From Directorforum Collaboration Wiki
--c := a * b SHR 8;
--result := c + a * (255 - ((255-a)*(255-b) SHR 8)-c) SHR 8;
-- ******** XFader soft light mode - better than photoshop's version *******
on softlightBlendMode (image1, image2)
-- using the source image for the width and height
-- you can add your own code here to determine how
-- you want to handle this.
theWidth = image1.width - 1
theHeight = image1.height - 1
theImage = image(theWidth + 1, theHeight + 1, 32)
theDivisor = 255.0
-- running through each pixel in theImage
repeat with x = 0 to theWidth
repeat with y = 0 to theHeight
theColour1 = image1.getPixel(point(x,y))
theColour2 = image2.getPixel(point(x,y))
theComp = theColour1.red * theColour2.red / theDivisor
theRed = theComp + theColour1.red * (255 - ((255 - theColour1.red) * (255 - theColour2.red) / theDivisor) - theComp) / theDivisor
theComp = theColour1.green * theColour2.green / theDivisor
theGreen = theComp + theColour1.green * (255 - ((255 - theColour1.green) * (255 - theColour2.green) / theDivisor) - theComp) / theDivisor
theComp = theColour1.blue * theColour2.blue / theDivisor
theBlue = theComp + theColour1.blue * (255 - ((255 - theColour1.blue ) * (255 - theColour2.blue ) / theDivisor) - theComp) / theDivisor
theNewColour = color(theRed, theGreen, theBlue)
theImage.setPixel(point(x, y), theNewColour)
end repeat
end repeat
return theImage
end