Simple blendmode equations

multiply	a * b
screen	1 - (1 - a) * (1 - b)
darken	min(a, b)
lighten	max(a, b)
difference	abs(a - b)
negation	1 - abs(1 - a - b)
exclusion	a + b - 2 * a * b
overlay	a < .5 ? (2 * a * b) : (1 - 2 * (1 - a) * (1 - b))
hard light	b < .5 ? (2 * a * b) : (1 - 2 * (1 - a) * (1 - b))
soft light	b < .5 ? (2 * a * b + a * a * (1 - 2 * b)) : (sqrt(a) * (2 * b - 1) + (2 * a) * (1 - b))
dodge	a / (1 - b)
burn	1 - (1 - a) / b

taken from here, which in turn references
http://www.blackpawn.com/blog/coding/2006/08/19/photoshop-blend-modes/
and http://www.pegtop.net/delphi/articles/blendmodes/