) / 255) * bottom[x,y,1] + 127) / a
					g = (top[x,y,0] * top[x,y,2] + (((255 - top[x,y,0]) * bottom[x,y,0] + 127) / 255) * bottom[x,y,2] + 127) / a
					b = (top[x,y,0] * top[x,y,3] + (((255 - top[x,y,0]) * bottom[x,y,0] + 127) / 255) * bottom[x,y,3] + 127) / a
				End If
				bottom[x,y,0] = a
				bottom[x,y,1] = r
				bottom[x,y,2] = g
				bottom[x,y,3] = b
			Next
		Next
	End Method
	
	Method SavePixelArray:Int(array:Int[,,],url:Object)
		Local x:Int
		Local y:Int
		Local savepixmap:tpixmap = CreatePixmap(width,height,PF_BGRA8888)
		
		For y = 0 Until height
			For x = 0 Until width
				WritePixel(savepixmap,x,y,GetRGBA(array[x,y,1],array[x,y,2],array[x,y,3],array[x,y,0]))
			Next
		Next
		
		SavePixmapPNG(savepixmap,url)
	End Method
	
	Method GetRGBA:Int(r:Int,g:Int,b:Int,a:Int=255)
		Return r | (g Shl 8) | (b Shl 16) | (a Shl 24)
	End Method
	
	Method GetA:Int(argb:Int)
		Return argb Shr 24 & %11111111
	End Method
	
	Method GetR:Int(argb:Int)
		Return argb & %11111111
	End Method
	
	Method GetG:Int(argb:Int)
		Return argb Shr 8 & %11111111
	End Method
	
	Method GetB:Int(argb:Int)
		Return argb Shr 16 & %11111111
	End Method
End Type

Function CreateTileMaker:ttilemaker()
	If appargs.length < 5 Return Null
	
	Local tilemaker:ttilemaker = New ttilemaker
	
	tilemaker.output = AppArgs[1]
	tilemaker.width = AppArgs[2].toint()
	tilemaker.height = AppArgs[3].toint()
	
	Local arg:Int = 4
	Local layer:tlayer
	
	While arg <= appargs.length - 3
		layer = New tlayer
		layer.pixmap = LoadPixmapPNG(AppArgs[arg])
		If layer.pixmap = Null Return Null
		layer.x = AppArgs[arg+1].toint()
		layer.y = AppArgs[arg+2].toint()
		tilemaker.layers.addlast(layer)
		arg:+3
	Wend
	
	tilemaker.pixmap = CreatePixmap(tilemaker.width,tilemaker.height,PF_BGRA8888)
	
	Return tilemaker
End Function

Type tlayer
	Field pixmap:tpixmap
	Field x:Int
	Field y:Int
End Type


'-----------------------------------------------------
Local tilemaker:ttilemaker = CreateTileMaker()

If tilemaker <> Null
	tilemaker.Save()
	Print "RESULT=1"
Else
	Print "RESULT=0"
End If