Code archives/Graphics/AlphaBlit

This code has not been declared by its author to be Public Domain code, and therefore should not be assumed to be so.

Download source code

AlphaBlit by WebDext
(Posted 25 years ago)
Somewhat more flexable, maybe faster, maybe not... You'll have to try and compare...

This function alpha blits any small image onto a desitantion
buffer of equal or lesser size, It has no error handling to speed up the
function.

DestBuffer is the pointer to the desitnation image.
ImgBuffer is the pointer to the image to be Alpha Blited.
xOff, yOff are the ofsets of the image on the destination image.
Alpha ranges from 0 to 1 and is the opacity of the image. 
Dim pixelData(500, 500)
Function AlphaBitBlit(DestBuffer, Image, xOff, yOff, Alpha#)
	;This function alpha blits any image smaller than 100,50 onto a desitantion
	;buffer of equal or lesser size, It has no error handling to speed up the 
	;function.
	
	;DestBuffer is the pointer to the desitnation image.
	;ImgBuffer is the pointer to the image to be Alpha Blited.
	;xOff, yOff are the ofsets of the image on the destination image.
	;Alpha ranges from 0 to 1 and is the opacity of the image.

	InvAlpha# 	= 1.0 - Alpha#
	ImgBuffer 	= ImageBuffer(Image)
	iWidth 		= ImageWidth(Image)
	iHeight 	= ImageHeight(Image)
	
	LockBuffer ImgBuffer
		For x = 0 To iWidth-1
			For y = 0 To iHeight-1
				pixelData(x, y) = ReadPixelFast(x,y,ImgBuffer)
			Next
		Next
	UnlockBuffer ImgBuffer
	LockBuffer DestBuffer
		For x = 0 To iWidth-1
			For y = 0 To iHeight-1
				PixA# = ReadPixelFast(x + xOff, y + yOff)
				PixB# = pixelData(x, y)
				
				;Calculate Alphas with the least number of computations
				aR = Int(PixA Sar 16)
				aRI = aR Shl 16
				aG = Int((PixA - aRI) Sar 8)
				aB = PixA - (aRI + (aG Shl 8))

				;Calculate Alphas with the least number of computations
				bR = Int(PixB Sar 16)
				bRI = bR Shl 16
				bG = Int((PixB - bRI) Sar 8)
				bB = PixB - (bRI + (bG Shl 8))
				
				;Apply Alphas
				If bR > 0 Or bG > 0 Or bB > 0
					nR = (aR * InvAlpha) + (bR * Alpha)
					nG = (aG * InvAlpha) + (bG * Alpha)
					nB = (aB * InvAlpha) + (bB * Alpha)
				
					;Write Pixel to Buffer
					WritePixelFast x + xOff, y + yOff, (nR Shl 16) + (nG Shl 8) + nB
				EndIf
			Next
		Next
	UnlockBuffer DestBuffer
End Function