MixImages function help

BlitzPlus Forums/BlitzPlus Programming/MixImages function help

Hello everybody! I need an help. I need to speed up a little this function...can anyone help me? Thanks!
Function MixImages(mix,ima1,ima2)

	b1=ImageBuffer(ima1)
	b2=ImageBuffer(ima2)
	bm=ImageBuffer(mix)

	LockBuffer b1
	LockBuffer b2
	LockBuffer bm
	
	For y=0 To Screen_Height
		For x=0 To Screen_Width 
			c1=ReadPixelFast(x,y,b1)
			c2=ReadPixelFast(x,y,b2)
			WritePixelFast x,y,c1+c2,bm
		Next
	Next

	UnlockBuffer bm
	UnlockBuffer b2
	UnlockBuffer b1
	
End Function